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bug#44120: 28.0.50; Animated GIFs sometimes leave "trails"
From: |
Alan Third |
Subject: |
bug#44120: 28.0.50; Animated GIFs sometimes leave "trails" |
Date: |
Wed, 21 Oct 2020 21:08:42 +0100 |
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