fix: only restore background when dispose is 2 or 3

Signed-off-by: Cocoa <[email protected]>

3rd_party/stb/stb_image.h

@@ -6840,8 +6840,10 @@ static stbi_uc *stbi__gif_load_next(stbi__context *s, stbi__gif *g, int *comp, i
          // 0:  not specified.
       }
 
-      // background is what out is after the undoing of the previou frame;
-      memcpy( g->background, g->out, 4 * g->w * g->h );
+      if (dispose == 2 || dispose == 3) {
+         // background is what out is after the undoing of the previou frame;
+         memcpy( g->background, g->out, 4 * g->w * g->h );
+      }
    }
 
    // clear my history;

test/stb_image_test.exs

@@ -279,4 +279,23 @@ defmodule StbImageTest do
       end
     end
   end
+
+  describe "GIF" do
+    test "only restores background when dispose is 2 or 3" do
+      file = Path.join(__DIR__, "stb-issue-1688-horse.gif")
+      binary = File.read!(file)
+
+      {:ok, frames, _} = StbImage.read_gif_binary(binary)
+      assert 34 == Enum.count(frames)
+
+      decoded = Enum.at(frames, 5)
+      # StbImage.write_file!(decoded, Path.join(__DIR__, "stb-issue-1688-horse-5.png"))
+      decoded = StbImage.to_binary(decoded, :png)
+
+      {:ok, expected_img} = StbImage.read_file(Path.join(__DIR__, "stb-issue-1688-expected.png"))
+      expected = StbImage.to_binary(expected_img, :png)
+
+      assert decoded == expected
+    end
+  end
 end