marfrit
5c69460722
FFmpeg 8.x dropped the H.264 decoder's low_delay code path — AV_CODEC_FLAG_LOW_DELAY no longer prevents h264_select_output_frame from running the display-order DPB output queue. The daedalus-v4l2 daemon's `ctx->flags |= AV_CODEC_FLAG_LOW_DELAY` at daemon/src/decoder.c:202 has been a silent no-op since the SONAME 61→62 jump landed in reauktion/daedalus-v4l2 PR #16; on Firefox YouTube this re-introduced the 2-1-4-3 B-frame pair-swap that PR #12's daemon flag was supposed to prevent. Fix lives in libavcodec, not the daemon: restore the documented LOW_DELAY semantics so the daemon (and any other V4L2-stateless- style consumer) keeps the one-frame-per-send_packet decode-order output contract it already declares. ## Patch 0006-h264-restore-low-delay.patch touches libavcodec/h264_slice.c: - h264_select_output_frame: early-exit when LOW_DELAY is set. Emit the just-decoded picture as next_output_pic, mirror the corruption / recovery-point tracking the main path performs, skip delayed_pic[] / POC reorder machinery entirely. - h264_field_start: suppress the SPS-driven `has_b_frames = sps->num_reorder_frames` clobber when LOW_DELAY is set. Without this the per-slice bitstream_restriction_flag re-pickup would reintroduce a nonzero reorder buffer mid-stream even after the daemon set has_b_frames=0 at avcodec_open2. ## Why not daemon-side A daemon SPS-rewrite (`num_reorder_frames=0`) was considered but rejected: it works only for the daemon's reconstructed SPS NAL, not for any in-band SPS the daemon dlopens libavformat to parse in other code paths. Restoring documented FFmpeg flag semantics is the smaller, more durable change and keeps the daemon interface stable. ## Packaging - PKGREL/pkgrel bump to 9. - No new build-deps, no Depends change. - Substitution arc cycles 6/7/8 unchanged. ## Refs - reauktion/daedalus-v4l2#11 / #12 (LOW_DELAY half-measure on daemon side, originally landed against FFmpeg 7.x). - daemon/src/decoder.c:202 (`ctx->flags |= AV_CODEC_FLAG_LOW_DELAY` for H.264 only — unchanged, but now actually has effect again).
83 lines
3.4 KiB
Diff
83 lines
3.4 KiB
Diff
From 0d1292ea99bc4e5fa2da438259fa01a2374e3e04 Mon Sep 17 00:00:00 2001
|
|
From: Markus Fritsche <mfritsche@reauktion.de>
|
|
Date: Fri, 22 May 2026 14:18:25 +0200
|
|
Subject: [PATCH] avcodec/h264: restore AV_CODEC_FLAG_LOW_DELAY semantics
|
|
MIME-Version: 1.0
|
|
Content-Type: text/plain; charset=UTF-8
|
|
Content-Transfer-Encoding: 8bit
|
|
|
|
FFmpeg 8.x dropped the H.264 decoder's low_delay path —
|
|
AV_CODEC_FLAG_LOW_DELAY no longer prevents
|
|
h264_select_output_frame from running the display-order DPB
|
|
output queue. V4L2-stateless-style consumers (daedalus-v4l2
|
|
daemon, libva-v4l2-request-fourier) that set the flag end up
|
|
seeing the 2-1-4-3 pair-swap pattern on B-frame streams again.
|
|
|
|
Restore the documented semantics:
|
|
|
|
- Early-exit at the top of h264_select_output_frame when the
|
|
flag is set: emit the just-decoded picture immediately as
|
|
next_output_pic, mirror the corruption / recovery-point
|
|
tracking the main path performs, and skip the entire
|
|
delayed_pic[] / POC reorder machinery.
|
|
|
|
- Suppress the SPS-driven has_b_frames clobber in
|
|
h264_field_start when the flag is set, so the per-slice
|
|
bitstream_restriction_flag re-pickup cannot reintroduce a
|
|
nonzero reorder buffer mid-stream.
|
|
|
|
This is a fork-only change required by the daedalus-v4l2 daemon's
|
|
one-frame-per-send_packet contract; upstream FFmpeg consumers that
|
|
expect display-order output remain untouched (flag default = off).
|
|
|
|
Refs reauktion/daedalus-v4l2#11 — substitution arc step 2 deblock
|
|
+ flag-restoration follow-up.
|
|
---
|
|
libavcodec/h264_slice.c | 23 +++++++++++++++++++++++
|
|
1 file changed, 23 insertions(+)
|
|
|
|
diff --git a/libavcodec/h264_slice.c b/libavcodec/h264_slice.c
|
|
index 97fab70..a7bfbd6 100644
|
|
--- a/libavcodec/h264_slice.c
|
|
+++ b/libavcodec/h264_slice.c
|
|
@@ -1308,6 +1308,28 @@ static int h264_select_output_frame(H264Context *h)
|
|
cur->mmco_reset = h->mmco_reset;
|
|
h->mmco_reset = 0;
|
|
|
|
+ /* AV_CODEC_FLAG_LOW_DELAY restore (FFmpeg 8.x dropped the H.264
|
|
+ * decoder's low_delay path). Bypass the display-order DPB
|
|
+ * output queue: emit the just-decoded picture immediately, in
|
|
+ * decode order, one per send_packet. V4L2-stateless-style
|
|
+ * consumers (daedalus-v4l2 daemon, libva-v4l2-request-fourier)
|
|
+ * do their own POC-based reorder downstream and require this
|
|
+ * behaviour. */
|
|
+ if (h->avctx->flags & AV_CODEC_FLAG_LOW_DELAY) {
|
|
+ h->next_output_pic = cur;
|
|
+ h->next_outputed_poc = cur->poc;
|
|
+ h->frame_recovered |= cur->recovered;
|
|
+ cur->recovered |= h->frame_recovered & FRAME_RECOVERED_SEI;
|
|
+ if (!cur->recovered) {
|
|
+ if (!(h->avctx->flags & AV_CODEC_FLAG_OUTPUT_CORRUPT) &&
|
|
+ !(h->avctx->flags2 & AV_CODEC_FLAG2_SHOW_ALL))
|
|
+ h->next_output_pic = NULL;
|
|
+ else
|
|
+ cur->f->flags |= AV_FRAME_FLAG_CORRUPT;
|
|
+ }
|
|
+ return 0;
|
|
+ }
|
|
+
|
|
if (sps->bitstream_restriction_flag ||
|
|
h->avctx->strict_std_compliance >= FF_COMPLIANCE_STRICT) {
|
|
h->avctx->has_b_frames = FFMAX(h->avctx->has_b_frames, sps->num_reorder_frames);
|
|
@@ -1415,6 +1437,7 @@ static int h264_field_start(H264Context *h, const H264SliceContext *sl,
|
|
sps = h->ps.sps;
|
|
|
|
if (sps->bitstream_restriction_flag &&
|
|
+ !(h->avctx->flags & AV_CODEC_FLAG_LOW_DELAY) &&
|
|
h->avctx->has_b_frames < sps->num_reorder_frames) {
|
|
h->avctx->has_b_frames = sps->num_reorder_frames;
|
|
}
|
|
--
|
|
2.47.3
|
|
|