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BMP enc: fix the transparency case

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When transparency is present, it's not enough to just write 32bpp samples.
One need to use the full BITMAPV3INFOHEADER syntax and specify the
masks for BGRA.

 see https://en.wikipedia.org/wiki/BMP_file_format#Pixel_storage

Also remove the height-flip trick and write samples bottom-to-top instead.

Change-Id: If5d92c11453b96764b5bfbf19e9678e632bc911f
This commit is contained in:
Pascal Massimino 2022-01-11 16:43:35 +01:00
parent 8ea81561d2
commit 480cd51de6
1 changed files with 23 additions and 12 deletions
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35
imageio/image_enc.c
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@ -280,7 +280,7 @@ int WebPWrite16bAsPGM(FILE* fout, const WebPDecBuffer* const buffer) {
}
//------------------------------------------------------------------------------
// BMP
// BMP (see https://en.wikipedia.org/wiki/BMP_file_format#Pixel_storage)
static void PutLE16(uint8_t* const dst, uint32_t value) {
dst[0] = (value >> 0) & 0xff;
@ -293,8 +293,11 @@ static void PutLE32(uint8_t* const dst, uint32_t value) {
}
#define BMP_HEADER_SIZE 54
#define BMP_HEADER_ALPHA_EXTRA_SIZE 16 // for alpha info
int WebPWriteBMP(FILE* fout, const WebPDecBuffer* const buffer) {
const int has_alpha = WebPIsAlphaMode(buffer->colorspace);
const int header_size =
BMP_HEADER_SIZE + (has_alpha ? BMP_HEADER_ALPHA_EXTRA_SIZE : 0);
const uint32_t width = buffer->width;
const uint32_t height = buffer->height;
const uint8_t* rgba = buffer->u.RGBA.rgba;
@ -303,8 +306,9 @@ int WebPWriteBMP(FILE* fout, const WebPDecBuffer* const buffer) {
uint32_t y;
const uint32_t line_size = bytes_per_px * width;
const uint32_t bmp_stride = (line_size + 3) & ~3; // pad to 4
const uint32_t total_size = bmp_stride * height + BMP_HEADER_SIZE;
uint8_t bmp_header[BMP_HEADER_SIZE] = { 0 };
const uint32_t image_size = bmp_stride * height;
const uint32_t total_size = image_size + header_size;
uint8_t bmp_header[BMP_HEADER_SIZE + BMP_HEADER_ALPHA_EXTRA_SIZE] = { 0 };
if (fout == NULL || buffer == NULL || rgba == NULL) return 0;
@ -312,30 +316,37 @@ int WebPWriteBMP(FILE* fout, const WebPDecBuffer* const buffer) {
PutLE16(bmp_header + 0, 0x4d42); // signature 'BM'
PutLE32(bmp_header + 2, total_size); // size including header
PutLE32(bmp_header + 6, 0); // reserved
PutLE32(bmp_header + 10, BMP_HEADER_SIZE); // offset to pixel array
PutLE32(bmp_header + 10, header_size); // offset to pixel array
// bitmap info header
PutLE32(bmp_header + 14, 40); // DIB header size
PutLE32(bmp_header + 14, header_size - 14); // DIB header size
PutLE32(bmp_header + 18, width); // dimensions
PutLE32(bmp_header + 22, -(int)height); // vertical flip!
PutLE32(bmp_header + 22, height); // no vertical flip
PutLE16(bmp_header + 26, 1); // number of planes
PutLE16(bmp_header + 28, bytes_per_px * 8); // bits per pixel
PutLE32(bmp_header + 30, 0); // no compression (BI_RGB)
PutLE32(bmp_header + 34, 0); // image size (placeholder)
PutLE32(bmp_header + 30, has_alpha ? 3 : 0); // BI_BITFIELDS or BI_RGB
PutLE32(bmp_header + 34, image_size);
PutLE32(bmp_header + 38, 2400); // x pixels/meter
PutLE32(bmp_header + 42, 2400); // y pixels/meter
PutLE32(bmp_header + 46, 0); // number of palette colors
PutLE32(bmp_header + 50, 0); // important color count
if (has_alpha) { // BITMAPV3INFOHEADER complement
PutLE32(bmp_header + 54, 0x00ff0000); // red mask
PutLE32(bmp_header + 58, 0x0000ff00); // green mask
PutLE32(bmp_header + 62, 0x000000ff); // blue mask
PutLE32(bmp_header + 66, 0xff000000); // alpha mask
}
// TODO(skal): color profile
// write header
if (fwrite(bmp_header, sizeof(bmp_header), 1, fout) != 1) {
if (fwrite(bmp_header, header_size, 1, fout) != 1) {
return 0;
}
// write pixel array
// write pixel array, bottom to top
for (y = 0; y < height; ++y) {
if (fwrite(rgba, line_size, 1, fout) != 1) {
const uint8_t* const src = &rgba[(uint64_t)(height - 1 - y) * stride];
if (fwrite(src, line_size, 1, fout) != 1) {
return 0;
}
// write padding zeroes
@ -345,11 +356,11 @@ int WebPWriteBMP(FILE* fout, const WebPDecBuffer* const buffer) {
return 0;
}
}
rgba += stride;
}
return 1;
}
#undef BMP_HEADER_SIZE
#undef BMP_HEADER_ALPHA_EXTRA_SIZE
//------------------------------------------------------------------------------
// TIFF