On Fri, 3 Jul 2020 at 12:16, <jcup...@gmail.com> wrote: > Thank you for libnsgif. I think I might have come across a gif that > fails to render correctly in libnsgif-0.2.1: > > https://user-images.githubusercontent.com/580843/86457777-18bd1400-bd1c-11ea-923a-adeed2eba031.gif > > Frame 6 is DISPOSE_PREVIOUS and should unpaint the tips of the wings. > libnsgif does not seem to handle this though, and the tips are left > behind.
Hello again, I spent a little time looking at the code. tldr: I think GIFs which have a COMBINE frame followed by a RESTORE frame will not render correctly in libnsgif. 1. The frames in this GIF (except for frame 5) have disposal_method 1, GIF_FRAME_COMBINE, ie. the frame is not cleared and pixels accumulate. 2. Frame 5 is GIF_FRAME_RESTORE, meaning that after frame 5 is rendered, the frame should be restored to frame 4. This is detected when frame 6 rendering starts here: https://source.netsurf-browser.org/libnsgif.git/tree/src/libnsgif.c#n794 And it calls gif_internal_decode_frame() recursively to put the pixels for frame 4 back. 3. However, because frame 4 is GIF_FRAME_COMBINE, the frame 4 pixels are incorrectly painted on top of frame 5. 4. gif_internal_decode_frame() then decode the pixels for frame 6, but because frame 4 has not been restored, the wingtips are left behind. The problem is that with GIF_FRAME_COMBINE, the state of a GIF frame depends on all the previous frames. You can't just call gif_internal_decode_frame() to put the pixels back --- you need to re-render the entire animation. It seems to me that the best way to implement GIF_FRAME_RESTORE is to have an extra bitmap in the gif struct to hold the restore pixels. When processing a frame with GIF_FRAME_RESTORE set, a copy should be made of the frame before painting in the new pixels. When processing a frame whose /previous/ frame is GIF_FRAME_RESTORE, the frame should be initialised from the previous bitmap. Does this sound reasonable? I could try to make a patch if there's interest. John _______________________________________________ netsurf-dev mailing list -- netsurf-dev@netsurf-browser.org To unsubscribe send an email to netsurf-dev-le...@netsurf-browser.org
