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feat: Add support for GIF frame disposal method 3 'Restore Previous' #244

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skidder merged 3 commits into from
Apr 29, 2025
Merged

feat: Add support for GIF frame disposal method 3 'Restore Previous' #244

skidder merged 3 commits into from
Apr 29, 2025

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@skidder skidder commented Apr 12, 2025

This PR implements support for the "Restore to Previous" disposal method in GIF decoding, addressing issue #234. The implementation stores the previous frame's content in a buffer to enable proper restoration when DISPOSE_PREVIOUS is encountered, and changes the default disposal method for unspecified frames to DISPOSE_NONE per the GIF89a specification (section 23):

            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.

While this increases memory usage by width * height * 4 bytes, it's a reasonable tradeoff for correct behavior and matches modern browser implementations. Tested with the sample GIF from the issue using GIF Inspector.

skidder added 3 commits April 12, 2025 07:21
@skidder
feat: Add support for GIF frame disposal method 3 'Restore Previous'
f7f1ae5
@skidder
Add tests for GIF disposal method 3 'Restore Previous'
12d61cd
@skidder
refactor: Improve memory safety and readability of GIF disposal code
893872f 
- Replace raw pointer with std::vector for safer memory management
- Add BYTES_PER_PIXEL constant to replace magic number 4
- Improve documentation of edge case handling with detailed comments
- Explicitly document handling of frames with negative offsets and bounds clipping
@skidder skidder marked this pull request as ready for review April 21, 2025 16:04
@skidder skidder merged commit 2cb03a6 into master Apr 29, 2025
5 checks passed
@skidder skidder deleted the skidder/additional-gif-disposal-methods branch April 29, 2025 16:32
@skidder skidder mentioned this pull request May 13, 2025
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@remiolivier-discord remiolivier-discord remiolivier-discord approved these changes

@salarkhan salarkhan Awaiting requested review from salarkhan

@nulltrope nulltrope Awaiting requested review from nulltrope

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