Expose background color for GIF inputs

giflib.cpp

@@ -18,6 +18,7 @@ struct giflib_decoder_struct {
     uint8_t bg_red;
     uint8_t bg_blue;
     uint8_t bg_alpha;
+    uint32_t background_color;
     bool have_read_first_frame;
     bool seek_clear_extensions;
 };
@@ -133,6 +134,11 @@ int giflib_decoder_get_prev_frame_disposal(const giflib_decoder d)
     }
 }
 
+uint32_t giflib_decoder_get_background_color(const giflib_decoder d)
+{
+    return d->background_color;
+}
+
 void giflib_decoder_release(giflib_decoder d)
 {
     if (d->pixels) {
@@ -564,6 +570,8 @@ bool giflib_decoder_decode_frame(giflib_decoder d, opencv_mat mat)
         else {
             d->bg_red = d->bg_green = d->bg_blue = d->bg_alpha = 255;
         }
+        d->background_color = (uint32_t)d->bg_alpha << 24 | ((uint32_t)d->bg_red << 16) |
+                              ((uint32_t)d->bg_green << 8) | (uint32_t)d->bg_blue;
     }
 
     if (!giflib_decoder_render_frame(d, &gcb, mat)) {

giflib.go

@@ -113,7 +113,7 @@ func (d *gifDecoder) Duration() time.Duration {
 }
 
 func (d *gifDecoder) BackgroundColor() uint32 {
-	return 0x00FFFFFF // use a non-opaque alpha value so we can see the background if needed
+	return uint32(C.giflib_decoder_get_background_color(d.decoder))
 }
 
 // readAnimationInfo reads the GIF info from the decoder and caches it

giflib.hpp

@@ -44,6 +44,7 @@ void giflib_encoder_release(giflib_encoder e);
 int giflib_encoder_get_output_length(giflib_encoder e);
 struct GifAnimationInfo giflib_decoder_get_animation_info(const giflib_decoder d);
 int giflib_decoder_get_prev_frame_disposal(const giflib_decoder d);
+uint32_t giflib_decoder_get_background_color(const giflib_decoder d);
 #ifdef __cplusplus
 }
 #endif