Investigation requested via /user 'mpeg2 next' after iter33 closed
MPEG-2 at libva==kdirect with libva!=SW gap. iter35 verifies:
1. libva==kdirect holds across:
- BBB 720p MPEG-2 240F (sha a75015b10fe205ea)
- Synthetic testsrc 720p MPEG-2 48F (sha 5557dae9fa99d01b)
Rules out fixture coincidence.
2. v4l2 ctrl structs (SEQUENCE + PICTURE + QUANTISATION) semantically
correct per V4L2 spec via iter35 env-gated DIAG dump. Intra Q
matrix in zigzag scan order; flags = PROGRESSIVE+FRAME_PRED_DCT.
3. HW vs SW gap quantified:
- mean byte-diff = 0.015, max = 3
- SSIM = 0.999923 (visually identical)
Within IEEE 1180 / ISO 13818-2 Annex A MPEG-2 IDCT precision
tolerance. Not fixable at V4L2/libva layer.
ffmpeg's own SW IDCT options (-idct auto/int/simple/...) produce 4
distinct hashes for the same bitstream — libavcodec internally has
the same precision divergence between impls. Hantro HW IDCT is yet
another impl, not matching any SW.
Backend tip 48fd028 (added env-gated LIBVA_MPEG2_DUMP_FRAME).
Memory: feedback_mpeg2_hw_sw_idct_precision.md added.
