Read GIF background color from GCB during metadata check.

Author: skidder

giflib.cpp

@@ -18,7 +18,6 @@ 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;
 };
@@ -134,11 +133,6 @@ 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) {
@@ -490,6 +484,23 @@ giflib_decoder_frame_state giflib_decoder_skip_frame(giflib_decoder d)
 static int interlace_offset[] = {0, 4, 2, 1};
 static int interlace_jumps[] = {8, 8, 4, 2};
 
+static void extract_background_color(GifFileType* gif, GraphicsControlBlock* gcb, 
+                                   uint8_t* r, uint8_t* g, uint8_t* b, uint8_t* a) {
+    bool have_transparency = (gcb->TransparentColor != NO_TRANSPARENT_COLOR);
+    if (have_transparency) {
+        *r = *g = *b = *a = 0;
+    }
+    else if (gif->SColorMap && gif->SColorMap->Colors) {
+        *r = gif->SColorMap->Colors[gif->SBackGroundColor].Red;
+        *g = gif->SColorMap->Colors[gif->SBackGroundColor].Green;
+        *b = gif->SColorMap->Colors[gif->SBackGroundColor].Blue;
+        *a = 255;
+    }
+    else {
+        *r = *g = *b = *a = 255;
+    }
+}
+
 // decode the full frame and write it into mat
 // decode_frame_header *must* be called before this function
 bool giflib_decoder_decode_frame(giflib_decoder d, opencv_mat mat)
@@ -557,21 +568,9 @@ bool giflib_decoder_decode_frame(giflib_decoder d, opencv_mat mat)
     giflib_get_frame_gcb(d->gif, &gcb);
 
     if (!d->have_read_first_frame) {
-        bool have_transparency = (gcb.TransparentColor != NO_TRANSPARENT_COLOR);
-        if (have_transparency) {
-            d->bg_red = d->bg_green = d->bg_blue = d->bg_alpha = 0;
-        }
-        else if (d->gif->SColorMap && d->gif->SColorMap->Colors) {
-            d->bg_red = d->gif->SColorMap->Colors[d->gif->SBackGroundColor].Red;
-            d->bg_green = d->gif->SColorMap->Colors[d->gif->SBackGroundColor].Green;
-            d->bg_blue = d->gif->SColorMap->Colors[d->gif->SBackGroundColor].Blue;
-            d->bg_alpha = 255;
-        }
-        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;
+        GraphicsControlBlock gcb;
+        giflib_get_frame_gcb(d->gif, &gcb);
+        extract_background_color(d->gif, &gcb, &d->bg_red, &d->bg_green, &d->bg_blue, &d->bg_alpha);
     }
 
     if (!giflib_decoder_render_frame(d, &gcb, mat)) {
@@ -1133,10 +1132,9 @@ int giflib_encoder_get_output_length(giflib_encoder e)
 
 struct GifAnimationInfo giflib_decoder_get_animation_info(const giflib_decoder d) {
     // Default to 1 loop (play once) if no NETSCAPE2.0 extension is found
-    GifAnimationInfo info = {1, 0};  // Initialize with defaults
+    GifAnimationInfo info = {1, 0, 255, 255, 255, 0};  // loop_count, frame_count, bg_r, bg_g, bg_b, bg_a
 
     // Create a temporary decoder to read extension blocks
-    // We need a separate decoder because reading extension blocks modifies decoder state
     giflib_decoder loopReader = new struct giflib_decoder_struct();
     if (!loopReader) {
         return info; // Return default on allocation failure
@@ -1153,21 +1151,31 @@ struct GifAnimationInfo giflib_decoder_get_animation_info(const giflib_decoder d
     }
 
     bool found_loop_count = false;
-    // Read all blocks until we hit end
+    bool found_gcb = false;
+    GraphicsControlBlock gcb = {};
     GifRecordType recordType;
+    
+    // Read all blocks until we hit end
     while (DGifGetRecordType(gif, &recordType) == GIF_OK) {
         switch (recordType) {
             case EXTENSION_RECORD_TYPE: {
                 GifByteType* ExtData;
                 int ExtFunction;
 
                 if (DGifGetExtension(gif, &ExtFunction, &ExtData) == GIF_OK && ExtData != NULL) {
+                    // Look for GraphicsControlBlock if we haven't found it yet
+                    if (!found_gcb && ExtFunction == GRAPHICS_EXT_FUNC_CODE) {
+                        found_gcb = true;
+                        DGifExtensionToGCB(ExtData[0], &ExtData[1], &gcb);
+                        // Get background color as soon as we have the GCB
+                        extract_background_color(gif, &gcb, &info.bg_red, &info.bg_green, 
+                                              &info.bg_blue, &info.bg_alpha);
+                    }
                     // Look for NETSCAPE2.0 extension
-                    if (!found_loop_count && 
+                    else if (!found_loop_count && 
                         ExtFunction == APPLICATION_EXT_FUNC_CODE && 
                         ExtData[0] >= 11 &&
                         memcmp(ExtData + 1, "NETSCAPE2.0", 11) == 0) {
-                        // Get the next block with loop count
                         if (DGifGetExtensionNext(gif, &ExtData) == GIF_OK && 
                             ExtData != NULL && 
                             ExtData[0] >= 3 && 
@@ -1215,6 +1223,12 @@ struct GifAnimationInfo giflib_decoder_get_animation_info(const giflib_decoder d
         }
     }
 
+    // If we never found a GCB, still need to set background color
+    if (!found_gcb) {
+        extract_background_color(gif, &gcb, &info.bg_red, &info.bg_green, 
+                               &info.bg_blue, &info.bg_alpha);
+    }
+
 cleanup:
     DGifCloseFile(gif, &error);
     delete loopReader;

giflib.go

@@ -19,6 +19,10 @@ type gifDecoder struct {
 	loopCount         int
 	frameCount        int
 	animationInfoRead bool
+	bgRed             uint8
+	bgGreen           uint8
+	bgBlue            uint8
+	bgAlpha           uint8
 }
 
 type gifEncoder struct {
@@ -113,7 +117,8 @@ func (d *gifDecoder) Duration() time.Duration {
 }
 
 func (d *gifDecoder) BackgroundColor() uint32 {
-	return uint32(C.giflib_decoder_get_background_color(d.decoder))
+	d.readAnimationInfo()
+	return uint32(d.bgRed)<<16 | uint32(d.bgGreen)<<8 | uint32(d.bgBlue) | uint32(d.bgAlpha)<<24
 }
 
 // readAnimationInfo reads the GIF info from the decoder and caches it
@@ -125,6 +130,10 @@ func (d *gifDecoder) readAnimationInfo() {
 		d.loopCount = int(info.loop_count)
 		d.frameCount = int(info.frame_count)
 		d.animationInfoRead = true
+		d.bgRed = uint8(info.bg_red)
+		d.bgGreen = uint8(info.bg_green)
+		d.bgBlue = uint8(info.bg_blue)
+		d.bgAlpha = uint8(info.bg_alpha)
 	}
 }
 

giflib.hpp

@@ -10,6 +10,10 @@ extern "C" {
 struct GifAnimationInfo {
     int loop_count;
     int frame_count;
+    uint8_t bg_red;
+    uint8_t bg_green;
+    uint8_t bg_blue;
+    uint8_t bg_alpha;
 };
 
 #define GIF_DISPOSE_NONE 0
@@ -44,7 +48,6 @@ 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