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Use "-dispose background" to improve reassembly of GIFs with transparency
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Authored by epriestley on Mar 13 2018, 4:07 PM.
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F14890129: D19214.id46012.diff
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Summary

Fixes T5741. We break GIFs apart with "-coalesce" which completely rasterizes each frame, but stitch them back together without specifying "-dispose".

This produces the default "-dispose none" behavior, which causes GIF frames to "pile up" if they contain transparency.

Instead, use "-dispose background" so that the previous frame is erased before each new frame is drawn.

Test Plan

See T5741 for additional details.

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