ffmpeg-v4l2-request-fourier: restore AV_CODEC_FLAG_LOW_DELAY in H.264 decoder
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).
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From 0d1292ea99bc4e5fa2da438259fa01a2374e3e04 Mon Sep 17 00:00:00 2001
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From: Markus Fritsche <mfritsche@reauktion.de>
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Date: Fri, 22 May 2026 14:18:25 +0200
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Subject: [PATCH] avcodec/h264: restore AV_CODEC_FLAG_LOW_DELAY semantics
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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FFmpeg 8.x dropped the H.264 decoder's low_delay path —
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AV_CODEC_FLAG_LOW_DELAY no longer prevents
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h264_select_output_frame from running the display-order DPB
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output queue. V4L2-stateless-style consumers (daedalus-v4l2
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daemon, libva-v4l2-request-fourier) that set the flag end up
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seeing the 2-1-4-3 pair-swap pattern on B-frame streams again.
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Restore the documented semantics:
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- Early-exit at the top of h264_select_output_frame when the
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flag is set: emit the just-decoded picture immediately as
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next_output_pic, mirror the corruption / recovery-point
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tracking the main path performs, and skip the entire
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delayed_pic[] / POC reorder machinery.
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- Suppress the SPS-driven has_b_frames clobber in
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h264_field_start when the flag is set, so the per-slice
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bitstream_restriction_flag re-pickup cannot reintroduce a
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nonzero reorder buffer mid-stream.
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This is a fork-only change required by the daedalus-v4l2 daemon's
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one-frame-per-send_packet contract; upstream FFmpeg consumers that
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expect display-order output remain untouched (flag default = off).
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Refs reauktion/daedalus-v4l2#11 — substitution arc step 2 deblock
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+ flag-restoration follow-up.
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---
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libavcodec/h264_slice.c | 23 +++++++++++++++++++++++
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1 file changed, 23 insertions(+)
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diff --git a/libavcodec/h264_slice.c b/libavcodec/h264_slice.c
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index 97fab70..a7bfbd6 100644
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--- a/libavcodec/h264_slice.c
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+++ b/libavcodec/h264_slice.c
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@@ -1308,6 +1308,28 @@ static int h264_select_output_frame(H264Context *h)
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cur->mmco_reset = h->mmco_reset;
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h->mmco_reset = 0;
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+ /* AV_CODEC_FLAG_LOW_DELAY restore (FFmpeg 8.x dropped the H.264
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+ * decoder's low_delay path). Bypass the display-order DPB
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+ * output queue: emit the just-decoded picture immediately, in
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+ * decode order, one per send_packet. V4L2-stateless-style
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+ * consumers (daedalus-v4l2 daemon, libva-v4l2-request-fourier)
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+ * do their own POC-based reorder downstream and require this
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+ * behaviour. */
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+ if (h->avctx->flags & AV_CODEC_FLAG_LOW_DELAY) {
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+ h->next_output_pic = cur;
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+ h->next_outputed_poc = cur->poc;
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+ h->frame_recovered |= cur->recovered;
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+ cur->recovered |= h->frame_recovered & FRAME_RECOVERED_SEI;
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+ if (!cur->recovered) {
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+ if (!(h->avctx->flags & AV_CODEC_FLAG_OUTPUT_CORRUPT) &&
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+ !(h->avctx->flags2 & AV_CODEC_FLAG2_SHOW_ALL))
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+ h->next_output_pic = NULL;
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+ else
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+ cur->f->flags |= AV_FRAME_FLAG_CORRUPT;
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+ }
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+ return 0;
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+ }
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+
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if (sps->bitstream_restriction_flag ||
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h->avctx->strict_std_compliance >= FF_COMPLIANCE_STRICT) {
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h->avctx->has_b_frames = FFMAX(h->avctx->has_b_frames, sps->num_reorder_frames);
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@@ -1415,6 +1437,7 @@ static int h264_field_start(H264Context *h, const H264SliceContext *sl,
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sps = h->ps.sps;
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if (sps->bitstream_restriction_flag &&
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+ !(h->avctx->flags & AV_CODEC_FLAG_LOW_DELAY) &&
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h->avctx->has_b_frames < sps->num_reorder_frames) {
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h->avctx->has_b_frames = sps->num_reorder_frames;
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}
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--
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2.47.3
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@@ -24,7 +24,7 @@ _srcname=FFmpeg
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_version='8.1'
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_commit='b57fbbe50c9b2656fad86a1a7eeabfd2b2a50935' # v4l2-request-n8.1 tip 2026-04-24
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pkgver=8.1.r123329.b57fbbe
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pkgrel=8 # pkgrel=8 — H.264 luma-v deblock daedalus-fourier substitution (cycle 8, 2026-05-22)
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pkgrel=9 # pkgrel=9 — restore AV_CODEC_FLAG_LOW_DELAY for H.264 (2026-05-22)
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epoch=2
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# daedalus-fourier pin — first kernel substitution in libavcodec
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@@ -92,8 +92,9 @@ source=("git+https://github.com/Kwiboo/FFmpeg.git#commit=${_commit}"
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'0002-nv15-to-p010-unpack.patch'
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'0003-h264-idct4-daedalus-fourier.patch'
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'0004-h264-idct8-daedalus-fourier.patch'
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'0005-h264-deblock-luma-v-daedalus-fourier.patch')
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sha256sums=('SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP')
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'0005-h264-deblock-luma-v-daedalus-fourier.patch'
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'0006-h264-restore-low-delay.patch')
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sha256sums=('SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP')
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pkgver() {
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cd "${_srcname}"
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@@ -109,6 +110,7 @@ prepare() {
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patch -Np1 -i "${srcdir}/0003-h264-idct4-daedalus-fourier.patch"
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patch -Np1 -i "${srcdir}/0004-h264-idct8-daedalus-fourier.patch"
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patch -Np1 -i "${srcdir}/0005-h264-deblock-luma-v-daedalus-fourier.patch"
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patch -Np1 -i "${srcdir}/0006-h264-restore-low-delay.patch"
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}
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build() {
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