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

[Mark Thompson]

On 09/01/2023 07:37, David Rosca wrote:
> On Mon, Jan 9, 2023 at 3:22 AM Xiang, Haihao <haihao.xiang at 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?

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