Android-encoded H.264 decoded cleanly with ffmpeg but showed diagonal
green/magenta banding on macOS. Root cause: shiguredo_video_toolbox's
I420Frame exposes y/u/v planes as bytes_per_row * height, including
CoreVideo's stride padding. VideoToolboxDecoder concatenated those
slices verbatim, then downstream code indexed the buffer as tight I420,
producing per-row drift that wrapped one full row every 16 chroma rows
(32 luma rows) at 960x540.
Add i420_frame_to_tight() helper that copies each plane row-by-row at
width / chroma_width using the plane's actual stride. All three macOS
decoders (H.264, HEVC, AV1) now call it. On first decode each logs the
real plane dimensions and strides at target wzp_video::videotoolbox so
future stride bugs are diagnosable from logs.
Verified mathematically against the corrupted dump:
band period = u_stride / (u_stride - chroma_width)
= 512 / (512 - 480) = 16 chroma rows = 32 luma rows
which matches the measured spacing exactly. 640x360 was unaffected
because chroma_width 320 is already 64-aligned.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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