Add version of PathArcTo() and PathArcToFast() with adaptive rendering quality. (#3491)

docs/CHANGELOG.txt

@@ -75,9 +75,13 @@ Other Changes:
   This can currently only ever be set by the Freetype renderer.
 - imgui_freetype: Added ImGuiFreeTypeBuilderFlags_Bitmap flag to request Freetype loading bitmap data.
   This may have an effect on size and must be called with correct size values. (#3879) [@metarutaiga]
+- ImDrawList: PathArcTo() now supports "int num_segments = 0" (new default) and adaptively tesselate.
+  The adapative tesselation uses look up tables, tends to be faster than old PathArcTo() while maintaining
+  quality for large arcs (tesselation quality derived from "style.CircleTessellationMaxError") (#3491) [@thedmd]
+- ImDrawList: PathArcToFast() also adaptively tesselate efficiently. This means that large rounded corners
+  in e.g. hi-dpi settings will generally look better. (#3491) [@thedmd]
 - ImDrawList: AddCircle, AddCircleFilled(): Tweaked default segment count calculation to honor MaxError
   with more accuracy. Made default segment count always even for better looking result. (#3808) [@thedmd]
-- ImDrawList: AddCircle, AddCircleFilled(): New default for style.
 - Backends: Android: Added native Android backend. (#3446) [@duddel]
 - Backends: Win32: Added ImGui_ImplWin32_EnableAlphaCompositing() to facilitate experimenting with
   alpha compositing and transparent windows. (#2766, #3447 etc.).

imgui.h

@@ -2436,7 +2436,7 @@ struct ImDrawList
     inline    void  PathLineToMergeDuplicate(const ImVec2& pos)                 { if (_Path.Size == 0 || memcmp(&_Path.Data[_Path.Size - 1], &pos, 8) != 0) _Path.push_back(pos); }
     inline    void  PathFillConvex(ImU32 col)                                   { AddConvexPolyFilled(_Path.Data, _Path.Size, col); _Path.Size = 0; }  // Note: Anti-aliased filling requires points to be in clockwise order.
     inline    void  PathStroke(ImU32 col, ImDrawFlags flags = 0, float thickness = 1.0f) { AddPolyline(_Path.Data, _Path.Size, col, flags, thickness); _Path.Size = 0; }
-    IMGUI_API void  PathArcTo(const ImVec2& center, float radius, float a_min, float a_max, int num_segments = 10);
+    IMGUI_API void  PathArcTo(const ImVec2& center, float radius, float a_min, float a_max, int num_segments = 0);
     IMGUI_API void  PathArcToFast(const ImVec2& center, float radius, int a_min_of_12, int a_max_of_12);                // Use precomputed angles for a 12 steps circle
     IMGUI_API void  PathBezierCubicCurveTo(const ImVec2& p2, const ImVec2& p3, const ImVec2& p4, int num_segments = 0); // Cubic Bezier (4 control points)
     IMGUI_API void  PathBezierQuadraticCurveTo(const ImVec2& p2, const ImVec2& p3, int num_segments = 0);               // Quadratic Bezier (3 control points)
@@ -2482,6 +2482,8 @@ struct ImDrawList
     IMGUI_API void  _OnChangedTextureID();
     IMGUI_API void  _OnChangedVtxOffset();
     IMGUI_API int   _CalcCircleAutoSegmentCount(float radius) const;
+    IMGUI_API void  _PathArcToFastEx(const ImVec2& center, float radius, int a_min_sample, int a_max_sample, int a_step);
+    IMGUI_API void  _PathArcToN(const ImVec2& center, float radius, float a_min, float a_max, int num_segments);
 };
 
 // All draw data to render a Dear ImGui frame

imgui_draw.cpp

@@ -374,18 +374,22 @@ ImDrawListSharedData::ImDrawListSharedData()
         const float a = ((float)i * 2 * IM_PI) / (float)IM_ARRAYSIZE(ArcFastVtx);
         ArcFastVtx[i] = ImVec2(ImCos(a), ImSin(a));
     }
+    ArcFastRadiusCutoff = IM_DRAWLIST_CIRCLE_AUTO_SEGMENT_CALC_R(IM_DRAWLIST_ARCFAST_SAMPLE_MAX, CircleSegmentMaxError);
 }
 
 void ImDrawListSharedData::SetCircleTessellationMaxError(float max_error)
 {
     if (CircleSegmentMaxError == max_error)
         return;
+
+    IM_ASSERT(max_error > 0.0f);
     CircleSegmentMaxError = max_error;
     for (int i = 0; i < IM_ARRAYSIZE(CircleSegmentCounts); i++)
     {
         const float radius = (float)i;
         CircleSegmentCounts[i] = (ImU8)((i > 0) ? IM_DRAWLIST_CIRCLE_AUTO_SEGMENT_CALC(radius, CircleSegmentMaxError) : 0);
     }
+    ArcFastRadiusCutoff = IM_DRAWLIST_CIRCLE_AUTO_SEGMENT_CALC_R(IM_DRAWLIST_ARCFAST_SAMPLE_MAX, CircleSegmentMaxError);
 }
 
 // Initialize before use in a new frame. We always have a command ready in the buffer.
@@ -1026,32 +1030,86 @@ void ImDrawList::AddConvexPolyFilled(const ImVec2* points, const int points_coun
     }
 }
 
-// 0: East, 3: South, 6: West, 9: North, 12: East
-void ImDrawList::PathArcToFast(const ImVec2& center, float radius, int a_min_of_12, int a_max_of_12)
+void ImDrawList::_PathArcToFastEx(const ImVec2& center, float radius, int a_min_sample, int a_max_sample, int a_step)
 {
     if (radius <= 0.0f)
     {
         _Path.push_back(center);
         return;
     }
-    IM_ASSERT(a_min_of_12 <= a_max_of_12);
+    IM_ASSERT(a_min_sample <= a_max_sample);
 
-    // For legacy reason the PathArcToFast() always takes angles where 2*PI is represented by 12,
-    // but it is possible to set IM_DRAWLIST_ARCFAST_TESSELATION_MULTIPLIER to a higher value. This should compile to a no-op otherwise.
-#if IM_DRAWLIST_ARCFAST_TESSELLATION_MULTIPLIER != 1
-    a_min_of_12 *= IM_DRAWLIST_ARCFAST_TESSELLATION_MULTIPLIER;
-    a_max_of_12 *= IM_DRAWLIST_ARCFAST_TESSELLATION_MULTIPLIER;
-#endif
+    // Calculate arc auto segment step size
+    if (a_step <= 0)
+        a_step = IM_DRAWLIST_ARCFAST_SAMPLE_MAX / _CalcCircleAutoSegmentCount(radius);
+
+    // Make sure we never do steps larger than one quarter of the circle
+    a_step = ImClamp(a_step, 1, IM_DRAWLIST_ARCFAST_TABLE_SIZE / 4);
+
+    // Normalize a_min_sample to always start lie in [0..IM_DRAWLIST_ARCFAST_SAMPLE_MAX] range.
+    if (a_min_sample < 0)
+    {
+        int normalized_sample = a_min_sample % IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
+        if (normalized_sample < 0)
+            normalized_sample += IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
+        a_max_sample += (normalized_sample - a_min_sample);
+        a_min_sample = normalized_sample;
+    }
+
+    const int sample_range = a_max_sample - a_min_sample;
+    const int a_next_step = a_step;
+
+    int samples = sample_range + 1;
+    bool extra_max_sample = false;
+    if (a_step > 1)
+    {
+        samples            = sample_range / a_step + 1;
+        const int overstep = sample_range % a_step;
+
+        if (overstep > 0)
+        {
+            extra_max_sample = true;
+            samples++;
 
-    _Path.reserve(_Path.Size + (a_max_of_12 - a_min_of_12 + 1));
-    for (int a = a_min_of_12; a <= a_max_of_12; a++)
+            // When we have overstep to avoid awkwardly looking one long line and one tiny one at the end,
+            // distribute first step range evenly between them by reducing first step size.
+            if (sample_range > 0)
+                a_step -= (a_step - overstep) / 2;
+        }
+    }
+
+    _Path.resize(_Path.Size + samples);
+    ImVec2* out_ptr = _Path.Data + (_Path.Size - samples);
+
+    int sample_index = a_min_sample;
+    for (int a = a_min_sample; a <= a_max_sample; a += a_step, sample_index += a_step, a_step = a_next_step)
+    {
+        // a_step is clamped to IM_DRAWLIST_ARCFAST_SAMPLE_MAX, so we have guaranteed that it will not wrap over range twice or more
+        if (sample_index >= IM_DRAWLIST_ARCFAST_SAMPLE_MAX)
+            sample_index -= IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
+
+        const ImVec2 s = _Data->ArcFastVtx[sample_index];
+        out_ptr->x = center.x + s.x * radius;
+        out_ptr->y = center.y + s.y * radius;
+        out_ptr++;
+    }
+
+    if (extra_max_sample)
     {
-        const ImVec2& c = _Data->ArcFastVtx[a % IM_ARRAYSIZE(_Data->ArcFastVtx)];
-        _Path.push_back(ImVec2(center.x + c.x * radius, center.y + c.y * radius));
+        int normalized_max_sample = a_max_sample % IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
+        if (normalized_max_sample < 0)
+            normalized_max_sample += IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
+
+        const ImVec2 s = _Data->ArcFastVtx[normalized_max_sample];
+        out_ptr->x = center.x + s.x * radius;
+        out_ptr->y = center.y + s.y * radius;
+        out_ptr++;
     }
+
+    IM_ASSERT_PARANOID(_Path.Data + _Path.Size == out_ptr);
 }
 
-void ImDrawList::PathArcTo(const ImVec2& center, float radius, float a_min, float a_max, int num_segments)
+void ImDrawList::_PathArcToN(const ImVec2& center, float radius, float a_min, float a_max, int num_segments)
 {
     if (radius <= 0.0f)
     {
@@ -1070,6 +1128,64 @@ void ImDrawList::PathArcTo(const ImVec2& center, float radius, float a_min, floa
     }
 }
 
+// 0: East, 3: South, 6: West, 9: North, 12: East
+void ImDrawList::PathArcToFast(const ImVec2& center, float radius, int a_min_of_12, int a_max_of_12)
+{
+    if (radius <= 0.0f)
+    {
+        _Path.push_back(center);
+        return;
+    }
+    IM_ASSERT(a_min_of_12 <= a_max_of_12);
+    _PathArcToFastEx(center, radius, a_min_of_12 * IM_DRAWLIST_ARCFAST_SAMPLE_MAX / 12, a_max_of_12 * IM_DRAWLIST_ARCFAST_SAMPLE_MAX / 12, 0);
+}
+
+void ImDrawList::PathArcTo(const ImVec2& center, float radius, float a_min, float a_max, int num_segments)
+{
+    if (radius <= 0.0f)
+    {
+        _Path.push_back(center);
+        return;
+    }
+    IM_ASSERT(a_min <= a_max);
+
+    if (num_segments > 0)
+    {
+        _PathArcToN(center, radius, a_min, a_max, num_segments);
+        return;
+    }
+
+    // Automatic segment count
+    if (radius <= _Data->ArcFastRadiusCutoff)
+    {
+        // We are going to use precomputed values for mid samples.
+        // Determine first and last sample in lookup table that belong to the arc.
+        const int a_min_sample = (int)ImCeil(IM_DRAWLIST_ARCFAST_SAMPLE_MAX * a_min / (IM_PI * 2.0f));
+        const int a_max_sample = (int)(      IM_DRAWLIST_ARCFAST_SAMPLE_MAX * a_max / (IM_PI * 2.0f));
+        const int a_mid_samples = ImMax(a_max_sample - a_min_sample, 0);
+
+        const float a_min_segment_angle = a_min_sample * IM_PI * 2.0f / IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
+        const float a_max_segment_angle = a_max_sample * IM_PI * 2.0f / IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
+        const bool a_emit_start = (a_min_segment_angle - a_min) > 0.0f;
+        const bool a_emit_end = (a_max - a_max_segment_angle) > 0.0f;
+
+        _Path.reserve(_Path.Size + (a_mid_samples + 1 + (a_emit_start ? 1 : 0) + (a_emit_end ? 1 : 0)));
+        if (a_emit_start)
+            _Path.push_back(ImVec2(center.x + ImCos(a_min) * radius, center.y + ImSin(a_min) * radius));
+        if (a_max_sample >= a_min_sample)
+            _PathArcToFastEx(center, radius, a_min_sample, a_max_sample, 0);
+        if (a_emit_end)
+            _Path.push_back(ImVec2(center.x + ImCos(a_max) * radius, center.y + ImSin(a_max) * radius));
+    }
+    else
+    {
+        const float arc_length = a_max - a_min;
+        const int circle_segment_count = _CalcCircleAutoSegmentCount(radius);
+        const int arc_segment_count = ImMax((int)ImCeil(circle_segment_count * arc_length / (IM_PI * 2.0f)), (int)(2.0f * IM_PI / arc_length));
+        _PathArcToN(center, radius, a_min, a_max, arc_segment_count);
+    }
+}
+
 ImVec2 ImBezierCubicCalc(const ImVec2& p1, const ImVec2& p2, const ImVec2& p3, const ImVec2& p4, float t)
 {
     float u = 1.0f - t;

imgui_internal.h

@@ -636,10 +636,15 @@ struct IMGUI_API ImChunkStream
 #define IM_DRAWLIST_CIRCLE_AUTO_SEGMENT_MAX                     512
 #define IM_DRAWLIST_CIRCLE_AUTO_SEGMENT_CALC(_RAD,_MAXERROR)    ImClamp(IM_ROUNDUP_TO_EVEN((int)ImCeil(IM_PI / ImAcos(1 - ImMin((_MAXERROR), (_RAD)) / (_RAD)))), IM_DRAWLIST_CIRCLE_AUTO_SEGMENT_MIN, IM_DRAWLIST_CIRCLE_AUTO_SEGMENT_MAX)
 
-// ImDrawList: You may set this to higher values (e.g. 2 or 3) to increase tessellation of fast rounded corners path.
-#ifndef IM_DRAWLIST_ARCFAST_TESSELLATION_MULTIPLIER
-#define IM_DRAWLIST_ARCFAST_TESSELLATION_MULTIPLIER             1
+// Raw equation from IM_DRAWLIST_CIRCLE_AUTO_SEGMENT_CALC rewritten for 'r' and 'error'.
+#define IM_DRAWLIST_CIRCLE_AUTO_SEGMENT_CALC_R(_N,_MAXERROR)    ((_MAXERROR) / (1 - ImCos(IM_PI / ImMax((float)(_N), IM_PI))))
+#define IM_DRAWLIST_CIRCLE_AUTO_SEGMENT_CALC_ERROR(_N,_RAD)     ((1 - ImCos(IM_PI / ImMax((float)(_N), IM_PI))) / (_RAD))
+
+// ImDrawList: Lookup table size for adaptive arc drawing, cover full circle.
+#ifndef IM_DRAWLIST_ARCFAST_TABLE_SIZE
+#define IM_DRAWLIST_ARCFAST_TABLE_SIZE                          48 // Number of samples in lookup table.
 #endif
+#define IM_DRAWLIST_ARCFAST_SAMPLE_MAX                          IM_DRAWLIST_ARCFAST_TABLE_SIZE // Sample index _PathArcToFastEx() for 360 angle.
 
 // Data shared between all ImDrawList instances
 // You may want to create your own instance of this if you want to use ImDrawList completely without ImGui. In that case, watch out for future changes to this structure.
@@ -654,7 +659,8 @@ struct IMGUI_API ImDrawListSharedData
     ImDrawListFlags InitialFlags;               // Initial flags at the beginning of the frame (it is possible to alter flags on a per-drawlist basis afterwards)
 
     // [Internal] Lookup tables
-    ImVec2          ArcFastVtx[12 * IM_DRAWLIST_ARCFAST_TESSELLATION_MULTIPLIER];  // FIXME: Bake rounded corners fill/borders in atlas
+    ImVec2          ArcFastVtx[IM_DRAWLIST_ARCFAST_TABLE_SIZE]; // Sample points on the quarter of the circle.
+    float           ArcFastRadiusCutoff;                        // Cutoff radius after which arc drawing will fallback to slower PathArcTo()
     ImU8            CircleSegmentCounts[64];    // Precomputed segment count for given radius before we calculate it dynamically (to avoid calculation overhead)
     const ImVec4*   TexUvLines;                 // UV of anti-aliased lines in the atlas