[FFmpeg-devel] [PATCH] avformat/movenc: Support alpha channel for AVIF

Wed Jun 1 22:47:01 EEST 2022

On Wed, Jun 1, 2022 at 11:52 AM xyesbee <xyesbee at protonmail.com> wrote:
>
> Why can it not accept it as one but two distinct streams? Why the extra hurdle of alphaextract? YUVA pixel format exists though.

Yes, while the YUVA pixel formats exist, AVIF specification requires
alpha channel to be a separate track (i.e.) we would need two AV1
encoders to make an AVIF image with alpha channel (one for the YUV
planes and one for the alpha plane encoded as a monochrome frame). The
underlying AV1 codec does not support the YUVA pixel format. So, in
order to be able to encode an AVIF image with alpha channel, we would
have to separate the alpha channel with the alphaextract filter and
pass it to a separate AV1 encoder.

If you already have the alpha channel as a separate input, then you
can simply pass it along as a second input stream and there will be no
need to use the alphaextract filter. I pointed out the use of
alphaextract filter simply because it is the simplest way to re-encode
existing PNG images with alpha channel into AVIF.

An alternative solution is to use the YUVA pixel format and internally
use two AV1 encoders in the libaom wrapper. But that requires more
changes and does not really make things any better. If you have a
better solution, please let me know and I can update this.

> -------- Original Message --------
> On Jun 1, 2022, 11:35 PM, Vignesh Venkatasubramanian wrote:
>
> > AVIF specification allows for alpha channel as an auxiliary item (in
> > case of still images) or as an auxiliary track (in case of animated
> > images). Add support for both of these. The AVIF muxer will take
> > exactly two streams (when alpha is present) as input (first one being
> > the YUV planes and the second one being the alpha plane).
> >
> > The input has to come from two different images (one of it color and
> > the other one being alpha), or it can come from a single file
> > source with the alpha channel extracted using the "alphaextract"
> > filter.
> >
> > Example using alphaextract:
> > ffmpeg -i rgba.png -filter_complex "[0:v]alphaextract[a]" -map 0 -map "[a]" -still-picture 1 avif_with_alpha.avif
> >
> > Example using two sources (first source can be in any pixel format and
> > the second source has to be in monochrome grey pixel format):
> > ffmpeg -i color.avif -i grey.avif -map 0 -map 1 -c copy avif_with_alpha.avif
> >
> > The generated files pass the compliance checks in Compliance Warden:
> > https://github.com/gpac/ComplianceWarden
> >
> > libavif (the reference avif library) is able to decode the files
> > generated using this patch.
> >
> > They also play back properly (with transparent background) in:
> > 1) Chrome
> > 2) Firefox (only still AVIF, no animation support)
> >
> > Signed-off-by: Vignesh Venkatasubramanian <vigneshv at google.com>
> > ---
> > libavformat/movenc.c | 188 +++++++++++++++++++++++++++++--------------
> > libavformat/movenc.h | 4 +-
> > 2 files changed, 130 insertions(+), 62 deletions(-)
> >
> > diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> > index de971f94e8..02eb4d0e9a 100644
> > --- a/libavformat/movenc.c
> > +++ b/libavformat/movenc.c
> > @@ -2852,7 +2852,7 @@ static int mov_write_hdlr_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *tra
> > hdlr = (track->mode == MODE_MOV) ? "mhlr" : "\0\0\0\0";
> > if (track->par->codec_type == AVMEDIA_TYPE_VIDEO) {
> > if (track->mode == MODE_AVIF) {
> > - hdlr_type = "pict";
> > + hdlr_type = (track == &mov->tracks[0]) ? "pict" : "auxv";
> > descr = "PictureHandler";
> > } else {
> > hdlr_type = "vide";
> > @@ -2940,57 +2940,83 @@ static int mov_write_iloc_tag(AVIOContext *pb, MOVMuxContext *mov, AVFormatConte
> > avio_wb32(pb, 0); /* Version & flags */
> > avio_w8(pb, (4 << 4) + 4); /* offset_size(4) and length_size(4) */
> > avio_w8(pb, 0); /* base_offset_size(4) and reserved(4) */
> > - avio_wb16(pb, 1); /* item_count */
> > + avio_wb16(pb, s->nb_streams); /* item_count */
> >
> > - avio_wb16(pb, 1); /* item_id */
> > - avio_wb16(pb, 0); /* data_reference_index */
> > - avio_wb16(pb, 1); /* extent_count */
> > - mov->avif_extent_pos = avio_tell(pb);
> > - avio_wb32(pb, 0); /* extent_offset (written later) */
> > - // For animated AVIF, we simply write the first packet's size.
> > - avio_wb32(pb, mov->avif_extent_length); /* extent_length */
> > + for (int i = 0; i < s->nb_streams; i++) {
> > + avio_wb16(pb, i + 1); /* item_id */
> > + avio_wb16(pb, 0); /* data_reference_index */
> > + avio_wb16(pb, 1); /* extent_count */
> > + mov->avif_extent_pos[i] = avio_tell(pb);
> > + avio_wb32(pb, 0); /* extent_offset (written later) */
> > + // For animated AVIF, we simply write the first packet's size.
> > + avio_wb32(pb, mov->avif_extent_length[i]); /* extent_length */
> > + }
> >
> > return update_size(pb, pos);
> > }
> >
> > static int mov_write_iinf_tag(AVIOContext *pb, MOVMuxContext *mov, AVFormatContext *s)
> > {
> > - int64_t infe_pos;
> > int64_t iinf_pos = avio_tell(pb);
> > avio_wb32(pb, 0); /* size */
> > ffio_wfourcc(pb, "iinf");
> > avio_wb32(pb, 0); /* Version & flags */
> > - avio_wb16(pb, 1); /* entry_count */
> > + avio_wb16(pb, s->nb_streams); /* entry_count */
> >
> > - infe_pos = avio_tell(pb);
> > - avio_wb32(pb, 0); /* size */
> > - ffio_wfourcc(pb, "infe");
> > - avio_w8(pb, 0x2); /* Version */
> > - avio_wb24(pb, 0); /* flags */
> > - avio_wb16(pb, 1); /* item_id */
> > - avio_wb16(pb, 0); /* item_protection_index */
> > - avio_write(pb, "av01", 4); /* item_type */
> > - avio_write(pb, "Color\0", 6); /* item_name */
> > - update_size(pb, infe_pos);
> > + for (int i = 0; i < s->nb_streams; i++) {
> > + int64_t infe_pos = avio_tell(pb);
> > + avio_wb32(pb, 0); /* size */
> > + ffio_wfourcc(pb, "infe");
> > + avio_w8(pb, 0x2); /* Version */
> > + avio_wb24(pb, 0); /* flags */
> > + avio_wb16(pb, i + 1); /* item_id */
> > + avio_wb16(pb, 0); /* item_protection_index */
> > + avio_write(pb, "av01", 4); /* item_type */
> > + avio_write(pb, !i ? "Color\0" : "Alpha\0", 6); /* item_name */
> > + update_size(pb, infe_pos);
> > + }
> >
> > return update_size(pb, iinf_pos);
> > }
> >
> > -static int mov_write_ispe_tag(AVIOContext *pb, MOVMuxContext *mov, AVFormatContext *s)
> > +
> > +static int mov_write_iref_tag(AVIOContext *pb, MOVMuxContext *mov, AVFormatContext *s)
> > +{
> > + int64_t auxl_pos;
> > + int64_t iref_pos = avio_tell(pb);
> > + avio_wb32(pb, 0); /* size */
> > + ffio_wfourcc(pb, "iref");
> > + avio_wb32(pb, 0); /* Version & flags */
> > +
> > + auxl_pos = avio_tell(pb);
> > + avio_wb32(pb, 0); /* size */
> > + ffio_wfourcc(pb, "auxl");
> > + avio_wb16(pb, 2); /* from_item_ID */
> > + avio_wb16(pb, 1); /* reference_count */
> > + avio_wb16(pb, 1); /* to_item_ID */
> > + update_size(pb, auxl_pos);
> > +
> > + return update_size(pb, iref_pos);
> > +}
> > +
> > +static int mov_write_ispe_tag(AVIOContext *pb, MOVMuxContext *mov, AVFormatContext *s,
> > + int stream_index)
> > {
> > int64_t pos = avio_tell(pb);
> > avio_wb32(pb, 0); /* size */
> > ffio_wfourcc(pb, "ispe");
> > avio_wb32(pb, 0); /* Version & flags */
> > - avio_wb32(pb, s->streams[0]->codecpar->width); /* image_width */
> > - avio_wb32(pb, s->streams[0]->codecpar->height); /* image_height */
> > + avio_wb32(pb, s->streams[stream_index]->codecpar->width); /* image_width */
> > + avio_wb32(pb, s->streams[stream_index]->codecpar->height); /* image_height */
> > return update_size(pb, pos);
> > }
> >
> > -static int mov_write_pixi_tag(AVIOContext *pb, MOVMuxContext *mov, AVFormatContext *s)
> > +static int mov_write_pixi_tag(AVIOContext *pb, MOVMuxContext *mov, AVFormatContext *s,
> > + int stream_index)
> > {
> > int64_t pos = avio_tell(pb);
> > - const AVPixFmtDescriptor *pixdesc = av_pix_fmt_desc_get(s->streams[0]->codecpar->format);
> > + const AVPixFmtDescriptor *pixdesc =
> > + av_pix_fmt_desc_get(s->streams[stream_index]->codecpar->format);
> > avio_wb32(pb, 0); /* size */
> > ffio_wfourcc(pb, "pixi");
> > avio_wb32(pb, 0); /* Version & flags */
> > @@ -3001,15 +3027,30 @@ static int mov_write_pixi_tag(AVIOContext *pb, MOVMuxContext *mov, AVFormatConte
> > return update_size(pb, pos);
> > }
> >
> > +static int mov_write_auxC_tag(AVIOContext *pb)
> > +{
> > + int64_t pos = avio_tell(pb);
> > + avio_wb32(pb, 0); /* size */
> > + ffio_wfourcc(pb, "auxC");
> > + avio_wb32(pb, 0); /* Version & flags */
> > + avio_write(pb, "urn:mpeg:mpegB:cicp:systems:auxiliary:alpha\0", 44);
> > + return update_size(pb, pos);
> > +}
> > +
> > static int mov_write_ipco_tag(AVIOContext *pb, MOVMuxContext *mov, AVFormatContext *s)
> > {
> > int64_t pos = avio_tell(pb);
> > avio_wb32(pb, 0); /* size */
> > ffio_wfourcc(pb, "ipco");
> > - mov_write_ispe_tag(pb, mov, s);
> > - mov_write_pixi_tag(pb, mov, s);
> > - mov_write_av1c_tag(pb, &mov->tracks[0]);
> > - mov_write_colr_tag(pb, &mov->tracks[0], 0);
> > + for (int i = 0; i < s->nb_streams; i++) {
> > + mov_write_ispe_tag(pb, mov, s, i);
> > + mov_write_pixi_tag(pb, mov, s, i);
> > + mov_write_av1c_tag(pb, &mov->tracks[i]);
> > + if (!i)
> > + mov_write_colr_tag(pb, &mov->tracks[0], 0);
> > + else
> > + mov_write_auxC_tag(pb);
> > + }
> > return update_size(pb, pos);
> > }
> >
> > @@ -3019,18 +3060,21 @@ static int mov_write_ipma_tag(AVIOContext *pb, MOVMuxContext *mov, AVFormatConte
> > avio_wb32(pb, 0); /* size */
> > ffio_wfourcc(pb, "ipma");
> > avio_wb32(pb, 0); /* Version & flags */
> > - avio_wb32(pb, 1); /* entry_count */
> > - avio_wb16(pb, 1); /* item_ID */
> > - avio_w8(pb, 4); /* association_count */
> > -
> > - // ispe association.
> > - avio_w8(pb, 1); /* essential and property_index */
> > - // pixi association.
> > - avio_w8(pb, 2); /* essential and property_index */
> > - // av1C association.
> > - avio_w8(pb, 0x80 | 3); /* essential and property_index */
> > - // colr association.
> > - avio_w8(pb, 4); /* essential and property_index */
> > + avio_wb32(pb, s->nb_streams); /* entry_count */
> > +
> > + for (int i = 0, index = 1; i < s->nb_streams; i++) {
> > + avio_wb16(pb, i + 1); /* item_ID */
> > + avio_w8(pb, 4); /* association_count */
> > +
> > + // ispe association.
> > + avio_w8(pb, index++); /* essential and property_index */
> > + // pixi association.
> > + avio_w8(pb, index++); /* essential and property_index */
> > + // av1C association.
> > + avio_w8(pb, 0x80 | index++); /* essential and property_index */
> > + // colr/auxC association.
> > + avio_w8(pb, index++); /* essential and property_index */
> > + }
> > return update_size(pb, pos);
> > }
> >
> > @@ -4112,6 +4156,8 @@ static int mov_write_meta_tag(AVIOContext *pb, MOVMuxContext *mov,
> > mov_write_pitm_tag(pb, 1);
> > mov_write_iloc_tag(pb, mov, s);
> > mov_write_iinf_tag(pb, mov, s);
> > + if (s->nb_streams > 1)
> > + mov_write_iref_tag(pb, mov, s);
> > mov_write_iprp_tag(pb, mov, s);
> > } else {
> > /* iTunes metadata tag */
> > @@ -6040,8 +6086,8 @@ int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
> > avio_write(pb, reformatted_data, size);
> > } else {
> > size = ff_av1_filter_obus(pb, pkt->data, pkt->size);
> > - if (trk->mode == MODE_AVIF && !mov->avif_extent_length) {
> > - mov->avif_extent_length = size;
> > + if (trk->mode == MODE_AVIF && !mov->avif_extent_length[pkt->stream_index]) {
> > + mov->avif_extent_length[pkt->stream_index] = size;
> > }
> > }
> >
> > @@ -6872,16 +6918,26 @@ static int mov_init(AVFormatContext *s)
> > return AVERROR(EINVAL);
> > }
> >
> > - /* AVIF output must have exactly one video stream */
> > + /* AVIF output must have at most two video streams (one for YUV and one for
> > + * alpha). */
> > if (mov->mode == MODE_AVIF) {
> > - if (s->nb_streams > 1) {
> > - av_log(s, AV_LOG_ERROR, "AVIF output requires exactly one stream\n");
> > + if (s->nb_streams > 2) {
> > + av_log(s, AV_LOG_ERROR, "AVIF output requires exactly one or two streams\n");
> > return AVERROR(EINVAL);
> > }
> > - if (s->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_VIDEO) {
> > - av_log(s, AV_LOG_ERROR, "AVIF output requires one video stream\n");
> > + if (s->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_VIDEO &&
> > + (s->nb_streams > 1 && s->streams[1]->codecpar->codec_type != AVMEDIA_TYPE_VIDEO)) {
> > + av_log(s, AV_LOG_ERROR, "AVIF output supports only video streams\n");
> > return AVERROR(EINVAL);
> > }
> > + if (s->nb_streams > 1) {
> > + const AVPixFmtDescriptor *pixdesc =
> > + av_pix_fmt_desc_get(s->streams[1]->codecpar->format);
> > + if (pixdesc->nb_components != 1) {
> > + av_log(s, AV_LOG_ERROR, "Second stream for AVIF (alpha) output must have exactly one plane\n");
> > + return AVERROR(EINVAL);
> > + }
> > + }
> > s->streams[0]->disposition |= AV_DISPOSITION_DEFAULT;
> > }
> >
> > @@ -7543,18 +7599,25 @@ static int avif_write_trailer(AVFormatContext *s)
> > {
> > AVIOContext *pb = s->pb;
> > MOVMuxContext *mov = s->priv_data;
> > - int64_t pos_backup, mdat_pos;
> > + int64_t pos_backup, extent_offsets[2];
> > uint8_t *buf;
> > - int buf_size, moov_size;
> > + int buf_size, moov_size, i;
> >
> > if (mov->moov_written) return 0;
> >
> > mov->is_animated_avif = s->streams[0]->nb_frames > 1;
> > + if (mov->is_animated_avif && s->nb_streams > 1) {
> > + // For animated avif with alpha channel, we need to write a tref tag
> > + // with type "auxl".
> > + mov->tracks[1].tref_tag = MKTAG('a', 'u', 'x', 'l');
> > + mov->tracks[1].tref_id = 1;
> > + }
> > mov_write_identification(pb, s);
> > mov_write_meta_tag(pb, mov, s);
> >
> > moov_size = get_moov_size(s);
> > - mov->tracks[0].data_offset = avio_tell(pb) + moov_size + 8;
> > + for (i = 0; i < s->nb_streams; i++)
> > + mov->tracks[i].data_offset = avio_tell(pb) + moov_size + 8;
> >
> > if (mov->is_animated_avif) {
> > int ret;
> > @@ -7565,19 +7628,24 @@ static int avif_write_trailer(AVFormatContext *s)
> > buf_size = avio_get_dyn_buf(mov->mdat_buf, &buf);
> > avio_wb32(pb, buf_size + 8);
> > ffio_wfourcc(pb, "mdat");
> > - mdat_pos = avio_tell(pb);
> >
> > - if (mdat_pos != (uint32_t)mdat_pos) {
> > - av_log(s, AV_LOG_ERROR, "mdat offset does not fit in 32 bits\n");
> > - return AVERROR_INVALIDDATA;
> > - }
> > + // The offset for the YUV planes is the starting position of mdat.
> > + extent_offsets[0] = avio_tell(pb);
> > + // The offset for alpha plane is YUV offset + YUV size.
> > + extent_offsets[1] = extent_offsets[0] + mov->avif_extent_length[0];
> >
> > avio_write(pb, buf, buf_size);
> >
> > - // write extent offset.
> > + // write extent offsets.
> > pos_backup = avio_tell(pb);
> > - avio_seek(pb, mov->avif_extent_pos, SEEK_SET);
> > - avio_wb32(pb, mdat_pos); /* rewrite offset */
> > + for (i = 0; i < s->nb_streams; i++) {
> > + if (extent_offsets[i] != (uint32_t)extent_offsets[i]) {
> > + av_log(s, AV_LOG_ERROR, "extent offset does not fit in 32 bits\n");
> > + return AVERROR_INVALIDDATA;
> > + }
> > + avio_seek(pb, mov->avif_extent_pos[i], SEEK_SET);
> > + avio_wb32(pb, extent_offsets[i]); /* rewrite offset */
> > + }
> > avio_seek(pb, pos_backup, SEEK_SET);
> >
> > return 0;
> > diff --git a/libavformat/movenc.h b/libavformat/movenc.h
> > index 281576cc66..e4550f7900 100644
> > --- a/libavformat/movenc.h
> > +++ b/libavformat/movenc.h
> > @@ -246,8 +246,8 @@ typedef struct MOVMuxContext {
> > int empty_hdlr_name;
> > int movie_timescale;
> >
> > - int64_t avif_extent_pos;
> > - int avif_extent_length;
> > + int64_t avif_extent_pos[2]; // index 0 is YUV and 1 is Alpha.
> > + int avif_extent_length[2]; // index 0 is YUV and 1 is Alpha.
> > int is_animated_avif;
> > } MOVMuxContext;
> >
> > --
> > 2.36.1.255.ge46751e96f-goog
> >
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-- 
Vignesh