Re: [FFmpeg-devel] [PATCH v6 3/3] avcodec/hevc: Add alpha layer support

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

> On Feb 11, 2025, at 20:12, Andreas Rheinhardt <andreas.rheinhardt@outlook.com> wrote:
> 
> Zhao Zhili:
>> 
>> 
>>> On Feb 11, 2025, at 19:23, 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    | 34 +++++++++++++++++-
>>>> 3 files changed, 107 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..da0bac8d9c 100644
>>>> --- a/libavcodec/hevc/refs.c
>>>> +++ b/libavcodec/hevc/refs.c
>>>> @@ -79,6 +79,30 @@ 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);
>>>> +    uint8_t *data = alpha->data[0];
>>>> +    int ret;
>>>> +
>>>> +    for (int i = 0; i < FF_ARRAY_ELEMS(alpha->buf) && alpha->buf[i]; i++) {
>>>> +        AVBufferRef *buf = alpha->buf[i];
>>>> +
>>>> +        if (data >= buf->data && data < buf->data + buf->size) {
>>> 
>>> Pointer comparisons (other than equality/inquality comparisons) are
>>> undefined behavior unless the two pointers point into the same array (or
>>> one past the last element of an array).
>> 
>> av_frame_get_plane_buffer does the same compare. Any idea to fix this UB?
> 
> ISO C does not have a portable way to achieve this (except iterating
> over buf->data[0..buf->size] and checking via ==).
> In practice (in a flat memory space) casting to uintptr_t should work.

What’s the difference with and without cast in practice?

> 
>> 
>>> 
>>>> +            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;
>>>> +        }
>>>> +    }
>>>> +
>>>> +    return AVERROR_BUG;
>>>> +}
>>>> +
>>>> static HEVCFrame *alloc_frame(HEVCContext *s, HEVCLayerContext *l)
>>>> {
>>>>    const HEVCVPS *vps = l->sps->vps;
>>>> @@ -103,7 +127,7 @@ 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 +185,15 @@ 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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