From 827a307fecdd11b6ff082ad3fb4ada134dcaea1d Mon Sep 17 00:00:00 2001
From: Pascal Massimino <skal@google.com>
Date: Tue, 11 Jan 2022 16:43:35 +0100
Subject: [PATCH] BMP enc: fix the transparency case

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
(cherry picked from commit 480cd51de6b5ed378b194ef77aa6d1bcdaf63da8)
---
 imageio/image_enc.c | 35 +++++++++++++++++++++++------------
 1 file changed, 23 insertions(+), 12 deletions(-)

diff --git a/imageio/image_enc.c b/imageio/image_enc.c
index 5a78cbda..e06bcaf5 100644
--- a/imageio/image_enc.c
+++ b/imageio/image_enc.c
@@ -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