Re: [FFmpeg-devel] [PATCH] vaapi_encode_h264: Only set pic_order_cnt_type to 0 with B-frames

[David Rosca <nowrep@gmail.com>]

On Mon, Jan 9, 2023 at 10:11 PM Mark Thompson <sw@jkqxz.net> wrote:
>
> On 09/01/2023 07:37, David Rosca wrote:
> > On Mon, Jan 9, 2023 at 3:22 AM Xiang, Haihao <haihao.xiang@intel.com> wrote:
> >>
> >> On Do, 2022-12-29 at 22:20 +0100, David Rosca wrote:
> >>> ---
> >>>   libavcodec/vaapi_encode_h264.c | 2 +-
> >>>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/libavcodec/vaapi_encode_h264.c b/libavcodec/vaapi_encode_h264.c
> >>> index dd17be2..d6926c4 100644
> >>> --- a/libavcodec/vaapi_encode_h264.c
> >>> +++ b/libavcodec/vaapi_encode_h264.c
> >>> @@ -350,7 +350,7 @@ static int
> >>> vaapi_encode_h264_init_sequence_params(AVCodecContext *avctx)
> >>>       sps->chroma_format_idc    = 1;
> >>>
> >>>       sps->log2_max_frame_num_minus4 = 4;
> >>> -    sps->pic_order_cnt_type        = 0;
> >>> +    sps->pic_order_cnt_type        = ctx->max_b_depth ? 0 : 2;
> >>
> >>
> >> pic_order_cnt_type (0) should work for ctx->max_b_depth == 0. If 2 is preferred
> >> for your vaapi driver, it would be better to query the capability from the
> >> driver, like as what commit 9f02e033875185409c861846f209b04a3be339d2 did.
> >
> > It's not about an encoder, but rather about decoder. Some decoders
> > (namely the Snapdragon HW decoder)
> > will buffer frames when pic_order_cnt_type == 0 (in case the frames
> > will be reordered?) which results
> > in undesired increased latency. Setting pic_order_cnt_type to 2 will
> > fix this problem, and it is also what
> > libx264 does [0].
>
> Has that decoder bug been reported to the vendor so that they can fix it?

I haven't and I'm not sure where I would check if it was already
reported by someone else. I wasn't even sure it's a bug since all
encoders I have tried so far (x264, nvenc, amf) will output
pic_order_cnt_type=2 when configured for low latency encoding.

>
> The decoder should be reading the stream buffering from max_num_reorder_frames in the VUI parameters, which is set at as expected <http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavcodec/vaapi_encode_h264.c;h=dd17be2190ab07331ed69cf830b8cea2584ad353;hb=HEAD#l478>.
>
> The change to using POC type 2 is not unreasonable to save a few bits, but to do it you will also need to fix all of the POC values to actually match the type 2 behaviour (the current type 0 setup steps by 1 on frames because interlacing is not supported, but type 2 always steps by 2 - see §8.2.1.3).
>
> - Mark
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