bug#44120: 28.0.50; Animated GIFs sometimes leave "trails"

[Alan Third <alan@idiocy.org>]

On Wed, Oct 21, 2020 at 09:17:28PM +0200, Lars Ingebrigtsen wrote:
> 
> M-x eww RET https://lh4.googleusercontent.com/TQ10szluPdXsKoYIeYe5ljxjVIoJzcCvLybUa3tEA24a6vISYkwiqAz9VymzgyNY_N8tfqHKvxSv9WhrcC-GvDc4uaiCE1T52y3C6xK1K--Lazicm9PSBiGxGVCyjFtDTBJaEOuExA
> 
> will give you an animated GIF that displays the problem: It seems like
> when repainting, the previous area that has changed isn't reset...  or
> something.
> 
> We're probably not following the GIF animation standard when applying
> the deltas?  

Well, I think this is the problem:

From image.c:

      /* From gif89a spec: 1 = "keep in place", 2 = "restore
	 to background".  Treat any other value like 2.  */

From the gif89a spec:

iv) Disposal Method - Indicates the way in which the graphic is to
            be treated after being displayed.

            Values :    0 -   No disposal specified. The decoder is
                              not required to take any action.
                        1 -   Do not dispose. The graphic is to be left
                              in place.
                        2 -   Restore to background color. The area used by the
                              graphic must be restored to the background color.
                        3 -   Restore to previous. The decoder is required to
                              restore the area overwritten by the graphic with
                              what was there prior to rendering the graphic.
                     4-7 -    To be defined.

That gif uses a disposal value of 3 quite a lot.

It looks like when a block is updated with a disposal value of 3 we
should hang onto the previous contents and then restore them in, I
guess, the next frame.
-- 
Alan Third