crop for animated image

Signed-off-by: songjiayang <songjiayang1@gmail.com>

vips/conversion.c

@@ -229,8 +229,40 @@ int smartcrop(VipsImage *in, VipsImage **out, int width, int height,
 
 int crop(VipsImage *in, VipsImage **out, int left, int top,
               int width, int height) {
-  return vips_crop(in, out, left, top, width, height,
-                        NULL);
+  // resolve image pages
+  int page_height = vips_image_get_page_height(in);
+  int n_pages = in->Ysize / page_height;
+  if (n_pages <= 1) {
+    return vips_crop(in, out, left, top, width, height, NULL);
+  }
+
+  int in_width = in->Xsize;
+  VipsObject *base = VIPS_OBJECT(vips_image_new());
+  VipsImage **page = (VipsImage **) vips_object_local_array(base, n_pages);
+  VipsImage **copy = (VipsImage **) vips_object_local_array(base, 1);
+  // split image into cropped frames
+  for (int i = 0; i < n_pages; i++) {
+    if (
+      vips_extract_area(in, &page[i], 0, page_height * i, in_width, page_height, NULL) ||
+      vips_crop(page[i], &page[i], left, top, width, height, NULL)
+    ) {
+      g_object_unref(base);
+      return -1;
+    }
+  }
+
+  // reassemble frames and set page height
+  // copy before modifying metadata
+  if(
+    vips_arrayjoin(page, &copy[0], n_pages, "across", 1, NULL) ||
+    vips_copy(copy[0], out, NULL)
+  ) {
+    g_object_unref(base);
+    return -1;
+  }
+  vips_image_set_int(*out, VIPS_META_PAGE_HEIGHT, height);
+  g_object_unref(base);
+  return 0;
 }
 
 int flatten_image(VipsImage *in, VipsImage **out, double r, double g,

vips/image_gif_test.go

@@ -115,6 +115,18 @@ func TestImage_GIF_Animated_ExtractArea(t *testing.T) {
 		nil)
 }
 
+func TestImage_GIF_Animated_Crop(t *testing.T) {
+	goldenAnimatedTest(t, resources+"gif-animated.gif",
+		-1,
+		func(img *ImageRef) error {
+			return img.Crop(10, 20, 20, 20)
+		},
+		func(img *ImageRef) {
+			assert.Equal(t, img.GetPageHeight(), 20)
+		},
+		nil)
+}
+
 func TestImage_GIF_Animated_PageDelay(t *testing.T) {
 	goldenAnimatedTest(t, resources+"gif-animated.gif",
 		-1,