feat(gif): Add GIF animation loop count detection for animated WebP conversion (#199)

Author: skidder

giflib.cpp

@@ -1112,3 +1112,77 @@ int giflib_encoder_get_output_length(giflib_encoder e)
 {
     return e->dst_offset;
 }
+
+int giflib_decoder_get_loop_count(const giflib_decoder d) {
+    // Default to 1 loop (play once) if no NETSCAPE2.0 extension is found
+    int loop_count = 1;
+
+    // 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 loop_count; // Return default on allocation failure
+    }
+
+    memset(loopReader, 0, sizeof(struct giflib_decoder_struct));
+    loopReader->mat = d->mat;  // Share the source data
+
+    int error = 0;
+    GifFileType* gif = DGifOpen(loopReader, decode_func, &error);
+    if (error) {
+        delete loopReader;
+        return loop_count;
+    }
+
+    // Read all blocks until we find NETSCAPE2.0 or hit end
+    GifRecordType recordType;
+    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 NETSCAPE2.0 extension
+                    if (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 && 
+                            ExtData[1] == 1) {
+                            loop_count = ExtData[2] | (ExtData[3] << 8);
+                            goto cleanup; // Found what we need
+                        }
+                    }
+                    
+                    // Skip any remaining extension blocks
+                    while (ExtData != NULL) {
+                        if (DGifGetExtensionNext(gif, &ExtData) != GIF_OK) {
+                            goto cleanup;
+                        }
+                    }
+                }
+                break;
+            }
+            
+            case IMAGE_DESC_RECORD_TYPE:
+                // Skip image data
+                if (DGifGetImageDesc(gif) != GIF_OK) {
+                    goto cleanup;
+                }
+                break;
+                
+            case TERMINATE_RECORD_TYPE:
+                goto cleanup;
+                
+            default:
+                break;
+        }
+    }
+
+cleanup:
+    DGifCloseFile(gif, &error);
+    delete loopReader;
+    return loop_count;
+}

giflib.go

@@ -12,10 +12,12 @@ import (
 )
 
 type gifDecoder struct {
-	decoder    C.giflib_decoder
-	mat        C.opencv_mat
-	buf        []byte
-	frameIndex int
+	decoder       C.giflib_decoder
+	mat           C.opencv_mat
+	buf           []byte
+	frameIndex    int
+	loopCount     int
+	loopCountRead bool
 }
 
 type gifEncoder struct {
@@ -114,7 +116,11 @@ func (d *gifDecoder) BackgroundColor() uint32 {
 }
 
 func (d *gifDecoder) LoopCount() int {
-	return 0 // loop indefinitely
+	if !d.loopCountRead {
+		d.loopCount = int(C.giflib_decoder_get_loop_count(d.decoder))
+		d.loopCountRead = true
+	}
+	return d.loopCount
 }
 
 func (d *gifDecoder) DecodeTo(f *Framebuffer) error {

giflib.hpp

@@ -34,6 +34,7 @@ bool giflib_encoder_encode_frame(giflib_encoder e, const giflib_decoder d, const
 bool giflib_encoder_flush(giflib_encoder e, const giflib_decoder d);
 void giflib_encoder_release(giflib_encoder e);
 int giflib_encoder_get_output_length(giflib_encoder e);
+int giflib_decoder_get_loop_count(const giflib_decoder d);
 
 #ifdef __cplusplus
 }

testdata/dispose_bgnd.gif

@@ -391,6 +391,7 @@ func testNewWebpEncoderWithAnimatedGIFSource(t *testing.T) {
 		height       int
 		quality      int
 		resizeMethod ImageOpsSizeMethod
+		wantLoops    int
 	}{
 		{
 			name:         "Animated GIF with alpha channel",
@@ -400,6 +401,37 @@ func testNewWebpEncoderWithAnimatedGIFSource(t *testing.T) {
 			height:       17,
 			quality:      80,
 			resizeMethod: ImageOpsResize,
+			wantLoops:    0,
+		},
+		{
+			name:         "Animated GIF with specific loop count",
+			inputPath:    "testdata/no-loop.gif",
+			outputPath:   "testdata/out/no-loop_out.webp",
+			width:        200,
+			height:       200,
+			quality:      80,
+			resizeMethod: ImageOpsResize,
+			wantLoops:    1,
+		},
+		{
+			name:         "Animated GIF with duplicate number of loop count, use the first loop count",
+			inputPath:    "testdata/duplicate_number_of_loops.gif",
+			outputPath:   "testdata/out/duplicate_number_of_loops.webp",
+			width:        200,
+			height:       200,
+			quality:      80,
+			resizeMethod: ImageOpsResize,
+			wantLoops:    2,
+		},
+		{
+			name:         "Animated GIF with multiple extension blocks",
+			inputPath:    "testdata/dispose_bgnd.gif",
+			outputPath:   "testdata/out/dispose_bgnd.webp",
+			width:        200,
+			height:       200,
+			quality:      80,
+			resizeMethod: ImageOpsResize,
+			wantLoops:    0,
 		},
 	}
 
@@ -422,6 +454,11 @@ func testNewWebpEncoderWithAnimatedGIFSource(t *testing.T) {
 				return
 			}
 
+			// Verify loop count
+			if decoder.LoopCount() != tc.wantLoops {
+				t.Errorf("Loop count = %d, want %d", decoder.LoopCount(), tc.wantLoops)
+			}
+
 			dstBuf := make([]byte, destinationBufferSize)
 
 			options := &ImageOptions{