Re: [FFmpeg-devel] [PATCH v7 3/4] avcodec/hevc: Add alpha layer support

[Zhao Zhili <quinkblack-at-foxmail.com@ffmpeg.org>]

> On Feb 11, 2025, at 23:37, Andreas Rheinhardt <andreas.rheinhardt@outlook.com> wrote:
> 
> Zhao Zhili:
>> From: Zhao Zhili <zhilizhao@tencent.com>
>> 
>> Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
>> ---
>> libavcodec/hevc/hevcdec.c | 73 ++++++++++++++++++++++++++++++++++++++-
>> libavcodec/hevc/hevcdec.h |  2 ++
>> libavcodec/hevc/refs.c    | 35 ++++++++++++++++++-
>> 3 files changed, 108 insertions(+), 2 deletions(-)
>> 
>> diff --git a/libavcodec/hevc/hevcdec.c b/libavcodec/hevc/hevcdec.c
>> index e9c045f7a1..f71edf213b 100644
>> --- a/libavcodec/hevc/hevcdec.c
>> +++ b/libavcodec/hevc/hevcdec.c
>> @@ -466,6 +466,25 @@ static int export_multilayer(HEVCContext *s, const HEVCVPS *vps)
>>     return 0;
>> }
>> 
>> +int ff_hevc_is_alpha_video(const HEVCContext *s)
>> +{
>> +    const HEVCVPS *vps = s->vps;
>> +    int ret = 0;
>> +
>> +    if (vps->nb_layers != 2 || !vps->layer_id_in_nuh[1])
>> +        return 0;
>> +
>> +    /* decode_vps_ext() guarantees that SCALABILITY_AUXILIARY with AuxId other
>> +     * than alpha cannot reach here.
>> +     */
>> +    ret = (s->vps->scalability_mask_flag & HEVC_SCALABILITY_AUXILIARY);
>> +
>> +    av_log(s->avctx, AV_LOG_DEBUG, "Multi layer video, %s alpha video\n",
>> +           ret ? "is" : "not");
>> +
>> +    return ret;
>> +}
>> +
>> static int setup_multilayer(HEVCContext *s, const HEVCVPS *vps)
>> {
>>     unsigned layers_active_output = 0, highest_layer;
>> @@ -473,6 +492,18 @@ static int setup_multilayer(HEVCContext *s, const HEVCVPS *vps)
>>     s->layers_active_output = 1;
>>     s->layers_active_decode = 1;
>> 
>> +    if (ff_hevc_is_alpha_video(s)) {
>> +        const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(s->avctx->pix_fmt);
>> +
>> +        if (!(desc->flags & AV_PIX_FMT_FLAG_ALPHA))
>> +            return 0;
>> +
>> +        s->layers_active_decode = (1 << vps->nb_layers) - 1;
>> +        s->layers_active_output = 1;
>> +
>> +        return 0;
>> +    }
>> +
>>     // nothing requested - decode base layer only
>>     if (!s->nb_view_ids)
>>         return 0;
>> @@ -530,6 +561,34 @@ static int setup_multilayer(HEVCContext *s, const HEVCVPS *vps)
>>     return 0;
>> }
>> 
>> +static enum AVPixelFormat map_to_alpha_format(HEVCContext *s,
>> +                                              enum AVPixelFormat pix_fmt)
>> +{
>> +    switch (pix_fmt) {
>> +    case AV_PIX_FMT_YUV420P:
>> +    case AV_PIX_FMT_YUVJ420P:
>> +        return AV_PIX_FMT_YUVA420P;
>> +    case AV_PIX_FMT_YUV420P10:
>> +        return AV_PIX_FMT_YUVA420P10;
>> +    case AV_PIX_FMT_YUV444P:
>> +        return AV_PIX_FMT_YUVA444P;
>> +    case AV_PIX_FMT_YUV422P:
>> +        return AV_PIX_FMT_YUVA422P;
>> +    case AV_PIX_FMT_YUV422P10LE:
>> +        return AV_PIX_FMT_YUVA422P10LE;
>> +    case AV_PIX_FMT_YUV444P10:
>> +        return AV_PIX_FMT_YUVA444P10;
>> +    case AV_PIX_FMT_YUV444P12:
>> +        return AV_PIX_FMT_YUVA444P12;
>> +    case AV_PIX_FMT_YUV422P12:
>> +        return AV_PIX_FMT_YUVA422P12;
>> +    default:
>> +        av_log(s->avctx, AV_LOG_WARNING, "No alpha pixel format map for %s\n",
>> +               av_get_pix_fmt_name(pix_fmt));
>> +        return AV_PIX_FMT_NONE;
>> +    }
>> +}
>> +
>> static enum AVPixelFormat get_format(HEVCContext *s, const HEVCSPS *sps)
>> {
>> #define HWACCEL_MAX (CONFIG_HEVC_DXVA2_HWACCEL + \
>> @@ -540,9 +599,13 @@ static enum AVPixelFormat get_format(HEVCContext *s, const HEVCSPS *sps)
>>                      CONFIG_HEVC_VIDEOTOOLBOX_HWACCEL + \
>>                      CONFIG_HEVC_VDPAU_HWACCEL + \
>>                      CONFIG_HEVC_VULKAN_HWACCEL)
>> -    enum AVPixelFormat pix_fmts[HWACCEL_MAX + 2], *fmt = pix_fmts;
>> +    enum AVPixelFormat pix_fmts[HWACCEL_MAX + 3], *fmt = pix_fmts;
>> +    enum AVPixelFormat alpha_fmt = AV_PIX_FMT_NONE;
>>     int ret;
>> 
>> +    if (ff_hevc_is_alpha_video(s))
>> +        alpha_fmt = map_to_alpha_format(s, sps->pix_fmt);
>> +
>>     switch (sps->pix_fmt) {
>>     case AV_PIX_FMT_YUV420P:
>>     case AV_PIX_FMT_YUVJ420P:
>> @@ -664,6 +727,8 @@ static enum AVPixelFormat get_format(HEVCContext *s, const HEVCSPS *sps)
>>         break;
>>     }
>> 
>> +    if (alpha_fmt != AV_PIX_FMT_NONE)
>> +        *fmt++ = alpha_fmt;
>>     *fmt++ = sps->pix_fmt;
>>     *fmt = AV_PIX_FMT_NONE;
>> 
>> @@ -3194,6 +3259,12 @@ static int hevc_frame_start(HEVCContext *s, HEVCLayerContext *l,
>>                 !sps->vui.common.video_signal_type_present_flag)
>>                 pix_fmt = sps_base->pix_fmt;
>> 
>> +            // Ignore range mismatch between base layer and alpha layer
>> +            if (ff_hevc_is_alpha_video(s) &&
>> +                sps_base->pix_fmt == AV_PIX_FMT_YUV420P &&
>> +                pix_fmt == AV_PIX_FMT_YUVJ420P)
>> +                pix_fmt = sps_base->pix_fmt;
>> +
>>             if (pix_fmt     != sps_base->pix_fmt ||
>>                 sps->width  != sps_base->width   ||
>>                 sps->height != sps_base->height) {
>> diff --git a/libavcodec/hevc/hevcdec.h b/libavcodec/hevc/hevcdec.h
>> index 4e95035688..b2b725b5cd 100644
>> --- a/libavcodec/hevc/hevcdec.h
>> +++ b/libavcodec/hevc/hevcdec.h
>> @@ -714,6 +714,8 @@ void ff_hevc_hls_residual_coding(HEVCLocalContext *lc, const HEVCPPS *pps,
>> 
>> void ff_hevc_hls_mvd_coding(HEVCLocalContext *lc, int x0, int y0, int log2_cb_size);
>> 
>> +int ff_hevc_is_alpha_video(const HEVCContext *s);
>> +
>> extern const uint8_t ff_hevc_qpel_extra_before[4];
>> extern const uint8_t ff_hevc_qpel_extra_after[4];
>> extern const uint8_t ff_hevc_qpel_extra[4];
>> diff --git a/libavcodec/hevc/refs.c b/libavcodec/hevc/refs.c
>> index dd7f7f95a8..6f10efd0ac 100644
>> --- a/libavcodec/hevc/refs.c
>> +++ b/libavcodec/hevc/refs.c
>> @@ -79,6 +79,31 @@ void ff_hevc_flush_dpb(HEVCContext *s)
>>     }
>> }
>> 
>> +static int replace_alpha_plane(AVFrame *alpha, AVFrame *base)
>> +{
>> +    AVBufferRef *base_a = av_frame_get_plane_buffer(base, 3);
>> +    uintptr_t data = (uintptr_t)alpha->data[0];
>> +    int ret;
>> +
>> +    for (int i = 0; i < FF_ARRAY_ELEMS(alpha->buf) && alpha->buf[i]; i++) {
>> +        AVBufferRef *buf = alpha->buf[i];
>> +        uintptr_t buf_begin = (uintptr_t)buf->data;
>> +
>> +        if (data >=  buf_begin && data < buf_begin + buf->size) {
>> +            ret = av_buffer_replace(&alpha->buf[i], base_a);
>> +            if (ret < 0)
>> +                return ret;
>> +
>> +            alpha->linesize[0] = base->linesize[3];
>> +            alpha->data[0] = base->data[3];
>> +
>> +            return 0;
>> +        }
>> +    }
> 
> Why does the decoding process actually need multiple references to the
> buffer of the alpha plane?

I’m not sure if I understand your question correctly.

1. Both the base layer and the alpha layer are decoded as yuv420p.
2. The y plane of alpha layer is actually alpha.
3. The decoder finally output yuva, not two yuv420p.

So here make y plane of alpha frame reference the alpha plane of base frame.
When output, the base frame will contain YUV and alpha from two layers.

> 
>> +
>> +    return AVERROR_BUG;
>> +}
>> +
>> static HEVCFrame *alloc_frame(HEVCContext *s, HEVCLayerContext *l)
>> {
>>     const HEVCVPS *vps = l->sps->vps;
>> @@ -103,7 +128,7 @@ static HEVCFrame *alloc_frame(HEVCContext *s, HEVCLayerContext *l)
>>         }
>> 
>>         // add view ID side data if it's nontrivial
>> -        if (vps->nb_layers > 1 || view_id) {
>> +        if (!ff_hevc_is_alpha_video(s) && (vps->nb_layers > 1 || view_id)) {
>>             HEVCSEITDRDI *tdrdi = &s->sei.tdrdi;
>>             AVFrameSideData *sd = av_frame_side_data_new(&frame->f->side_data,
>>                                                          &frame->f->nb_side_data,
>> @@ -161,7 +186,15 @@ static HEVCFrame *alloc_frame(HEVCContext *s, HEVCLayerContext *l)
>>         if (ret < 0)
>>             goto fail;
>> 
>> +
>>         frame->pps = av_refstruct_ref_c(s->pps);
>> +        if (l != &s->layers[0] && ff_hevc_is_alpha_video(s)) {
>> +            AVFrame *alpha = frame->f;
>> +            AVFrame *base = s->layers[0].cur_frame->f;
>> +            ret = replace_alpha_plane(alpha, base);
>> +            if (ret < 0)
>> +                goto fail;
>> +        }
>> 
>>         return frame;
>> fail:
> 
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