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bug#44120: 28.0.50; Animated GIFs sometimes leave "trails"
From: |
Eli Zaretskii |
Subject: |
bug#44120: 28.0.50; Animated GIFs sometimes leave "trails" |
Date: |
Thu, 22 Oct 2020 15:54:29 +0300 |
> Date: Wed, 21 Oct 2020 21:08:42 +0100
> From: Alan Third <alan@idiocy.org>
> Cc: 44120@debbugs.gnu.org
>
> >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.
FTR, the image renders correctly on MS-Windows when
w32-use-native-image-API is non-nil, so this indeed seems like a
problem with our GIF code.