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

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

> On Feb 12, 2025, at 00:10, Andreas Rheinhardt <andreas.rheinhardt@outlook.com> wrote:
> 
> Zhao Zhili:
>> 
>> 
>>> 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.
>> 
> 
> The base layer AVFrame already contains a reference to the alpha plane;
> so why do we need another one? Does the base AVFrame get unreferenced
> before the alpha plane is finished decoding?

In the decoding process, it’s almost the same as MV-HEVC:
1. The base layer is decoded to the YUV of base frame, without touching the alpha plane.
2. The alpha layer is decoded to YUV of alpha frame.

We can output the two frames like MV-HEVC, but it’s user friendly to output in YUVA format.
So I’m letting the alpha layer decoded to the alpha plane of the base layer. Or I can do the
reverse. It’s the same.

We can skip the alpha frame totally. Then the decoding process and DPB buffer management
will be different. Current method is simple but use more memory. I can work on that lately.

> 
>>> 
>>>> +
>>>> +    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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