Fix APNG push/sequential frame-count divergence (#891) - #893
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When a leading fcTL has an incorrect frame size it is ignored and PNG_FIRST_FRAME_HIDDEN is set. The progressive reader was still calling frame_end_fn after the default IDAT, completing 1 frame while the sequential reader completed 0. Skip the frame_end callback for the hidden default image to align both read paths.
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There are a whole lot of bugs reported here in the Stepin APNG reader. Perhaps someone would like to enter the arena as a maintainer? @ProgramMax |
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Fixes #891
Problem
When an APNG has a leading
fcTLwhose width/height does not matchIHDR, libpng warns and ignores it (PNG_FIRST_FRAME_HIDDEN). The two read APIs then disagree on completed frame count for the same bytes:png_read_frame_head): 0 frames completedpng_process_data): 1 frame completed (incorrectframe_endcallback)Root cause
In
png_push_read_IDAT(), after the default IDAT finishes (num_frames_read == 0), the progressive reader always callsframe_end_fn. The sequential reader does not treat the hidden default image as a completed animation frame.Fix
Skip
frame_end_fnwhenPNG_FIRST_FRAME_HIDDENis set at thenum_frames_read == 0completion path.num_frames_readis still incremented to match sequential behavior.Verification
POC compares
frame_endcallback counts between sequential and progressive readers on a malformed APNG (leadingfcTL8×1 vs IHDR 8×8):Before:
sequential_frames_completed=0 progressive_frames_completed=1 BUG_PRESENT=yes
After:
sequential_frames_completed=0 progressive_frames_completed=0 BUG_PRESENT=no