Re: possible bug: libnsgif does not handle successive RESTORE frames correctly

Hi John,

Thanks for investigating and for the patch. It looks good!
I've applied it and its on master now.

Thanks again,

Michael


On 07/03/2021 14:39, jcupitt@gmail.com wrote:
> Here's a possible patch:
>
> https://github.com/libvips/libvips/commit/bf522eeffc5aa84a275b1d53e24d0daace59fa3e
>
>
> On Sun, 7 Mar 2021 at 14:31, <jcupitt@gmail.com> wrote:
>>
>> Hello again everyone,
>>
>> I think I have a GIF which libnsgif does not handle correctly:
>>
>> https://github.com/libvips/libvips/blob/master/test/test-suite/images/dispose-previous.gif
>>
>> This GIF has three frames which draw three concentric red circles. The
>> GIF does this:
>>
>> - clear frame
>> frame 0:
>> - disposal_method COMBINE
>> - draw outermost red circle
>> frame 1:
>> - disposal_method RESTORE
>> - draw middle red circle
>> frame 2:
>> - disposal_method RESTORE
>> - draw inner red circle
>>
>> So the viewer should see this:
>>
>> https://github.com/libvips/libvips/blob/master/test/test-suite/images/dispose-previous.png
>>
>> frame 0:
>> - outer circle
>> frame 1:
>> - outer + middle
>> frame 2:
>> - outer + inner
>>
>> However, if you view with libnsgif, you'll see frame 2 has all three
>> circles, ie. the middle frame is not removed.
>>
>> Looking at the code, the problem seems to be that
>> gif_internal_decode_frame() calls gif__record_previous_frame() before
>> it calls gif__recover_previous_frame(). This means that if you have
>> two frames with RESTORE dispose in a row, the second restore will be
>> missed, since it will save the frame (overwriting the restore) before
>> it tries to restore it.
>>
>> The simplest fix is probably to simply move the record handler to just
>> after the recover. I think this makes intuitive sense -- you want to
>> finish disposing of the previous frame before you record the current
>> state.
>>
>> John
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--
Michael Drake https://www.codethink.co.uk/
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