BMP format improvements including alpha support for MTSDF

core/save-bmp.cpp

@@ -14,7 +14,6 @@ namespace msdfgen {
     typedef int int32_t;
     typedef unsigned uint32_t;
     typedef unsigned short uint16_t;
-    typedef unsigned char uint8_t;
 }
 #endif
 
@@ -22,44 +21,70 @@ namespace msdfgen {
 
 namespace msdfgen {
 
+static const byte BMP_LINEAR_COLOR_SPACE_SPECIFICATION[48] = {
+    0xf8, 0xc2, 0x64, 0x1a, 0x08, 0x3d, 0x9b, 0x0d, 0x11, 0x36, 0x3c, 0x01,
+    0x1c, 0xeb, 0xe2, 0x16, 0x39, 0xd6, 0xc5, 0x2d, 0x09, 0xf9, 0xa0, 0x07,
+    0xdf, 0x4f, 0x8d, 0x0b, 0xc0, 0xec, 0x9e, 0x04, 0xf4, 0xfd, 0xd4, 0x3c,
+    0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x01, 0x00,
+};
+
 template <typename T>
-static bool writeValue(FILE *file, T value) {
-    #ifdef __BIG_ENDIAN__
-        T reverse = 0;
-        for (int i = 0; i < sizeof(T); ++i) {
-            reverse <<= 8;
-            reverse |= value&T(0xff);
-            value >>= 8;
-        }
-        return fwrite(&reverse, sizeof(T), 1, file) == 1;
-    #else
-        return fwrite(&value, sizeof(T), 1, file) == 1;
-    #endif
+bool writeLE(FILE *file, T value);
+
+template <>
+bool writeLE(FILE *f, uint16_t value) {
+    byte bytes[2] = { byte(value), byte(value>>8) };
+    return fwrite(bytes, 1, 2, f) == 2;
+}
+template <>
+bool writeLE(FILE *f, uint32_t value) {
+    byte bytes[4] = { byte(value), byte(value>>8), byte(value>>16), byte(value>>24) };
+    return fwrite(bytes, 1, 4, f) == 4;
+}
+template <>
+bool writeLE(FILE *f, int32_t value) {
+    byte bytes[4] = { byte(value), byte(value>>8), byte(value>>16), byte(value>>24) };
+    return fwrite(bytes, 1, 4, f) == 4;
 }
 
-static bool writeBmpHeader(FILE *file, int width, int height, int &paddedWidth) {
-    paddedWidth = (3*width+3)&~3;
-    const uint32_t bitmapStart = 54;
-    const uint32_t bitmapSize = paddedWidth*height;
-    const uint32_t fileSize = bitmapStart+bitmapSize;
-
-    writeValue<uint16_t>(file, 0x4d42u);
-    writeValue<uint32_t>(file, fileSize);
-    writeValue<uint16_t>(file, 0);
-    writeValue<uint16_t>(file, 0);
-    writeValue<uint32_t>(file, bitmapStart);
-
-    writeValue<uint32_t>(file, 40);
-    writeValue<int32_t>(file, width);
-    writeValue<int32_t>(file, height);
-    writeValue<uint16_t>(file, 1);
-    writeValue<uint16_t>(file, 24);
-    writeValue<uint32_t>(file, 0);
-    writeValue<uint32_t>(file, bitmapSize);
-    writeValue<uint32_t>(file, 2835);
-    writeValue<uint32_t>(file, 2835);
-    writeValue<uint32_t>(file, 0);
-    writeValue<uint32_t>(file, 0);
+static bool writeBmpHeader(FILE *file, int bytesPerPixel, int width, int height, int &paddedWidth) {
+    paddedWidth = (bytesPerPixel*width+3)&~3; // Ceil to multiple of 4 bytes
+    uint32_t colorTableEntries = bytesPerPixel == 1 ? 256 : 0;
+    uint32_t bitmapStart = 14+108+4*colorTableEntries;
+    uint32_t bitmapSize = paddedWidth*height;
+    uint32_t fileSize = bitmapStart+bitmapSize;
+
+    // BMP file header
+    writeLE<uint16_t>(file, 0x4d42u); // "BM"
+    writeLE<uint32_t>(file, fileSize);
+    writeLE<uint16_t>(file, 0);
+    writeLE<uint16_t>(file, 0);
+    writeLE<uint32_t>(file, bitmapStart);
+
+    // DIB header (BITMAPV4HEADER)
+    writeLE<uint32_t>(file, 108);
+    writeLE<int32_t>(file, width);
+    writeLE<int32_t>(file, height);
+    writeLE<uint16_t>(file, 1); // planes
+    writeLE<uint16_t>(file, uint16_t(8*bytesPerPixel));
+    writeLE<uint32_t>(file, bytesPerPixel == 4 ? 3 : 0); // 0 = BI_RGB, 3 = BI_BITFIELDS
+    writeLE<uint32_t>(file, bitmapSize);
+    writeLE<uint32_t>(file, 2835); // 72 dpi as pixels/meter
+    writeLE<uint32_t>(file, 2835);
+    writeLE<uint32_t>(file, colorTableEntries);
+    writeLE<uint32_t>(file, colorTableEntries);
+    writeLE<uint32_t>(file, 0x00ff0000u); // Red channel bit mask
+    writeLE<uint32_t>(file, 0x0000ff00u); // Green channel bit mask
+    writeLE<uint32_t>(file, 0x000000ffu); // Blue channel bit mask
+    writeLE<uint32_t>(file, bytesPerPixel == 4 ? 0xff000000u : 0u); // Alpha channel bit mask
+    writeLE<uint32_t>(file, 0); // LCS_CALIBRATED_RGB
+    fwrite(BMP_LINEAR_COLOR_SPACE_SPECIFICATION, 1, 48, file);
+
+    // Use indexed color for single channel - color table just lists grayscale colors (#000000, #010101, ... #FFFFFF)
+    if (bytesPerPixel == 1) {
+        for (uint32_t grayColor = 0; grayColor < 0x01000000u; grayColor += 0x00010101u)
+            writeLE<uint32_t>(file, grayColor|0xff000000u);
+    }
 
     return true;
 }
@@ -70,17 +95,12 @@ bool saveBmp(const BitmapConstRef<byte, 1> &bitmap, const char *filename) {
         return false;
 
     int paddedWidth;
-    writeBmpHeader(file, bitmap.width, bitmap.height, paddedWidth);
+    writeBmpHeader(file, 1, bitmap.width, bitmap.height, paddedWidth);
 
-    const uint8_t padding[4] = { };
-    int padLength = paddedWidth-3*bitmap.width;
+    byte padding[4] = { };
+    int padLength = paddedWidth-bitmap.width;
     for (int y = 0; y < bitmap.height; ++y) {
-        for (int x = 0; x < bitmap.width; ++x) {
-            uint8_t px = (uint8_t) *bitmap(x, y);
-            fwrite(&px, sizeof(uint8_t), 1, file);
-            fwrite(&px, sizeof(uint8_t), 1, file);
-            fwrite(&px, sizeof(uint8_t), 1, file);
-        }
+        fwrite(bitmap(0, y), sizeof(byte), bitmap.width, file);
         fwrite(padding, 1, padLength, file);
     }
 
@@ -93,18 +113,18 @@ bool saveBmp(const BitmapConstRef<byte, 3> &bitmap, const char *filename) {
         return false;
 
     int paddedWidth;
-    writeBmpHeader(file, bitmap.width, bitmap.height, paddedWidth);
+    writeBmpHeader(file, 3, bitmap.width, bitmap.height, paddedWidth);
 
-    const uint8_t padding[4] = { };
+    byte padding[4] = { };
     int padLength = paddedWidth-3*bitmap.width;
     for (int y = 0; y < bitmap.height; ++y) {
         for (int x = 0; x < bitmap.width; ++x) {
-            uint8_t bgr[3] = {
-                (uint8_t) bitmap(x, y)[2],
-                (uint8_t) bitmap(x, y)[1],
-                (uint8_t) bitmap(x, y)[0]
+            byte bgr[3] = {
+                bitmap(x, y)[2],
+                bitmap(x, y)[1],
+                bitmap(x, y)[0]
             };
-            fwrite(bgr, sizeof(uint8_t), 3, file);
+            fwrite(bgr, sizeof(byte), 3, file);
         }
         fwrite(padding, 1, padLength, file);
     }
@@ -113,8 +133,26 @@ bool saveBmp(const BitmapConstRef<byte, 3> &bitmap, const char *filename) {
 }
 
 bool saveBmp(const BitmapConstRef<byte, 4> &bitmap, const char *filename) {
-    // RGBA not supported by the BMP format
-    return false;
+    FILE *file = fopen(filename, "wb");
+    if (!file)
+        return false;
+
+    int dummyPaddedWidth;
+    writeBmpHeader(file, 4, bitmap.width, bitmap.height, dummyPaddedWidth);
+
+    for (int y = 0; y < bitmap.height; ++y) {
+        for (int x = 0; x < bitmap.width; ++x) {
+            byte bgra[4] = {
+                bitmap(x, y)[2],
+                bitmap(x, y)[1],
+                bitmap(x, y)[0],
+                bitmap(x, y)[3]
+            };
+            fwrite(bgra, sizeof(byte), 4, file);
+        }
+    }
+
+    return !fclose(file);
 }
 
 bool saveBmp(const BitmapConstRef<float, 1> &bitmap, const char *filename) {
@@ -123,16 +161,14 @@ bool saveBmp(const BitmapConstRef<float, 1> &bitmap, const char *filename) {
         return false;
 
     int paddedWidth;
-    writeBmpHeader(file, bitmap.width, bitmap.height, paddedWidth);
+    writeBmpHeader(file, 1, bitmap.width, bitmap.height, paddedWidth);
 
-    const uint8_t padding[4] = { };
-    int padLength = paddedWidth-3*bitmap.width;
+    byte padding[4] = { };
+    int padLength = paddedWidth-bitmap.width;
     for (int y = 0; y < bitmap.height; ++y) {
         for (int x = 0; x < bitmap.width; ++x) {
-            uint8_t px = (uint8_t) pixelFloatToByte(*bitmap(x, y));
-            fwrite(&px, sizeof(uint8_t), 1, file);
-            fwrite(&px, sizeof(uint8_t), 1, file);
-            fwrite(&px, sizeof(uint8_t), 1, file);
+            byte px = pixelFloatToByte(*bitmap(x, y));
+            fwrite(&px, sizeof(byte), 1, file);
         }
         fwrite(padding, 1, padLength, file);
     }
@@ -146,18 +182,18 @@ bool saveBmp(const BitmapConstRef<float, 3> &bitmap, const char *filename) {
         return false;
 
     int paddedWidth;
-    writeBmpHeader(file, bitmap.width, bitmap.height, paddedWidth);
+    writeBmpHeader(file, 3, bitmap.width, bitmap.height, paddedWidth);
 
-    const uint8_t padding[4] = { };
+    byte padding[4] = { };
     int padLength = paddedWidth-3*bitmap.width;
     for (int y = 0; y < bitmap.height; ++y) {
         for (int x = 0; x < bitmap.width; ++x) {
-            uint8_t bgr[3] = {
-                (uint8_t) pixelFloatToByte(bitmap(x, y)[2]),
-                (uint8_t) pixelFloatToByte(bitmap(x, y)[1]),
-                (uint8_t) pixelFloatToByte(bitmap(x, y)[0])
+            byte bgr[3] = {
+                pixelFloatToByte(bitmap(x, y)[2]),
+                pixelFloatToByte(bitmap(x, y)[1]),
+                pixelFloatToByte(bitmap(x, y)[0])
             };
-            fwrite(bgr, sizeof(uint8_t), 3, file);
+            fwrite(bgr, sizeof(byte), 3, file);
         }
         fwrite(padding, 1, padLength, file);
     }
@@ -166,8 +202,26 @@ bool saveBmp(const BitmapConstRef<float, 3> &bitmap, const char *filename) {
 }
 
 bool saveBmp(const BitmapConstRef<float, 4> &bitmap, const char *filename) {
-    // RGBA not supported by the BMP format
-    return false;
+    FILE *file = fopen(filename, "wb");
+    if (!file)
+        return false;
+
+    int dummyPaddedWidth;
+    writeBmpHeader(file, 4, bitmap.width, bitmap.height, dummyPaddedWidth);
+
+    for (int y = 0; y < bitmap.height; ++y) {
+        for (int x = 0; x < bitmap.width; ++x) {
+            byte bgra[4] = {
+                pixelFloatToByte(bitmap(x, y)[2]),
+                pixelFloatToByte(bitmap(x, y)[1]),
+                pixelFloatToByte(bitmap(x, y)[0]),
+                pixelFloatToByte(bitmap(x, y)[3])
+            };
+            fwrite(bgra, sizeof(byte), 4, file);
+        }
+    }
+
+    return !fclose(file);
 }
 
 }

main.cpp

@@ -1012,8 +1012,6 @@ int main(int argc, const char *const *argv) {
             ABORT("No input specified! See -help.");
         #endif
     }
-    if (mode == MULTI_AND_TRUE && (format == BMP || (format == AUTO && output && cmpExtension(output, ".bmp"))))
-        ABORT("Incompatible image format. A BMP file cannot contain alpha channel, which is required in mtsdf mode.");
     Shape shape;
     switch (inputType) {
     #if defined(MSDFGEN_EXTENSIONS) && !defined(MSDFGEN_DISABLE_SVG)