[PATCH 0/1] MR9486: windowscodecs: correct GIF LZW compression initialization

[Jiajin Cui (＠jin-king1)]

The GIF LZW compression algorithm was incorrectly initializing the code
size to 8 bits regardless of the actual color depth. This caused issues
with images that have fewer colors requiring smaller initial code sizes.
The fix calculates the proper initial code size based on the actual
number of colors in the image, using the formula: minimum code size =
max(2, ceil(log2(color_count))). This ensures proper compression for
images with small color palettes and maintains compatibility with GIF
decoders that expect correct LZW initialization parameters.

Log: Fixed GIF compression for images with small color palettes

Signed-off-by: Jiajin Cui

[PATCH 1/1] windowscodecs: correct GIF LZW compression initialization

[Jiajin Cui]

From: Jiajin Cui

The GIF LZW compression algorithm was incorrectly initializing the code
size to 8 bits regardless of the actual color depth. This caused issues
with images that have fewer colors requiring smaller initial code sizes.
The fix calculates the proper initial code size based on the actual
number of colors in the image, using the formula: minimum code size =
max(2, ceil(log2(color_count))). This ensures proper compression for
images with small color palettes and maintains compatibility with GIF
decoders that expect correct LZW initialization parameters.

Log: Fixed GIF compression for images with small color palettes

Signed-off-by: Jiajin Cui 
---
 dlls/windowscodecs/gifformat.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/dlls/windowscodecs/gifformat.c b/dlls/windowscodecs/gifformat.c
index 4fde4f81bba..989ae3f3568 100644
--- a/dlls/windowscodecs/gifformat.c
+++ b/dlls/windowscodecs/gifformat.c
@@ -1415,7 +1415,7 @@ static inline int read_byte(struct input_stream *in, unsigned char *byte)
     return 0;
 }

-static HRESULT gif_compress(IStream *out_stream, const BYTE *in_data, ULONG in_size)
+static HRESULT gif_compress(IStream *out_stream, const BYTE *in_data, ULONG in_size, int color_bits)
 {
     struct input_stream in;
     struct output_stream out;
@@ -1429,7 +1429,7 @@ static HRESULT gif_compress(IStream *out_stream, const BYTE *in_data, ULONG in_s
     out.gif_block.len = 0;
     out.out = out_stream;

-    init_code_bits = suffix = 8;
+    init_code_bits = suffix = max(2, color_bits);
     if (IStream_Write(out.out, &suffix, sizeof(suffix), NULL) != S_OK)
         return E_FAIL;

@@ -1563,12 +1563,24 @@ static HRESULT WINAPI GifFrameEncode_Commit(IWICBitmapFrameEncode *iface)
                     hr = IStream_Write(This->encoder->stream, gif_palette, sizeof(gif_palette), NULL);
                     if (hr == S_OK)
                     {
+                        int color_bits = 1;
+                        while ((1 << color_bits) < This->colors) color_bits++;
+
                         /* Image Data */
-                        hr = gif_compress(This->encoder->stream, This->image_data, This->width * This->height);
+                        hr = gif_compress(This->encoder->stream, This->image_data, This->width * This->height, color_bits);
                         if (hr == S_OK)
                             This->committed = TRUE;
                     }
                 }
+                else if (hr == S_OK)
+                {
+                    int color_bits = 1;
+                    while ((1 << color_bits) < This->encoder->colors) color_bits++;
+
+                    hr = gif_compress(This->encoder->stream, This->image_data, This->width * This->height, color_bits);
+                    if (hr == S_OK)
+                        This->committed = TRUE;
+                }
             }
         }
     }

[Dmitry Timoshkov (＠dmitry)]

Dmitry Timoshkov (@dmitry) commented about dlls/windowscodecs/gifformat.c:
>                      /* Image Data */
> -
>                    hr = gif_compress(This->encoder->stream, This->image_data, This->width * This->height);
> -
>                    hr = gif_compress(This->encoder->stream, This->image_data, This->width * This->height, color_bits);
>                    if (hr == S_OK)
>                        This->committed = TRUE;
>                }
>            }
> -
>            else if (hr == S_OK)
> -
>            {
> -
>                int color_bits = 1;
> -
>                while ((1 << color_bits) < This->encoder->colors) color_bits++;
> -
> -
>                hr = gif_compress(This->encoder->stream, This->image_data, This->width * This->height, color_bits);
> -
>                if (hr == S_OK)
> -
>                    This->committed = TRUE;
> -
>            }

It should be possible to avoid the code duplication, for instance by writing palette only when necessary.