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fix: Preserve GIF transparent color across frames #255

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Merged
skidder merged 2 commits into from
May 16, 2025
Merged

fix: Preserve GIF transparent color across frames #255

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f1db6ca
fix: Preserve GIF transparent color across frames
skidder May 16, 2025
8b653b4
Preserve unspecified disposal method behavior from earlier.
skidder May 16, 2025
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54 changes: 40 additions & 14 deletions giflib.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -146,7 +146,7 @@ int giflib_decoder_get_prev_frame_disposal(const giflib_decoder d)
case DISPOSE_PREVIOUS:
return GIF_DISPOSE_PREVIOUS;
default: // DISPOSAL_UNSPECIFIED
return GIF_DISPOSE_BACKGROUND;
return GIF_DISPOSE_NONE;
}
}

Expand Down Expand Up @@ -206,7 +206,6 @@ static bool giflib_get_frame_gcb(GifFileType* gif, GraphicsControlBlock* gcb)
gcb->DisposalMode = DISPOSAL_UNSPECIFIED;
gcb->UserInputFlag = false;
gcb->DelayTime = 0;
gcb->TransparentColor = NO_TRANSPARENT_COLOR;

bool success = true;
for (int i = 0; i < gif->ExtensionBlockCount; i++) {
Expand Down Expand Up @@ -538,17 +537,38 @@ static void extract_background_color(GifFileType* gif,
uint8_t* a)
{
bool have_transparency = (gcb->TransparentColor != NO_TRANSPARENT_COLOR);

if (have_transparency) {
*r = *g = *b = *a = 0;
// For transparent GIFs, use background color with alpha=0 to preserve color information
if (gif->SColorMap && gif->SColorMap->Colors &&
gif->SBackGroundColor >= 0 &&
gif->SBackGroundColor < gif->SColorMap->ColorCount) {
*r = gif->SColorMap->Colors[gif->SBackGroundColor].Red;
*g = gif->SColorMap->Colors[gif->SBackGroundColor].Green;
*b = gif->SColorMap->Colors[gif->SBackGroundColor].Blue;
} else {
// Default to white (with alpha=0) if no valid background color index
*r = 255;
*g = 255;
*b = 255;
}
*a = 0; // Always transparent if GCB has transparency
}
else if (gif->SColorMap && gif->SColorMap->Colors) {
else if (gif->SColorMap && gif->SColorMap->Colors &&
gif->SBackGroundColor >= 0 &&
gif->SBackGroundColor < gif->SColorMap->ColorCount) {
// For non-transparent GIFs with valid background color, use opaque background
*r = gif->SColorMap->Colors[gif->SBackGroundColor].Red;
*g = gif->SColorMap->Colors[gif->SBackGroundColor].Green;
*b = gif->SColorMap->Colors[gif->SBackGroundColor].Blue;
*a = 255;
*a = 255; // Fully opaque
}
else {
*r = *g = *b = *a = 255;
// Default to white background if no color map or invalid background color index
*r = 255;
*g = 255;
*b = 255;
*a = 255;
}
}

Expand Down Expand Up @@ -743,6 +763,13 @@ bool giflib_encoder_init(giflib_encoder e, const giflib_decoder d, int width, in
memmove(e->gif->SColorMap->Colors,
d->gif->SColorMap->Colors,
e->gif->SColorMap->ColorCount * sizeof(GifColorType));

if (d->gif->SBackGroundColor >= 0 &&
d->gif->SBackGroundColor < d->gif->SColorMap->ColorCount) {
e->gif->SBackGroundColor = d->gif->SBackGroundColor;
} else {
e->gif->SBackGroundColor = 0;
}
}

int res = EGifPutScreenDesc(e->gif,
Expand Down Expand Up @@ -806,16 +833,15 @@ static bool giflib_encoder_setup_frame(giflib_encoder e, const giflib_decoder d)
// Fix GCB disposal mode for DISPOSAL_UNSPECIFIED
GraphicsControlBlock gcb;
if (giflib_get_frame_gcb(e->gif, &gcb)) {
if (gcb.DisposalMode == DISPOSAL_UNSPECIFIED) {
// For backward compatibility, treat DISPOSAL_UNSPECIFIED as DISPOSE_BACKGROUND
gcb.DisposalMode = DISPOSE_BACKGROUND;
giflib_set_frame_gcb(e->gif, &gcb);
}

// If the transparent color is the background color, remove transparency
// GIF transparency optimization: If the transparent color matches the background color,
// we can remove the transparency, but only when the background is fully opaque (alpha=255).
// This prevents incorrect removal of transparency when the background has transparency.
if (gcb.TransparentColor != NO_TRANSPARENT_COLOR) {
ColorMapObject* color_map = e->frame_color_map ? e->frame_color_map : e->gif->SColorMap;
if (color_map && gcb.TransparentColor == e->gif->SBackGroundColor) {

if (color_map &&
gcb.TransparentColor == e->gif->SBackGroundColor &&
d->bg_alpha == 255) {
gcb.TransparentColor = NO_TRANSPARENT_COLOR;
giflib_set_frame_gcb(e->gif, &gcb);
}
Expand Down