par/par_msquares.h

// MSQUARES :: https://github.com/prideout/par
// Converts fp32 grayscale images, or 8-bit color images, into triangles.
//
// For grayscale images, a threshold is specified to determine insideness.
// For color images, an exact color is specified to determine insideness.
// Color images can be r8, rg16, rgb24, or rgba32. For a visual overview of
// the API and all the flags, see:
//
//     http://github.prideout.net/marching-squares/
//
// The MIT License
// Copyright (c) 2015 Philip Rideout

#include <stdint.h>

// -----------------------------------------------------------------------------
// BEGIN PUBLIC API
// -----------------------------------------------------------------------------

typedef uint8_t par_byte;

typedef struct par_msquares_meshlist_s par_msquares_meshlist;

// Results of a marching squares operation.  Triangles are counter-clockwise.
typedef struct {
    float* points;        // pointer to XY (or XYZ) vertex coordinates
    int npoints;          // number of vertex coordinates
    uint16_t* triangles;  // pointer to 3-tuples of vertex indices
    int ntriangles;       // number of 3-tuples
    int dim;              // number of floats per point (either 2 or 3)
    uint32_t color;       // used only with par_msquares_color_multi
} par_msquares_mesh;

// Polyline boundary extracted from a mesh, composed of one or more chains.
// Counterclockwise chains are solid, clockwise chains are holes.  So, when
// serializing to SVG, all chains can be aggregated in a single <path>,
// provided they each terminate with a "Z" and use the default fill rule.
typedef struct {
    float* points;        // list of XY vertex coordinates
    int npoints;          // number of vertex coordinates
    float** chains;       // list of pointers to the start of each chain
    uint16_t* lengths;    // list of chain lengths
    int nchains;          // number of chains
} par_msquares_boundary;

// Reverses the "insideness" test.
#define PAR_MSQUARES_INVERT (1 << 0)

// Returns a meshlist with two meshes: one for the inside, one for the outside.
#define PAR_MSQUARES_DUAL (1 << 1)

// Requests that returned meshes have 3-tuple coordinates instead of 2-tuples.
// When using a color-based function, the Z coordinate represents the alpha
// value of the nearest pixel.
#define PAR_MSQUARES_HEIGHTS (1 << 2)

// Applies a step function to the Z coordinates.  Requires HEIGHTS and DUAL.
#define PAR_MSQUARES_SNAP (1 << 3)

// Adds extrusion triangles to each mesh other than the lowest mesh.  Requires
// the PAR_MSQUARES_HEIGHTS flag to be present.
#define PAR_MSQUARES_CONNECT (1 << 4)

// Enables quick & dirty (not best) simpification of the returned mesh.
#define PAR_MSQUARES_SIMPLIFY (1 << 5)

// Indicates that the "color" argument is ABGR instead of ARGB.
#define PAR_MSQUARES_SWIZZLE (1 << 6)

// Ensures there are no T-junction vertices. (par_msquares_color_multi only)
// Requires the PAR_MSQUARES_SIMPLIFY flag to be disabled.
#define PAR_MSQUARES_CLEAN (1 << 7)

par_msquares_meshlist* par_msquares_grayscale(float const* data, int width,
    int height, int cellsize, float threshold, int flags);

par_msquares_meshlist* par_msquares_color(par_byte const* data, int width,
    int height, int cellsize, uint32_t color, int bpp, int flags);

par_msquares_mesh const* par_msquares_get_mesh(par_msquares_meshlist*, int n);

int par_msquares_get_count(par_msquares_meshlist*);

void par_msquares_free(par_msquares_meshlist*);

void par_msquares_free_boundary(par_msquares_boundary*);

typedef int (*par_msquares_inside_fn)(int, void*);
typedef float (*par_msquares_height_fn)(float, float, void*);

par_msquares_meshlist* par_msquares_function(int width, int height,
    int cellsize, int flags, void* context, par_msquares_inside_fn insidefn,
    par_msquares_height_fn heightfn);

par_msquares_meshlist* par_msquares_grayscale_multi(float const* data,
    int width, int height, int cellsize, float const* thresholds,
    int nthresholds, int flags);

par_msquares_meshlist* par_msquares_color_multi(par_byte const* data, int width,
    int height, int cellsize, int bpp, int flags);

par_msquares_boundary* par_msquares_extract_boundary(par_msquares_mesh const* );

#ifndef PAR_HELPERS
#define PAR_HELPERS 1
#define PAR_PI (3.14159265359)
#define PAR_MIN(a, b) (a > b ? b : a)
#define PAR_MAX(a, b) (a > b ? a : b)
#define PAR_CLAMP(v, lo, hi) PAR_MAX(lo, PAR_MIN(hi, v))
#define PAR_MALLOC(T, N) ((T*) malloc(N * sizeof(T)))
#define PAR_CALLOC(T, N) ((T*) calloc(N * sizeof(T), 1))
#define PAR_REALLOC(T, BUF, N) ((T*) realloc(BUF, sizeof(T) * N))
#define PAR_FREE(BUF) free(BUF)
#define PAR_SWAP(T, A, B) { T tmp = B; B = A; A = tmp; }
#define PAR_SQR(a) (a * a)
#endif

// -----------------------------------------------------------------------------
// END PUBLIC API
// -----------------------------------------------------------------------------

#ifdef PAR_MSQUARES_IMPLEMENTATION

#include <stdlib.h>
#include <assert.h>
#include <float.h>
#include <string.h>

typedef struct {
    uint16_t* values;
    size_t count;
    size_t capacity;
} par__uint16list;

typedef struct {
    float* points;
    int npoints;
    uint16_t* triangles;
    int ntriangles;
    int dim;
    uint32_t color;
    int nconntriangles;
    uint16_t* conntri;
    par__uint16list* tjunctions;
} par_msquares__mesh;

struct par_msquares_meshlist_s {
    int nmeshes;
    par_msquares__mesh** meshes;
};

static int** par_msquares_binary_point_table = 0;
static int** par_msquares_binary_triangle_table = 0;
static int* par_msquares_quaternary_triangle_table[64][4];
static int* par_msquares_quaternary_boundary_table[64][4];

static par_msquares_meshlist* par_msquares__merge(par_msquares_meshlist** lists,
    int count, int snap);

static void par_init_tables()
{
    char const* BINARY_TABLE =
        "0"
        "1017"
        "1123"
        "2023370"
        "1756"
        "2015560"
        "2123756"
        "3023035056"
        "1345"
        "4013034045057"
        "2124451"
        "3024045057"
        "2734467"
        "3013034046"
        "3124146167"
        "2024460";
    char const* binary_token = BINARY_TABLE;

    par_msquares_binary_point_table = PAR_CALLOC(int*, 16);
    par_msquares_binary_triangle_table = PAR_CALLOC(int*, 16);
    for (int i = 0; i < 16; i++) {
        int ntris = *binary_token - '0';
        binary_token++;
        par_msquares_binary_triangle_table[i] =
            PAR_CALLOC(int, (ntris + 1) * 3);
        int* sqrtris = par_msquares_binary_triangle_table[i];
        sqrtris[0] = ntris;
        int mask = 0;
        int* sqrpts = par_msquares_binary_point_table[i] = PAR_CALLOC(int, 7);
        sqrpts[0] = 0;
        for (int j = 0; j < ntris * 3; j++, binary_token++) {
            int midp = *binary_token - '0';
            int bit = 1 << midp;
            if (!(mask & bit)) {
                mask |= bit;
                sqrpts[++sqrpts[0]] = midp;
            }
            sqrtris[j + 1] = midp;
        }
    }

    char const* QUATERNARY_TABLE =
        "2024046000"
        "3346360301112300"
        "3346360301112300"
        "3346360301112300"
        "3560502523013450"
        "2015056212414500"
        "4018087785756212313828348450"
        "4018087785756212313828348450"
        "3560502523013450"
        "4018087785756212313828348450"
        "2015056212414500"
        "4018087785756212313828348450"
        "3560502523013450"
        "4018087785756212313828348450"
        "4018087785756212313828348450"
        "2015056212414500"
        "3702724745001756"
        "2018087212313828348452785756"
        "4013034045057112301756"
        "4013034045057112301756"
        "2023037027347460"
        "1701312414616700"
        "2018087212313847857568348450"
        "2018087212313847857568348450"
        "4018087123138028348452785756"
        "1701467161262363513450"
        "2018087412313883484502785756"
        "2018087212313828348452785756"
        "4018087123138028348452785756"
        "1701467161262363513450"
        "2018087212313828348452785756"
        "2018087412313883484502785756"
        "3702724745001756"
        "4013034045057112301756"
        "2018087212313828348452785756"
        "4013034045057112301756"
        "4018087123138028348452785756"
        "2018087412313883484502785756"
        "1701467161262363513450"
        "2018087212313828348452785756"
        "2023037027347460"
        "2018087212313847857568348450"
        "1701312414616700"
        "2018087212313847857568348450"
        "4018087123138028348452785756"
        "2018087212313828348452785756"
        "1701467161262363513450"
        "2018087412313883484502785756"
        "3702724745001756"
        "4013034045057112301756"
        "4013034045057112301756"
        "2018087212313828348452785756"
        "4018087123138028348452785756"
        "2018087412313883484502785756"
        "2018087212313828348452785756"
        "1701467161262363513450"
        "4018087123138028348452785756"
        "2018087212313828348452785756"
        "2018087412313883484502785756"
        "1701467161262363513450"
        "2023037027347460"
        "2018087212313847857568348450"
        "2018087212313847857568348450"
        "1701312414616700";
    char const* quaternary_token = QUATERNARY_TABLE;

    int* quaternary_values = PAR_CALLOC(int, strlen(QUATERNARY_TABLE));
    int* vals = quaternary_values;
    for (int i = 0; i < 64; i++) {
        int ntris = *quaternary_token++ - '0';
        *vals = ntris;
        par_msquares_quaternary_triangle_table[i][0] = vals++;
        for (int j = 0; j < ntris * 3; j++) {
            int pt = *quaternary_token++ - '0';
            assert(pt >= 0 && pt < 9);
            *vals++ = pt;
        }
        ntris = *quaternary_token++ - '0';
        *vals = ntris;
        par_msquares_quaternary_triangle_table[i][1] = vals++;
        for (int j = 0; j < ntris * 3; j++) {
            int pt = *quaternary_token++ - '0';
            assert(pt >= 0 && pt < 9);
            *vals++ = pt;
        }
        ntris = *quaternary_token++ - '0';
        *vals = ntris;
        par_msquares_quaternary_triangle_table[i][2] = vals++;
        for (int j = 0; j < ntris * 3; j++) {
            int pt = *quaternary_token++ - '0';
            assert(pt >= 0 && pt < 9);
            *vals++ = pt;
        }
        ntris = *quaternary_token++ - '0';
        *vals = ntris;
        par_msquares_quaternary_triangle_table[i][3] = vals++;
        for (int j = 0; j < ntris * 3; j++) {
            int pt = *quaternary_token++ - '0';
            assert(pt >= 0 && pt < 9);
            *vals++ = pt;
        }
    }
    assert(vals == quaternary_values + strlen(QUATERNARY_TABLE));

    char const* QUATERNARY_EDGES =
        "0000"
        "11313100113131001131310013501530"
        "115151002188523881258830218852388125883013501530"
        "218852388125883011515100218852388125883013501530"
        "218852388125883021885238812588301151510015700175"
        "2188723881258832788521357131017521357131017513701730"
        "11717100218872388127883021887238812788302388702588327885"
        "1172713515302188725881027885218872388125883278852388702588327885"
        "11727135153021887238812588327885218872588102788515700175"
        "213571310175218872388125883278852135713101752388702588327885"
        "21887258810278851172713515302188723881258832788513701730"
        "21887238812788301171710021887238812788302388702588327885"
        "21887238812588327885117271351530218872588102788515700175"
        "213571310175213571310175218872388125883278852388702588327885"
        "2188725881027885218872388125883278851172713515302388702588327885"
        "21887238812588327885218872588102788511727135153013701730"
        "2188723881278830218872388127883011717100";
    quaternary_token = QUATERNARY_EDGES;

    quaternary_values = PAR_CALLOC(int, strlen(QUATERNARY_EDGES));
    vals = quaternary_values;
    for (int i = 0; i < 64; i++) {
        int nedges = *quaternary_token++ - '0';
        *vals = nedges;
        par_msquares_quaternary_boundary_table[i][0] = vals++;
        for (int j = 0; j < nedges * 2; j++) {
            int pt = *quaternary_token++ - '0';
            assert(pt >= 0 && pt < 9);
            *vals++ = pt;
        }
        nedges = *quaternary_token++ - '0';
        *vals = nedges;
        par_msquares_quaternary_boundary_table[i][1] = vals++;
        for (int j = 0; j < nedges * 2; j++) {
            int pt = *quaternary_token++ - '0';
            assert(pt >= 0 && pt < 9);
            *vals++ = pt;
        }
        nedges = *quaternary_token++ - '0';
        *vals = nedges;
        par_msquares_quaternary_boundary_table[i][2] = vals++;
        for (int j = 0; j < nedges * 2; j++) {
            int pt = *quaternary_token++ - '0';
            assert(pt >= 0 && pt < 9);
            *vals++ = pt;
        }
        nedges = *quaternary_token++ - '0';
        *vals = nedges;
        par_msquares_quaternary_boundary_table[i][3] = vals++;
        for (int j = 0; j < nedges * 2; j++) {
            int pt = *quaternary_token++ - '0';
            assert(pt >= 0 && pt < 9);
            *vals++ = pt;
        }
    }
    assert(vals == quaternary_values + strlen(QUATERNARY_EDGES));
}

typedef struct {
    float const* data;
    float threshold;
    float lower_bound;
    float upper_bound;
    int width;
    int height;
} par_gray_context;

static int gray_inside(int location, void* contextptr)
{
    par_gray_context* context = (par_gray_context*) contextptr;
    return context->data[location] > context->threshold;
}

static int gray_multi_inside(int location, void* contextptr)
{
    par_gray_context* context = (par_gray_context*) contextptr;
    float val = context->data[location];
    float upper = context->upper_bound;
    float lower = context->lower_bound;
    return val >= lower && val < upper;
}

static float gray_height(float x, float y, void* contextptr)
{
    par_gray_context* context = (par_gray_context*) contextptr;
    int i = PAR_CLAMP(context->width * x, 0, context->width - 1);
    int j = PAR_CLAMP(context->height * y, 0, context->height - 1);
    return context->data[i + j * context->width];
}

typedef struct {
    par_byte const* data;
    par_byte color[4];
    int bpp;
    int width;
    int height;
} par_color_context;

static int color_inside(int location, void* contextptr)
{
    par_color_context* context = (par_color_context*) contextptr;
    par_byte const* data = context->data + location * context->bpp;
    for (int i = 0; i < context->bpp; i++) {
        if (data[i] != context->color[i]) {
            return 0;
        }
    }
    return 1;
}

static float color_height(float x, float y, void* contextptr)
{
    par_color_context* context = (par_color_context*) contextptr;
    assert(context->bpp == 4);
    int i = PAR_CLAMP(context->width * x, 0, context->width - 1);
    int j = PAR_CLAMP(context->height * y, 0, context->height - 1);
    int k = i + j * context->width;
    return context->data[k * 4 + 3] / 255.0;
}

par_msquares_meshlist* par_msquares_color(par_byte const* data, int width,
    int height, int cellsize, uint32_t color, int bpp, int flags)
{
    par_color_context context;
    context.bpp = bpp;
    if (flags & PAR_MSQUARES_SWIZZLE) {
        context.color[0] = (color >>  0) & 0xff;
        context.color[1] = (color >>  8) & 0xff;
        context.color[2] = (color >> 16) & 0xff;
        context.color[3] = (color >> 24) & 0xff;
    } else {
        context.color[0] = (color >> 16) & 0xff;
        context.color[1] = (color >>  8) & 0xff;
        context.color[2] = (color >>  0) & 0xff;
        context.color[3] = (color >> 24) & 0xff;
    }
    context.data = data;
    context.width = width;
    context.height = height;
    return par_msquares_function(
        width, height, cellsize, flags, &context, color_inside, color_height);
}

par_msquares_meshlist* par_msquares_grayscale(float const* data, int width,
    int height, int cellsize, float threshold, int flags)
{
    par_gray_context context;
    context.width = width;
    context.height = height;
    context.data = data;
    context.threshold = threshold;
    return par_msquares_function(
        width, height, cellsize, flags, &context, gray_inside, gray_height);
}

par_msquares_meshlist* par_msquares_grayscale_multi(float const* data,
    int width, int height, int cellsize, float const* thresholds,
    int nthresholds, int flags)
{
    par_msquares_meshlist* mlists[2];
    mlists[0] = PAR_CALLOC(par_msquares_meshlist, 1);
    int connect = flags & PAR_MSQUARES_CONNECT;
    int snap = flags & PAR_MSQUARES_SNAP;
    int heights = flags & PAR_MSQUARES_HEIGHTS;
    if (!heights) {
        snap = connect = 0;
    }
    flags &= ~PAR_MSQUARES_INVERT;
    flags &= ~PAR_MSQUARES_DUAL;
    flags &= ~PAR_MSQUARES_CONNECT;
    flags &= ~PAR_MSQUARES_SNAP;
    par_gray_context context;
    context.width = width;
    context.height = height;
    context.data = data;
    context.lower_bound = -FLT_MAX;
    for (int i = 0; i <= nthresholds; i++) {
        int mergeconf = i > 0 ? connect : 0;
        if (i == nthresholds) {
            context.upper_bound = FLT_MAX;
            mergeconf |= snap;
        } else {
            context.upper_bound = thresholds[i];
        }
        mlists[1] = par_msquares_function(width, height, cellsize, flags,
            &context, gray_multi_inside, gray_height);
        mlists[0] = par_msquares__merge(mlists, 2, mergeconf);
        context.lower_bound = context.upper_bound;
        flags |= connect;
    }
    return mlists[0];
}

par_msquares_mesh const* par_msquares_get_mesh(
    par_msquares_meshlist* mlist, int mindex)
{
    assert(mlist && mindex < mlist->nmeshes);
    return (par_msquares_mesh const*) mlist->meshes[mindex];
}

int par_msquares_get_count(par_msquares_meshlist* mlist)
{
    assert(mlist);
    return mlist->nmeshes;
}

void par_msquares_free(par_msquares_meshlist* mlist)
{
    if (!mlist) {
        return;
    }
    par_msquares__mesh** meshes = mlist->meshes;
    for (int i = 0; i < mlist->nmeshes; i++) {
        free(meshes[i]->points);
        free(meshes[i]->triangles);
        free(meshes[i]);
    }
    free(meshes);
    free(mlist);
}

// Combine multiple meshlists by moving mesh pointers, and optionally apply
// a "snap" operation that assigns a single Z value across all verts in each
// mesh.  The Z value determined by the mesh's position in the final mesh list.
static par_msquares_meshlist* par_msquares__merge(par_msquares_meshlist** lists,
    int count, int snap)
{
    par_msquares_meshlist* merged = PAR_CALLOC(par_msquares_meshlist, 1);
    merged->nmeshes = 0;
    for (int i = 0; i < count; i++) {
        merged->nmeshes += lists[i]->nmeshes;
    }
    merged->meshes = PAR_CALLOC(par_msquares__mesh*, merged->nmeshes);
    par_msquares__mesh** pmesh = merged->meshes;
    for (int i = 0; i < count; i++) {
        par_msquares_meshlist* meshlist = lists[i];
        for (int j = 0; j < meshlist->nmeshes; j++) {
            *pmesh++ = meshlist->meshes[j];
        }
        free(meshlist);
    }
    if (!snap) {
        return merged;
    }
    pmesh = merged->meshes;
    float zmin = FLT_MAX;
    float zmax = -zmin;
    for (int i = 0; i < merged->nmeshes; i++, pmesh++) {
        float* pzed = (*pmesh)->points + 2;
        for (int j = 0; j < (*pmesh)->npoints; j++, pzed += 3) {
            zmin = PAR_MIN(*pzed, zmin);
            zmax = PAR_MAX(*pzed, zmax);
        }
    }
    float zextent = zmax - zmin;
    pmesh = merged->meshes;
    for (int i = 0; i < merged->nmeshes; i++, pmesh++) {
        float* pzed = (*pmesh)->points + 2;
        float zed = zmin + zextent * i / (merged->nmeshes - 1);
        for (int j = 0; j < (*pmesh)->npoints; j++, pzed += 3) {
            *pzed = zed;
        }
    }
    if (!(snap & PAR_MSQUARES_CONNECT)) {
        return merged;
    }
    for (int i = 1; i < merged->nmeshes; i++) {
        par_msquares__mesh* mesh = merged->meshes[i];

        // Find all extrusion points.  This is tightly coupled to the
        // tessellation code, which generates two "connector" triangles for each
        // extruded edge.  The first two verts of the second triangle are the
        // verts that need to be displaced.
        char* markers = PAR_CALLOC(char, mesh->npoints);
        int tri = mesh->ntriangles - mesh->nconntriangles;
        while (tri < mesh->ntriangles) {
            markers[mesh->triangles[tri * 3 + 3]] = 1;
            markers[mesh->triangles[tri * 3 + 4]] = 1;
            tri += 2;
        }

        // Displace all extrusion points down to the previous level.
        float zed = zmin + zextent * (i - 1) / (merged->nmeshes - 1);
        float* pzed = mesh->points + 2;
        for (int j = 0; j < mesh->npoints; j++, pzed += 3) {
            if (markers[j]) {
                *pzed = zed;
            }
        }
        free(markers);
    }
    return merged;
}

static void par_remove_unreferenced_verts(par_msquares__mesh* mesh)
{
    if (mesh->npoints == 0) {
        return;
    }
    char* markers = PAR_CALLOC(char, mesh->npoints);
    uint16_t const* ptris = mesh->triangles;
    int newnpts = 0;
    for (int i = 0; i < mesh->ntriangles * 3; i++, ptris++) {
        if (!markers[*ptris]) {
            newnpts++;
            markers[*ptris] = 1;
        }
    }
    float* newpts = PAR_CALLOC(float, newnpts * mesh->dim);
    uint16_t* mapping = PAR_CALLOC(uint16_t, mesh->npoints);
    float const* ppts = mesh->points;
    float* pnewpts = newpts;
    int j = 0;
    if (mesh->dim == 3) {
        for (int i = 0; i < mesh->npoints; i++, ppts += 3) {
            if (markers[i]) {
                *pnewpts++ = ppts[0];
                *pnewpts++ = ppts[1];
                *pnewpts++ = ppts[2];
                mapping[i] = j++;
            }
        }
    } else {
        for (int i = 0; i < mesh->npoints; i++, ppts += 2) {
            if (markers[i]) {
                *pnewpts++ = ppts[0];
                *pnewpts++ = ppts[1];
                mapping[i] = j++;
            }
        }
    }
    free(mesh->points);
    free(markers);
    mesh->points = newpts;
    mesh->npoints = newnpts;
    for (int i = 0; i < mesh->ntriangles * 3; i++) {
        mesh->triangles[i] = mapping[mesh->triangles[i]];
    }
    free(mapping);
}

par_msquares_meshlist* par_msquares_function(int width, int height,
    int cellsize, int flags, void* context, par_msquares_inside_fn insidefn,
    par_msquares_height_fn heightfn)
{
    assert(width > 0 && width % cellsize == 0);
    assert(height > 0 && height % cellsize == 0);

    if (flags & PAR_MSQUARES_DUAL) {
        int connect = flags & PAR_MSQUARES_CONNECT;
        int snap = flags & PAR_MSQUARES_SNAP;
        int heights = flags & PAR_MSQUARES_HEIGHTS;
        if (!heights) {
            snap = connect = 0;
        }
        flags ^= PAR_MSQUARES_INVERT;
        flags &= ~PAR_MSQUARES_DUAL;
        flags &= ~PAR_MSQUARES_CONNECT;
        par_msquares_meshlist* m[2];
        m[0] = par_msquares_function(width, height, cellsize, flags,
            context, insidefn, heightfn);
        flags ^= PAR_MSQUARES_INVERT;
        if (connect) {
            flags |= PAR_MSQUARES_CONNECT;
        }
        m[1] = par_msquares_function(width, height, cellsize, flags,
            context, insidefn, heightfn);
        return par_msquares__merge(m, 2, snap | connect);
    }

    int invert = flags & PAR_MSQUARES_INVERT;

    // Create the two code tables if we haven't already.  These are tables of
    // fixed constants, so it's embarassing that we use dynamic memory
    // allocation for them.  However it's easy and it's one-time-only.
    if (!par_msquares_binary_point_table) {
        par_init_tables();
    }

    // Allocate the meshlist and the first mesh.
    par_msquares_meshlist* mlist = PAR_CALLOC(par_msquares_meshlist, 1);
    mlist->nmeshes = 1;
    mlist->meshes = PAR_CALLOC(par_msquares__mesh*, 1);
    mlist->meshes[0] = PAR_CALLOC(par_msquares__mesh, 1);
    par_msquares__mesh* mesh = mlist->meshes[0];
    mesh->dim = (flags & PAR_MSQUARES_HEIGHTS) ? 3 : 2;
    int ncols = width / cellsize;
    int nrows = height / cellsize;

    // Worst case is four triangles and six verts per cell, so allocate that
    // much.
    int maxtris = ncols * nrows * 4;
    int maxpts = ncols * nrows * 6;
    int maxedges = ncols * nrows * 2;

    // However, if we include extrusion triangles for boundary edges,
    // we need space for another 4 triangles and 4 points per cell.
    uint16_t* conntris = 0;
    int nconntris = 0;
    uint16_t* edgemap = 0;
    if (flags & PAR_MSQUARES_CONNECT) {
        conntris = PAR_CALLOC(uint16_t, maxedges * 6);
        maxtris +=  maxedges * 2;
        maxpts += maxedges * 2;
        edgemap = PAR_CALLOC(uint16_t, maxpts);
        for (int i = 0; i < maxpts; i++) {
            edgemap[i] = 0xffff;
        }
    }
    uint16_t* tris = PAR_CALLOC(uint16_t, maxtris * 3);
    int ntris = 0;
    float* pts = PAR_CALLOC(float, maxpts * mesh->dim);
    int npts = 0;

    // The "verts" x/y/z arrays are the 4 corners and 4 midpoints around the
    // square, in counter-clockwise order.  The origin of "triangle space" is at
    // the lower-left, although we expect the image data to be in raster order
    // (starts at top-left).
    float vertsx[8], vertsy[8];
    float normalization = 1.0f / PAR_MAX(width, height);
    float normalized_cellsize = cellsize * normalization;
    int maxrow = (height - 1) * width;
    uint16_t* ptris = tris;
    uint16_t* pconntris = conntris;
    float* ppts = pts;
    uint8_t* prevrowmasks = PAR_CALLOC(uint8_t, ncols);
    int* prevrowinds = PAR_CALLOC(int, ncols * 3);

    // If simplification is enabled, we need to track all 'F' cells and their
    // repsective triangle indices.
    uint8_t* simplification_codes = 0;
    uint16_t* simplification_tris = 0;
    uint8_t* simplification_ntris = 0;
    if (flags & PAR_MSQUARES_SIMPLIFY) {
        simplification_codes = PAR_CALLOC(uint8_t, nrows * ncols);
        simplification_tris = PAR_CALLOC(uint16_t, nrows * ncols);
        simplification_ntris = PAR_CALLOC(uint8_t, nrows * ncols);
    }

    // Do the march!
    for (int row = 0; row < nrows; row++) {
        vertsx[0] = vertsx[6] = vertsx[7] = 0;
        vertsx[1] = vertsx[5] = 0.5 * normalized_cellsize;
        vertsx[2] = vertsx[3] = vertsx[4] = normalized_cellsize;
        vertsy[0] = vertsy[1] = vertsy[2] = normalized_cellsize * (row + 1);
        vertsy[4] = vertsy[5] = vertsy[6] = normalized_cellsize * row;
        vertsy[3] = vertsy[7] = normalized_cellsize * (row + 0.5);

        int northi = row * cellsize * width;
        int southi = PAR_MIN(northi + cellsize * width, maxrow);
        int northwest = invert ^ insidefn(northi, context);
        int southwest = invert ^ insidefn(southi, context);
        int previnds[8] = {0};
        uint8_t prevmask = 0;

        for (int col = 0; col < ncols; col++) {
            northi += cellsize;
            southi += cellsize;
            if (col == ncols - 1) {
                northi--;
                southi--;
            }

            int northeast = invert ^ insidefn(northi, context);
            int southeast = invert ^ insidefn(southi, context);
            int code = southwest | (southeast << 1) | (northwest << 2) |
                (northeast << 3);

            int const* pointspec = par_msquares_binary_point_table[code];
            int ptspeclength = *pointspec++;
            int currinds[8] = {0};
            uint8_t mask = 0;
            uint8_t prevrowmask = prevrowmasks[col];
            while (ptspeclength--) {
                int midp = *pointspec++;
                int bit = 1 << midp;
                mask |= bit;

                // The following six conditionals perform welding to reduce the
                // number of vertices.  The first three perform welding with the
                // cell to the west; the latter three perform welding with the
                // cell to the north.
                if (bit == 1 && (prevmask & 4)) {
                    currinds[midp] = previnds[2];
                    continue;
                }
                if (bit == 128 && (prevmask & 8)) {
                    currinds[midp] = previnds[3];
                    continue;
                }
                if (bit == 64 && (prevmask & 16)) {
                    currinds[midp] = previnds[4];
                    continue;
                }
                if (bit == 16 && (prevrowmask & 4)) {
                    currinds[midp] = prevrowinds[col * 3 + 2];
                    continue;
                }
                if (bit == 32 && (prevrowmask & 2)) {
                    currinds[midp] = prevrowinds[col * 3 + 1];
                    continue;
                }
                if (bit == 64 && (prevrowmask & 1)) {
                    currinds[midp] = prevrowinds[col * 3 + 0];
                    continue;
                }

                ppts[0] = vertsx[midp];
                ppts[1] = vertsy[midp];

                // Adjust the midpoints to a more exact crossing point.
                if (midp == 1) {
                    int begin = southi - cellsize / 2;
                    int previous = 0;
                    for (int i = 0; i < cellsize; i++) {
                        int offset = begin + i / 2 * ((i % 2) ? -1 : 1);
                        int inside = insidefn(offset, context);
                        if (i > 0 && inside != previous) {
                            ppts[0] = normalization *
                                (col * cellsize + offset - southi + cellsize);
                            break;
                        }
                        previous = inside;
                    }
                } else if (midp == 5) {
                    int begin = northi - cellsize / 2;
                    int previous = 0;
                    for (int i = 0; i < cellsize; i++) {
                        int offset = begin + i / 2 * ((i % 2) ? -1 : 1);
                        int inside = insidefn(offset, context);
                        if (i > 0 && inside != previous) {
                            ppts[0] = normalization *
                                (col * cellsize + offset - northi + cellsize);
                            break;
                        }
                        previous = inside;
                    }
                } else if (midp == 3) {
                    int begin = northi + width * cellsize / 2;
                    int previous = 0;
                    for (int i = 0; i < cellsize; i++) {
                        int offset = begin +
                            width * (i / 2 * ((i % 2) ? -1 : 1));
                        int inside = insidefn(offset, context);
                        if (i > 0 && inside != previous) {
                            ppts[1] = normalization *
                                (row * cellsize +
                                (offset - northi) / (float) width);
                            break;
                        }
                        previous = inside;
                    }
                } else if (midp == 7) {
                    int begin = northi + width * cellsize / 2 - cellsize;
                    int previous = 0;
                    for (int i = 0; i < cellsize; i++) {
                        int offset = begin +
                            width * (i / 2 * ((i % 2) ? -1 : 1));
                        int inside = insidefn(offset, context);
                        if (i > 0 && inside != previous) {
                            ppts[1] = normalization *
                                (row * cellsize +
                                (offset - northi - cellsize) / (float) width);
                            break;
                        }
                        previous = inside;
                    }
                }

                if (mesh->dim == 3) {
                    ppts[2] = heightfn(ppts[0], ppts[1], context);
                }

                ppts += mesh->dim;
                currinds[midp] = npts++;
            }

            int const* trianglespec = par_msquares_binary_triangle_table[code];
            int trispeclength = *trianglespec++;

            if (flags & PAR_MSQUARES_SIMPLIFY) {
                simplification_codes[ncols * row + col] = code;
                simplification_tris[ncols * row + col] = ntris;
                simplification_ntris[ncols * row + col] = trispeclength;
            }

            // Add triangles.
            while (trispeclength--) {
                int a = *trianglespec++;
                int b = *trianglespec++;
                int c = *trianglespec++;
                *ptris++ = currinds[c];
                *ptris++ = currinds[b];
                *ptris++ = currinds[a];
                ntris++;
            }

            // Create two extrusion triangles for each boundary edge.
            if (flags & PAR_MSQUARES_CONNECT) {
                trianglespec = par_msquares_binary_triangle_table[code];
                trispeclength = *trianglespec++;
                while (trispeclength--) {
                    int a = *trianglespec++;
                    int b = *trianglespec++;
                    int c = *trianglespec++;
                    int i = currinds[a];
                    int j = currinds[b];
                    int k = currinds[c];
                    int u = 0, v = 0, w = 0;
                    if ((a % 2) && (b % 2)) {
                        u = v = 1;
                    } else if ((a % 2) && (c % 2)) {
                        u = w = 1;
                    } else if ((b % 2) && (c % 2)) {
                        v = w = 1;
                    } else {
                        continue;
                    }
                    if (u && edgemap[i] == 0xffff) {
                        for (int d = 0; d < mesh->dim; d++) {
                            *ppts++ = pts[i * mesh->dim + d];
                        }
                        edgemap[i] = npts++;
                    }
                    if (v && edgemap[j] == 0xffff) {
                        for (int d = 0; d < mesh->dim; d++) {
                            *ppts++ = pts[j * mesh->dim + d];
                        }
                        edgemap[j] = npts++;
                    }
                    if (w && edgemap[k] == 0xffff) {
                        for (int d = 0; d < mesh->dim; d++) {
                            *ppts++ = pts[k * mesh->dim + d];
                        }
                        edgemap[k] = npts++;
                    }
                    if ((a % 2) && (b % 2)) {
                        *pconntris++ = i;
                        *pconntris++ = j;
                        *pconntris++ = edgemap[j];
                        *pconntris++ = edgemap[j];
                        *pconntris++ = edgemap[i];
                        *pconntris++ = i;
                    } else if ((a % 2) && (c % 2)) {
                        *pconntris++ = edgemap[k];
                        *pconntris++ = k;
                        *pconntris++ = i;
                        *pconntris++ = edgemap[i];
                        *pconntris++ = edgemap[k];
                        *pconntris++ = i;
                    } else if ((b % 2) && (c % 2)) {
                        *pconntris++ = j;
                        *pconntris++ = k;
                        *pconntris++ = edgemap[k];
                        *pconntris++ = edgemap[k];
                        *pconntris++ = edgemap[j];
                        *pconntris++ = j;
                    }
                    nconntris += 2;
                }
            }

            // Prepare for the next cell.
            prevrowmasks[col] = mask;
            prevrowinds[col * 3 + 0] = currinds[0];
            prevrowinds[col * 3 + 1] = currinds[1];
            prevrowinds[col * 3 + 2] = currinds[2];
            prevmask = mask;
            northwest = northeast;
            southwest = southeast;
            for (int i = 0; i < 8; i++) {
                previnds[i] = currinds[i];
                vertsx[i] += normalized_cellsize;
            }
        }
    }
    free(edgemap);
    free(prevrowmasks);
    free(prevrowinds);

    // Perform quick-n-dirty simplification by iterating two rows at a time.
    // In no way does this create the simplest possible mesh, but at least it's
    // fast and easy.
    if (flags & PAR_MSQUARES_SIMPLIFY) {
        int in_run = 0, start_run;

        // First figure out how many triangles we can eliminate.
        int neliminated_triangles = 0;
        for (int row = 0; row < nrows - 1; row += 2) {
            for (int col = 0; col < ncols; col++) {
                int a = simplification_codes[ncols * row + col] == 0xf;
                int b = simplification_codes[ncols * row + col + ncols] == 0xf;
                if (a && b) {
                    if (!in_run) {
                        in_run = 1;
                        start_run = col;
                    }
                    continue;
                }
                if (in_run) {
                    in_run = 0;
                    int run_width = col - start_run;
                    neliminated_triangles += run_width * 4 - 2;
                }
            }
            if (in_run) {
                in_run = 0;
                int run_width = ncols - start_run;
                neliminated_triangles += run_width * 4 - 2;
            }
        }

        // Build a new index array cell-by-cell.  If any given cell is 'F' and
        // its neighbor to the south is also 'F', then it's part of a run.
        int nnewtris = ntris + nconntris - neliminated_triangles;
        uint16_t* newtris = PAR_CALLOC(uint16_t, nnewtris * 3);
        uint16_t* pnewtris = newtris;
        in_run = 0;
        for (int row = 0; row < nrows - 1; row += 2) {
            for (int col = 0; col < ncols; col++) {
                int cell = ncols * row + col;
                int south = cell + ncols;
                int a = simplification_codes[cell] == 0xf;
                int b = simplification_codes[south] == 0xf;
                if (a && b) {
                    if (!in_run) {
                        in_run = 1;
                        start_run = col;
                    }
                    continue;
                }
                if (in_run) {
                    in_run = 0;
                    int nw_cell = ncols * row + start_run;
                    int ne_cell = ncols * row + col - 1;
                    int sw_cell = nw_cell + ncols;
                    int se_cell = ne_cell + ncols;
                    int nw_tri = simplification_tris[nw_cell];
                    int ne_tri = simplification_tris[ne_cell];
                    int sw_tri = simplification_tris[sw_cell];
                    int se_tri = simplification_tris[se_cell];
                    int nw_corner = nw_tri * 3 + 4;
                    int ne_corner = ne_tri * 3 + 0;
                    int sw_corner = sw_tri * 3 + 2;
                    int se_corner = se_tri * 3 + 1;
                    *pnewtris++ = tris[se_corner];
                    *pnewtris++ = tris[sw_corner];
                    *pnewtris++ = tris[nw_corner];
                    *pnewtris++ = tris[nw_corner];
                    *pnewtris++ = tris[ne_corner];
                    *pnewtris++ = tris[se_corner];
                }
                int ncelltris = simplification_ntris[cell];
                int celltri = simplification_tris[cell];
                for (int t = 0; t < ncelltris; t++, celltri++) {
                    *pnewtris++ = tris[celltri * 3];
                    *pnewtris++ = tris[celltri * 3 + 1];
                    *pnewtris++ = tris[celltri * 3 + 2];
                }
                ncelltris = simplification_ntris[south];
                celltri = simplification_tris[south];
                for (int t = 0; t < ncelltris; t++, celltri++) {
                    *pnewtris++ = tris[celltri * 3];
                    *pnewtris++ = tris[celltri * 3 + 1];
                    *pnewtris++ = tris[celltri * 3 + 2];
                }
            }
            if (in_run) {
                in_run = 0;
                int nw_cell = ncols * row + start_run;
                int ne_cell = ncols * row + ncols - 1;
                int sw_cell = nw_cell + ncols;
                int se_cell = ne_cell + ncols;
                int nw_tri = simplification_tris[nw_cell];
                int ne_tri = simplification_tris[ne_cell];
                int sw_tri = simplification_tris[sw_cell];
                int se_tri = simplification_tris[se_cell];
                int nw_corner = nw_tri * 3 + 4;
                int ne_corner = ne_tri * 3 + 0;
                int sw_corner = sw_tri * 3 + 2;
                int se_corner = se_tri * 3 + 1;
                *pnewtris++ = tris[se_corner];
                *pnewtris++ = tris[sw_corner];
                *pnewtris++ = tris[nw_corner];
                *pnewtris++ = tris[nw_corner];
                *pnewtris++ = tris[ne_corner];
                *pnewtris++ = tris[se_corner];
            }
        }
        ptris = pnewtris;
        ntris -= neliminated_triangles;
        free(tris);
        tris = newtris;
        free(simplification_codes);
        free(simplification_tris);
        free(simplification_ntris);

        // Remove unreferenced points.
        char* markers = PAR_CALLOC(char, npts);
        ptris = tris;
        int newnpts = 0;
        for (int i = 0; i < ntris * 3; i++, ptris++) {
            if (!markers[*ptris]) {
                newnpts++;
                markers[*ptris] = 1;
            }
        }
        for (int i = 0; i < nconntris * 3; i++) {
            if (!markers[conntris[i]]) {
                newnpts++;
                markers[conntris[i]] = 1;
            }
        }
        float* newpts = PAR_CALLOC(float, newnpts * mesh->dim);
        uint16_t* mapping = PAR_CALLOC(uint16_t, npts);
        ppts = pts;
        float* pnewpts = newpts;
        int j = 0;
        if (mesh->dim == 3) {
            for (int i = 0; i < npts; i++, ppts += 3) {
                if (markers[i]) {
                    *pnewpts++ = ppts[0];
                    *pnewpts++ = ppts[1];
                    *pnewpts++ = ppts[2];
                    mapping[i] = j++;
                }
            }
        } else {
            for (int i = 0; i < npts; i++, ppts += 2) {
                if (markers[i]) {
                    *pnewpts++ = ppts[0];
                    *pnewpts++ = ppts[1];
                    mapping[i] = j++;
                }
            }
        }
        free(pts);
        free(markers);
        pts = newpts;
        npts = newnpts;
        for (int i = 0; i < ntris * 3; i++) {
            tris[i] = mapping[tris[i]];
        }
        for (int i = 0; i < nconntris * 3; i++) {
            conntris[i] = mapping[conntris[i]];
        }
        free(mapping);
    }

    // Append all extrusion triangles to the main triangle array.
    // We need them to be last so that they form a contiguous sequence.
    pconntris = conntris;
    for (int i = 0; i < nconntris; i++) {
        *ptris++ = *pconntris++;
        *ptris++ = *pconntris++;
        *ptris++ = *pconntris++;
        ntris++;
    }
    free(conntris);

    // Final cleanup and return.
    assert(npts <= maxpts);
    assert(ntris <= maxtris);
    mesh->npoints = npts;
    mesh->points = pts;
    mesh->ntriangles = ntris;
    mesh->triangles = tris;
    mesh->nconntriangles = nconntris;
    return mlist;
}

typedef struct {
    uint16_t outera;
    uint16_t outerb;
    uint16_t innera;
    uint16_t innerb;
    char i;
    char j;
    par_msquares__mesh* mesh;
    int mesh_index;
} par_connector;

static par_connector* par_conn_find(par_connector* conns, int nconns,
    char i, char j)
{
    for (int c = 0; c < nconns; c++) {
        if (conns[c].i == i && conns[c].j == j) {
            return conns + c;
        }
    }
    return 0;
}

static int par_msquares_cmp(const void *a, const void *b)
{
    uint32_t arg1 = *((uint32_t const*) a);
    uint32_t arg2 = *((uint32_t const*) b);
    if (arg1 < arg2) return -1;
    if (arg1 > arg2) return 1;
    return 0;
}

typedef int (*par_msquares_code_fn)(int, int, int, int, void*);

static int par_msquares_multi_code(int sw, int se, int ne, int nw)
{
    int code[4];
    int ncols = 0;
    code[0] = ncols++;
    if (se == sw) {
        code[1] = code[0];
    } else {
        code[1] = ncols++;
    }
    if (ne == se) {
        code[2] = code[1];
    } else if (ne == sw) {
        code[2] = code[0];
    } else {
        code[2] = ncols++;
    }
    if (nw == ne) {
        code[3] = code[2];
    } else if (nw == se) {
        code[3] = code[1];
    } else if (nw == sw) {
        code[3] = code[0];
    } else {
        code[3] = ncols++;
    }
    return code[0] | (code[1] << 2) | (code[2] << 4) | (code[3] << 6);
}

static uint32_t par_msquares_argb(par_byte const* pdata, int bpp)
{
    uint32_t color = 0;
    if (bpp == 4) {
        color |= pdata[2];
        color |= pdata[1] << 8;
        color |= pdata[0] << 16;
        color |= pdata[3] << 24;
        return color;
    }
    for (int j = 0; j < bpp; j++) {
        color <<= 8;
        color |= pdata[j];
    }
    return color;
}

// Merge connective triangles into the primary triangle list.
static void par_msquares__finalize(par_msquares_meshlist* mlist)
{
    if (mlist->nmeshes < 2 || mlist->meshes[1]->nconntriangles == 0) {
        return;
    }
    for (int m = 1; m < mlist->nmeshes; m++) {
        par_msquares__mesh* mesh = mlist->meshes[m];
        int ntris = mesh->ntriangles + mesh->nconntriangles;
        uint16_t* triangles = PAR_CALLOC(uint16_t, ntris * 3);
        uint16_t* dst = triangles;
        uint16_t const* src = mesh->triangles;
        for (int t = 0; t < mesh->ntriangles; t++) {
            *dst++ = *src++;
            *dst++ = *src++;
            *dst++ = *src++;
        }
        src = mesh->conntri;
        for (int t = 0; t < mesh->nconntriangles; t++) {
            *dst++ = *src++;
            *dst++ = *src++;
            *dst++ = *src++;
        }
        free(mesh->triangles);
        free(mesh->conntri);
        mesh->triangles = triangles;
        mesh->ntriangles = ntris;
        mesh->conntri = 0;
        mesh->nconntriangles = 0;
    }
}

static par__uint16list* par__uint16list_create()
{
    par__uint16list* list = PAR_CALLOC(par__uint16list, 1);
    list->count = 0;
    list->capacity = 32;
    list->values = PAR_CALLOC(uint16_t, list->capacity);
    return list;
}

static void par__uint16list_add3(par__uint16list* list,
    uint16_t a, uint16_t b, uint16_t c)
{
    if (list->count + 3 > list->capacity) {
        list->capacity *= 2;
        list->values = PAR_REALLOC(uint16_t, list->values, list->capacity);
    }
    list->values[list->count++] = a;
    list->values[list->count++] = b;
    list->values[list->count++] = c;
}

static void par__uint16list_free(par__uint16list* list)
{
    if (list) {
        PAR_FREE(list->values);
        PAR_FREE(list);
    }
}

static void par_msquares__repair_tjunctions(par_msquares_meshlist* mlist)
{
    for (int m = 0; m < mlist->nmeshes; m++) {
        par_msquares__mesh* mesh = mlist->meshes[m];
        par__uint16list* tjunctions = mesh->tjunctions;
        int njunctions = (int) tjunctions->count / 3;
        if (njunctions == 0) {
            continue;
        }
        int ntriangles = mesh->ntriangles + njunctions;
        mesh->triangles = PAR_REALLOC(uint16_t, mesh->triangles,
            ntriangles * 3);
        uint16_t const* jun = tjunctions->values;
        uint16_t* new_triangles = mesh->triangles + mesh->ntriangles * 3;
        int ncreated = 0;
        for (int j = 0; j < njunctions; j++, jun += 3) {
            uint16_t* tri = mesh->triangles;
            int t;
            for (t = 0; t < mesh->ntriangles; t++, tri += 3) {
                int i = -1;
                if (tri[0] == jun[0] && tri[1] == jun[1]) {
                    i = 0;
                } else if (tri[1] == jun[0] && tri[2] == jun[1]) {
                    i = 1;
                } else if (tri[2] == jun[0] && tri[0] == jun[1]) {
                    i = 2;
                } else {
                    continue;
                }
                new_triangles[0] = tri[(i + 0) % 3];
                new_triangles[1] = jun[2];
                new_triangles[2] = tri[(i + 2) % 3];
                tri[(i + 0) % 3] = jun[2];
                new_triangles += 3;
                ncreated++;
                break;
            }
            // TODO: Need to investigate the "msquares_multi_diagram.obj" test.
            assert(t != mesh->ntriangles &&
                "Error with T-Junction repair; please disable the CLEAN flag.");
        }
        mesh->ntriangles += ncreated;
    }
}

par_msquares_meshlist* par_msquares_color_multi(par_byte const* data, int width,
    int height, int cellsize, int bpp, int flags)
{
    if (!par_msquares_binary_point_table) {
        par_init_tables();
    }
    const int ncols = width / cellsize;
    const int nrows = height / cellsize;
    const int maxrow = (height - 1) * width;
    const int ncells = ncols * nrows;
    const int dim = (flags & PAR_MSQUARES_HEIGHTS) ? 3 : 2;
    const int west_to_east[9] =   {  2, -1, -1, -1, -1, -1,  4,  3, -1 };
    const int north_to_south[9] = { -1, -1, -1, -1,  2,  1,  0, -1, -1 };
    assert(!(flags & PAR_MSQUARES_HEIGHTS) || bpp == 4);
    assert(bpp > 0 && bpp <= 4 && "Bytes per pixel must be 1, 2, 3, or 4.");
    assert(!(flags & PAR_MSQUARES_CLEAN) || !(flags & PAR_MSQUARES_SIMPLIFY));
    assert(!(flags & PAR_MSQUARES_SNAP) &&
        "SNAP is not supported with color_multi");
    assert(!(flags & PAR_MSQUARES_INVERT) &&
        "INVERT is not supported with color_multi");
    assert(!(flags & PAR_MSQUARES_DUAL) &&
        "DUAL is not supported with color_multi");

    // Find all unique colors and ensure there are no more than 256 colors.
    uint32_t colors[256];
    int ncolors = 0;
    par_byte const* pdata = data;
    for (int i = 0; i < width * height; i++, pdata += bpp) {
        uint32_t color = par_msquares_argb(pdata, bpp);
        if (0 == bsearch(&color, colors, ncolors, 4, par_msquares_cmp)) {
            assert(ncolors < 256);
            colors[ncolors++] = color;
            qsort(colors, ncolors, sizeof(uint32_t), par_msquares_cmp);
        }
    }

    // Convert the color image to grayscale using the mapping table.
    par_byte* pixels = PAR_CALLOC(par_byte, width * height);
    pdata = data;
    for (int i = 0; i < width * height; i++, pdata += bpp) {
        uint32_t color = par_msquares_argb(pdata, bpp);
        void* result = bsearch(&color, colors, ncolors, 4, par_msquares_cmp);
        pixels[i] = (uint32_t*) result - &colors[0];
    }

    // Allocate 1 mesh for each color.
    par_msquares_meshlist* mlist = PAR_CALLOC(par_msquares_meshlist, 1);
    mlist->nmeshes = ncolors;
    mlist->meshes = PAR_CALLOC(par_msquares__mesh*, ncolors);
    par_msquares__mesh* mesh;
    int maxtris_per_cell = 6;
    int maxpts_per_cell = 9;
    if (flags & PAR_MSQUARES_CONNECT) {
        maxpts_per_cell += 6;
    }
    for (int i = 0; i < ncolors; i++) {
        mesh = mlist->meshes[i] = PAR_CALLOC(par_msquares__mesh, 1);
        mesh->color = colors[i];
        mesh->points = PAR_CALLOC(float, ncells * maxpts_per_cell * dim);
        mesh->triangles = PAR_CALLOC(uint16_t, ncells * maxtris_per_cell * 3);
        mesh->dim = dim;
        mesh->tjunctions = par__uint16list_create();
        if (flags & PAR_MSQUARES_CONNECT) {
            mesh->conntri = PAR_CALLOC(uint16_t, ncells * 8 * 3);
        }
    }

    // The "verts" x/y/z arrays are the 4 corners and 4 midpoints around the
    // square, in counter-clockwise order, starting at the lower-left.  The
    // ninth vert is the center point.

    float vertsx[9], vertsy[9];
    float normalization = 1.0f / PAR_MAX(width, height);
    float normalized_cellsize = cellsize * normalization;
    uint8_t cella[256];
    uint8_t cellb[256];
    uint8_t* currcell = cella;
    uint8_t* prevcell = cellb;
    uint16_t inds0[256 * 9];
    uint16_t inds1[256 * 9];
    uint16_t* currinds = inds0;
    uint16_t* previnds = inds1;
    uint16_t* rowindsa = PAR_CALLOC(uint16_t, ncols * 3 * 256);
    uint8_t* rowcellsa = PAR_CALLOC(uint8_t, ncols * 256);
    uint16_t* rowindsb = PAR_CALLOC(uint16_t, ncols * 3 * 256);
    uint8_t* rowcellsb = PAR_CALLOC(uint8_t, ncols * 256);
    uint16_t* prevrowinds = rowindsa;
    uint16_t* currrowinds = rowindsb;
    uint8_t* prevrowcells = rowcellsa;
    uint8_t* currrowcells = rowcellsb;
    uint32_t* simplification_words = 0;
    if (flags & PAR_MSQUARES_SIMPLIFY) {
        simplification_words = PAR_CALLOC(uint32_t, 2 * nrows * ncols);
    }

    // Do the march!
    for (int row = 0; row < nrows; row++) {
        vertsx[0] = vertsx[6] = vertsx[7] = 0;
        vertsx[1] = vertsx[5] = vertsx[8] = 0.5 * normalized_cellsize;
        vertsx[2] = vertsx[3] = vertsx[4] = normalized_cellsize;
        vertsy[0] = vertsy[1] = vertsy[2] = normalized_cellsize * (row + 1);
        vertsy[4] = vertsy[5] = vertsy[6] = normalized_cellsize * row;
        vertsy[3] = vertsy[7] = vertsy[8] = normalized_cellsize * (row + 0.5);
        int northi = row * cellsize * width;
        int southi = PAR_MIN(northi + cellsize * width, maxrow);
        int nwval = pixels[northi];
        int swval = pixels[southi];
        memset(currrowcells, 0, ncols * 256);

        for (int col = 0; col < ncols; col++) {
            northi += cellsize;
            southi += cellsize;
            if (col == ncols - 1) {
                northi--;
                southi--;
            }

            // Obtain 8-bit code and grab the four corresponding triangle lists.
            int neval = pixels[northi];
            int seval = pixels[southi];
            int code = par_msquares_multi_code(swval, seval, neval, nwval) >> 2;
            int const* trispecs[4] = {
                par_msquares_quaternary_triangle_table[code][0],
                par_msquares_quaternary_triangle_table[code][1],
                par_msquares_quaternary_triangle_table[code][2],
                par_msquares_quaternary_triangle_table[code][3]
            };
            int ntris[4] = {
                *trispecs[0]++,
                *trispecs[1]++,
                *trispecs[2]++,
                *trispecs[3]++
            };
            int const* edgespecs[4] = {
                par_msquares_quaternary_boundary_table[code][0],
                par_msquares_quaternary_boundary_table[code][1],
                par_msquares_quaternary_boundary_table[code][2],
                par_msquares_quaternary_boundary_table[code][3]
            };
            int nedges[4] = {
                *edgespecs[0]++,
                *edgespecs[1]++,
                *edgespecs[2]++,
                *edgespecs[3]++
            };
            int vals[4] = { swval, seval, neval, nwval };

            // Gather topology information.
            par_connector edges[16];
            int ncedges = 0;
            for (int c = 0; c < 4; c++) {
                int color = vals[c];
                par_msquares__mesh* mesh = mlist->meshes[color];
                par_connector edge;
                for (int e = 0; e < nedges[c]; e++) {
                    char previndex = edgespecs[c][e * 2];
                    char currindex = edgespecs[c][e * 2 + 1];
                    edge.i = previndex;
                    edge.j = currindex;
                    edge.mesh_index = color;
                    edge.mesh = mesh;
                    edges[ncedges++] = edge;
                }
            }
            assert(ncedges < 16);

            // Push triangles and points into the four affected meshes.
            for (int m = 0; m < ncolors; m++) {
                currcell[m] = 0;
            }
            uint32_t colors = 0;
            uint32_t counts = 0;
            uint16_t* conntris_start[4];
            for (int c = 0; c < 4; c++) {
                int color = vals[c];
                colors |= color << (8 * c);
                counts |= ntris[c] << (8 * c);
                par_msquares__mesh* mesh = mlist->meshes[color];
                float height = (mesh->color >> 24) / 255.0;
                conntris_start[c] = mesh->conntri + mesh->nconntriangles * 3;
                int usedpts[9] = {0};
                uint16_t* pcurrinds = currinds + 9 * color;
                uint16_t const* pprevinds = previnds + 9 * color;
                uint16_t const* pprevrowinds =
                    prevrowinds + ncols * 3 * color + col * 3;
                uint8_t prevrowcell = prevrowcells[color * ncols + col];
                float* pdst = mesh->points + mesh->npoints * mesh->dim;
                int previndex, prevflag;
                for (int t = 0; t < ntris[c] * 3; t++) {
                    uint16_t index = trispecs[c][t];
                    if (usedpts[index]) {
                        continue;
                    }
                    usedpts[index] = 1;
                    if (index < 8) {
                        currcell[color] |= 1 << index;
                    }

                    // Vertical welding.
                    previndex = north_to_south[index];
                    prevflag = (previndex > -1) ? (1 << previndex) : 0;
                    if (row > 0 && (prevrowcell & prevflag)) {
                        pcurrinds[index] = pprevrowinds[previndex];
                        continue;
                    }

                    // Horizontal welding.
                    previndex = west_to_east[index];
                    prevflag = (previndex > -1) ? (1 << previndex) : 0;
                    if (col > 0 && (prevcell[color] & prevflag)) {
                        pcurrinds[index] = pprevinds[previndex];
                        continue;
                    }

                    // Insert brand new point.
                    float* vertex = pdst;
                    *pdst++ = vertsx[index];
                    *pdst++ = 1 - vertsy[index];
                    if (mesh->dim == 3) {
                        *pdst++ = height;
                    }
                    pcurrinds[index] = mesh->npoints++;

                    // If this is a midpoint, nudge it to the intersection.
                    if (index == 1) {
                        int begin = southi - cellsize;
                        for (int i = 1; i < cellsize + 1; i++) {
                            int val = pixels[begin + i];
                            if (val != pixels[begin]) {
                                vertex[0] = vertsx[0] + normalized_cellsize *
                                    (float) i / cellsize;
                                break;
                            }
                        }
                    } else if (index == 3) {
                        int begin = northi;
                        for (int i = 1; i < cellsize + 1; i++) {
                            int val = pixels[begin + i * width];
                            if (val != pixels[begin]) {
                                vertex[1] = (1 - vertsy[4]) -
                                    normalized_cellsize * (float) i / cellsize;
                                break;
                            }
                        }
                    }
                }

                // Look for T junctions and note them for later repairs.
                uint8_t prc = prevrowcell;
                if (usedpts[4] && !usedpts[5] && usedpts[6] && (prc & 2)) {
                    // Above cell had a middle vert, current cell straddles it.
                    par__uint16list_add3(mesh->tjunctions,
                        pcurrinds[4], pcurrinds[6], pprevrowinds[1]);
                } else if ((prc & 1) && !(prc & 2) && (prc & 4) && usedpts[5]) {
                    // Current cell has a middle vert, above cell straddles it.
                    par__uint16list_add3(mesh->tjunctions,
                        pprevrowinds[0], pprevrowinds[2], pcurrinds[5]);
                }
                uint8_t pcc = col > 0 ? prevcell[color] : 0;
                if (usedpts[0] && !usedpts[7] && usedpts[6] && (pcc & 8)) {
                    // Left cell had a middle vert, current cell straddles it.
                    par__uint16list_add3(mesh->tjunctions,
                        pcurrinds[6], pcurrinds[0], pprevinds[3]);
                }
                if ((pcc & 4) && !(pcc & 8) && (pcc & 16) && usedpts[7]) {
                    // Current cell has a middle vert, left cell straddles it.
                    par__uint16list_add3(mesh->tjunctions,
                        pprevinds[2], pprevinds[4], pcurrinds[7]);
                }

                // Stamp out the cell's triangle indices for this color.
                uint16_t* tdst = mesh->triangles + mesh->ntriangles * 3;
                mesh->ntriangles += ntris[c];
                for (int t = 0; t < ntris[c] * 3; t++) {
                    uint16_t index = trispecs[c][t];
                    *tdst++ = pcurrinds[index];
                }

                // Add extrusion points and connective triangles if requested.
                if (!(flags & PAR_MSQUARES_CONNECT)) {
                    continue;
                }
                for (int e = 0; e < nedges[c]; e++) {
                    int previndex = edgespecs[c][e * 2];
                    int currindex = edgespecs[c][e * 2 + 1];
                    par_connector* thisedge = par_conn_find(edges,
                        ncedges, previndex, currindex);
                    thisedge->innera = pcurrinds[previndex];
                    thisedge->innerb = pcurrinds[currindex];
                    thisedge->outera = mesh->npoints;
                    thisedge->outerb = mesh->npoints + 1;
                    par_connector* oppedge = par_conn_find(edges,
                        ncedges, currindex, previndex);
                    if (oppedge->mesh_index > color) continue;
                    *pdst++ = vertsx[previndex];
                    *pdst++ = 1 - vertsy[previndex];
                    if (mesh->dim == 3) {
                        *pdst++ = height;
                    }
                    mesh->npoints++;
                    *pdst++ = vertsx[currindex];
                    *pdst++ = 1 - vertsy[currindex];
                    if (mesh->dim == 3) {
                        *pdst++ = height;
                    }
                    mesh->npoints++;
                    uint16_t i0 = mesh->npoints - 1;
                    uint16_t i1 = mesh->npoints - 2;
                    uint16_t i2 = pcurrinds[previndex];
                    uint16_t i3 = pcurrinds[currindex];
                    uint16_t* ptr = mesh->conntri +
                        mesh->nconntriangles * 3;
                    *ptr++ = i2; *ptr++ = i1; *ptr++ = i0;
                    *ptr++ = i0; *ptr++ = i3; *ptr++ = i2;
                    mesh->nconntriangles += 2;
                }
            }

            // Adjust the positions of the extrusion verts.
            if (flags & PAR_MSQUARES_CONNECT) {
                for (int c = 0; c < 4; c++) {
                    int color = vals[c];
                    uint16_t* pconninds = conntris_start[c];
                    par_msquares__mesh* mesh = mlist->meshes[color];
                    for (int e = 0; e < nedges[c]; e++) {
                        int previndex = edgespecs[c][e * 2];
                        int currindex = edgespecs[c][e * 2 + 1];
                        uint16_t i1 = pconninds[1];
                        uint16_t i0 = pconninds[2];
                        par_connector const* oppedge = par_conn_find(edges,
                            ncedges, currindex, previndex);
                        if (oppedge->mesh_index > color) continue;
                        int d = mesh->dim;
                        float* dst = mesh->points;
                        float const* src = oppedge->mesh->points;
                        dst[i0 * d + 0] = src[oppedge->innera * d + 0];
                        dst[i0 * d + 1] = src[oppedge->innera * d + 1];
                        dst[i1 * d + 0] = src[oppedge->innerb * d + 0];
                        dst[i1 * d + 1] = src[oppedge->innerb * d + 1];
                        if (d == 3) {
                            dst[i0 * d + 2] = src[oppedge->innera * d + 2];
                            dst[i1 * d + 2] = src[oppedge->innerb * d + 2];
                        }
                        pconninds += 6;
                    }
                }
            }

            // Stash the bottom indices for each mesh in this cell to enable
            // vertical as-you-go welding.
            uint8_t* pcurrrowcells = currrowcells;
            uint16_t* pcurrrowinds = currrowinds;
            uint16_t const* pcurrinds = currinds;
            for (int color = 0; color < ncolors; color++) {
                pcurrrowcells[col] = currcell[color];
                pcurrrowcells += ncols;
                pcurrrowinds[col * 3 + 0] = pcurrinds[0];
                pcurrrowinds[col * 3 + 1] = pcurrinds[1];
                pcurrrowinds[col * 3 + 2] = pcurrinds[2];
                pcurrrowinds += ncols * 3;
                pcurrinds += 9;
            }

            // Stash some information later used by simplification.
            if (flags & PAR_MSQUARES_SIMPLIFY) {
                int cell = col + row * ncols;
                simplification_words[cell * 2] = colors;
                simplification_words[cell * 2 + 1] = counts;
            }

            // Advance the cursor.
            nwval = neval;
            swval = seval;
            for (int i = 0; i < 9; i++) {
                vertsx[i] += normalized_cellsize;
            }
            PAR_SWAP(uint8_t*, prevcell, currcell);
            PAR_SWAP(uint16_t*, previnds, currinds);
        }
        PAR_SWAP(uint8_t*, prevrowcells, currrowcells);
        PAR_SWAP(uint16_t*, prevrowinds, currrowinds);
    }
    free(prevrowinds);
    free(prevrowcells);
    free(pixels);

    if (flags & PAR_MSQUARES_CLEAN) {
        par_msquares__repair_tjunctions(mlist);
    }
    for (int m = 0; m < mlist->nmeshes; m++) {
        par_msquares__mesh* mesh = mlist->meshes[m];
        par__uint16list_free(mesh->tjunctions);
    }
    if (!(flags & PAR_MSQUARES_SIMPLIFY)) {
        par_msquares__finalize(mlist);
        return mlist;
    }

    uint8_t* simplification_blocks = PAR_CALLOC(uint8_t, nrows * ncols);
    uint32_t* simplification_tris = PAR_CALLOC(uint32_t, nrows * ncols);
    uint8_t* simplification_ntris = PAR_CALLOC(uint8_t, nrows * ncols);

    // Perform quick-n-dirty simplification by iterating two rows at a time.
    // In no way does this create the simplest possible mesh, but at least it's
    // fast and easy.
    for (uint32_t color = 0; color < (uint32_t) ncolors; color++) {
        par_msquares__mesh* mesh = mlist->meshes[color];

        // Populate the per-mesh info grids.
        int ntris = 0;
        for (int row = 0; row < nrows; row++) {
            for (int col = 0; col < ncols; col++) {
                int cell = ncols * row + col;
                uint32_t colors = simplification_words[cell * 2];
                uint32_t counts = simplification_words[cell * 2 + 1];
                int ncelltris = 0;
                int ncorners = 0;
                if ((colors & 0xff) == color) {
                    ncelltris = counts & 0xff;
                    ncorners++;
                }
                if (((colors >> 8) & 0xff) == color) {
                    ncelltris += (counts >> 8) & 0xff;
                    ncorners++;
                }
                if (((colors >> 16) & 0xff) == color) {
                    ncelltris += (counts >> 16) & 0xff;
                    ncorners++;
                }
                if (((colors >> 24) & 0xff) == color) {
                    ncelltris += (counts >> 24) & 0xff;
                    ncorners++;
                }
                simplification_ntris[cell] = ncelltris;
                simplification_tris[cell] = ntris;
                simplification_blocks[cell] = ncorners == 4;
                ntris += ncelltris;
            }
        }

        // First figure out how many triangles we can eliminate.
        int in_run = 0, start_run;
        int neliminated_triangles = 0;
        for (int row = 0; row < nrows - 1; row += 2) {
            for (int col = 0; col < ncols; col++) {
                int cell = ncols * row + col;
                int a = simplification_blocks[cell];
                int b = simplification_blocks[cell + ncols];
                if (a && b) {
                    if (!in_run) {
                        in_run = 1;
                        start_run = col;
                    }
                    continue;
                }
                if (in_run) {
                    in_run = 0;
                    int run_width = col - start_run;
                    neliminated_triangles += run_width * 4 - 2;
                }
            }
            if (in_run) {
                in_run = 0;
                int run_width = ncols - start_run;
                neliminated_triangles += run_width * 4 - 2;
            }
        }
        if (neliminated_triangles == 0) {
            continue;
        }

        // Build a new index array cell-by-cell.  If any given cell is 'F' and
        // its neighbor to the south is also 'F', then it's part of a run.
        int nnewtris = mesh->ntriangles - neliminated_triangles;
        uint16_t* newtris = PAR_CALLOC(uint16_t, nnewtris * 3);
        uint16_t* pnewtris = newtris;
        in_run = 0;
        uint16_t* tris = mesh->triangles;
        for (int row = 0; row < nrows - 1; row += 2) {
            for (int col = 0; col < ncols; col++) {
                int cell = ncols * row + col;
                int south = cell + ncols;
                int a = simplification_blocks[cell];
                int b = simplification_blocks[south];
                if (a && b) {
                    if (!in_run) {
                        in_run = 1;
                        start_run = col;
                    }
                    continue;
                }
                if (in_run) {
                    in_run = 0;
                    int nw_cell = ncols * row + start_run;
                    int ne_cell = ncols * row + col - 1;
                    int sw_cell = nw_cell + ncols;
                    int se_cell = ne_cell + ncols;
                    int nw_tri = simplification_tris[nw_cell];
                    int ne_tri = simplification_tris[ne_cell];
                    int sw_tri = simplification_tris[sw_cell];
                    int se_tri = simplification_tris[se_cell];
                    int nw_corner = nw_tri * 3 + 5;
                    int ne_corner = ne_tri * 3 + 2;
                    int sw_corner = sw_tri * 3 + 0;
                    int se_corner = se_tri * 3 + 1;
                    *pnewtris++ = tris[nw_corner];
                    *pnewtris++ = tris[sw_corner];
                    *pnewtris++ = tris[se_corner];
                    *pnewtris++ = tris[se_corner];
                    *pnewtris++ = tris[ne_corner];
                    *pnewtris++ = tris[nw_corner];
                }
                int ncelltris = simplification_ntris[cell];
                int celltri = simplification_tris[cell];
                for (int t = 0; t < ncelltris; t++, celltri++) {
                    *pnewtris++ = tris[celltri * 3];
                    *pnewtris++ = tris[celltri * 3 + 1];
                    *pnewtris++ = tris[celltri * 3 + 2];
                }
                ncelltris = simplification_ntris[south];
                celltri = simplification_tris[south];
                for (int t = 0; t < ncelltris; t++, celltri++) {
                    *pnewtris++ = tris[celltri * 3];
                    *pnewtris++ = tris[celltri * 3 + 1];
                    *pnewtris++ = tris[celltri * 3 + 2];
                }
            }
            if (in_run) {
                in_run = 0;
                int nw_cell = ncols * row + start_run;
                int ne_cell = ncols * row + ncols - 1;
                int sw_cell = nw_cell + ncols;
                int se_cell = ne_cell + ncols;
                int nw_tri = simplification_tris[nw_cell];
                int ne_tri = simplification_tris[ne_cell];
                int sw_tri = simplification_tris[sw_cell];
                int se_tri = simplification_tris[se_cell];
                int nw_corner = nw_tri * 3 + 5;
                int ne_corner = ne_tri * 3 + 2;
                int sw_corner = sw_tri * 3 + 0;
                int se_corner = se_tri * 3 + 1;
                *pnewtris++ = tris[nw_corner];
                *pnewtris++ = tris[sw_corner];
                *pnewtris++ = tris[se_corner];
                *pnewtris++ = tris[se_corner];
                *pnewtris++ = tris[ne_corner];
                *pnewtris++ = tris[nw_corner];
            }
        }
        mesh->ntriangles -= neliminated_triangles;
        free(mesh->triangles);
        mesh->triangles = newtris;
    }

    free(simplification_blocks);
    free(simplification_ntris);
    free(simplification_tris);
    free(simplification_words);

    par_msquares__finalize(mlist);
    for (int i = 0; i < mlist->nmeshes; i++) {
        par_remove_unreferenced_verts(mlist->meshes[i]);
    }
    return mlist;
}

void par_msquares_free_boundary(par_msquares_boundary* polygon)
{
    free(polygon->points);
    free(polygon->chains);
    free(polygon->lengths);
    free(polygon);
}

typedef struct par__hedge_s {
    uint32_t key;
    struct par__hvert_s* endvert;
    struct par__hedge_s* opposite;
    struct par__hedge_s* next;
    struct par__hedge_s* prev;
} par__hedge;

typedef struct par__hvert_s {
    par__hedge* incoming;
} par__hvert;

typedef struct {
    par_msquares_mesh const* mesh;
    par__hvert* verts;
    par__hedge* edges;
    par__hedge** sorted_edges;
} par__hemesh;

static int par__hedge_cmp(const void *arg0, const void *arg1)
{
    par__hedge* he0 = *((par__hedge**) arg0);
    par__hedge* he1 = *((par__hedge**) arg1);
    if (he0->key < he1->key) return -1;
    if (he0->key > he1->key) return 1;
    return 0;
}

static par__hedge* par__hedge_find(par__hemesh* hemesh, uint32_t key)
{
    par__hedge target = {0};
    target.key = key;
    par__hedge* ptarget = &target;
    int nedges = hemesh->mesh->ntriangles * 3;
    par__hedge** result = (par__hedge**) bsearch(&ptarget, hemesh->sorted_edges,
        nedges, sizeof(par__hedge*), par__hedge_cmp);
    return result ? *result : 0;
}

static uint32_t par__hedge_key(par__hvert* a, par__hvert* b, par__hvert* s)
{
    uint32_t ai = a - s;
    uint32_t bi = b - s;
    return (ai << 16) | bi;
}

par_msquares_boundary* par_msquares_extract_boundary(
    par_msquares_mesh const* mesh)
{
    par_msquares_boundary* result = PAR_CALLOC(par_msquares_boundary, 1);
    par__hemesh hemesh = {0};
    hemesh.mesh = mesh;
    int nedges = mesh->ntriangles * 3;

    // Populate all fields of verts and edges, except opposite.
    hemesh.edges = PAR_CALLOC(par__hedge, nedges);
    par__hvert* hverts = hemesh.verts = PAR_CALLOC(par__hvert, mesh->npoints);
    par__hedge* edge = hemesh.edges;
    uint16_t const* tri = mesh->triangles;
    for (int n = 0; n < mesh->ntriangles; n++, edge += 3, tri += 3) {
        edge[0].endvert = hverts + tri[1];
        edge[1].endvert = hverts + tri[2];
        edge[2].endvert = hverts + tri[0];
        hverts[tri[1]].incoming = edge + 0;
        hverts[tri[2]].incoming = edge + 1;
        hverts[tri[0]].incoming = edge + 2;
        edge[0].next = edge + 1;
        edge[1].next = edge + 2;
        edge[2].next = edge + 0;
        edge[0].prev = edge + 2;
        edge[1].prev = edge + 0;
        edge[2].prev = edge + 1;
        edge[0].key = par__hedge_key(edge[2].endvert, edge[0].endvert, hverts);
        edge[1].key = par__hedge_key(edge[0].endvert, edge[1].endvert, hverts);
        edge[2].key = par__hedge_key(edge[1].endvert, edge[2].endvert, hverts);
    }

    // Sort the edges according to their key.
    hemesh.sorted_edges = PAR_CALLOC(par__hedge*, mesh->ntriangles * 3);
    for (int n = 0; n < nedges; n++) {
        hemesh.sorted_edges[n] = hemesh.edges + n;
    }
    qsort(hemesh.sorted_edges, nedges, sizeof(par__hedge*), par__hedge_cmp);

    // Populate the "opposite" field in each edge.
    for (int n = 0; n < nedges; n++) {
        par__hedge* edge = hemesh.edges + n;
        par__hedge* prev = edge->prev;
        par__hvert* start = edge->endvert;
        par__hvert* end = prev->endvert;
        uint32_t key = par__hedge_key(start, end, hverts);
        edge->opposite = par__hedge_find(&hemesh, key);
    }

    // Re-use the sorted_edges array, filling it with boundary edges only.
    // Also create a mapping table to consolidate all boundary verts.
    int nborders = 0;
    for (int n = 0; n < nedges; n++) {
        par__hedge* edge = hemesh.edges + n;
        if (!edge->opposite) {
            hemesh.sorted_edges[nborders++] = edge;
        }
    }

    // Allocate for the worst case (all separate triangles).
    // We'll adjust the lengths later.
    result->nchains = nborders / 3;
    result->npoints = nborders + result->nchains;
    result->points = PAR_CALLOC(float, 2 * result->npoints);
    result->chains = PAR_CALLOC(float*, result->nchains);
    result->lengths = PAR_CALLOC(uint16_t, result->nchains);

    // Iterate over each polyline.
    edge = hemesh.sorted_edges[0];
    int pt = 0;
    int nwritten = 0;
    int nchains = 0;
    while (1) {
        float* points = result->points;
        par__hedge* orig = edge;
        uint16_t index = edge->prev->endvert - hverts;
        result->chains[nchains] = points + pt;
        result->lengths[nchains]++;
        points[pt++] = mesh->points[index * mesh->dim];
        points[pt++] = mesh->points[index * mesh->dim + 1];
        while (1) {
            index = edge->endvert - hverts;
            edge->key = 0;
            nwritten++;
            result->lengths[nchains]++;
            points[pt++] = mesh->points[index * mesh->dim];
            points[pt++] = mesh->points[index * mesh->dim + 1];
            par__hedge* next = edge->next;
            while (next != edge) {
                if (!next->opposite) {
                    break;
                }
                next = next->opposite->next;
            }
            edge = next;
            if (edge == orig) {
                break;
            }
        }
        nchains++;
        if (nwritten >= nborders) {
            break;
        }
        for (int i = 0; i < nborders; i++) {
            edge = hemesh.sorted_edges[i];
            if (edge->key) {
                break;
            }
        }
    }

    result->npoints = pt / 2;
    result->nchains = nchains;
    free(hemesh.verts);
    free(hemesh.edges);
    free(hemesh.sorted_edges);
    return result;
}

#endif