[FFmpeg-devel] [PATCH 2/2] lavc/videotoolboxenc: enable low-latency mode for HEVC

["zhilizhao(赵志立)"]

> On Sep 1, 2023, at 09:13, mypopy at gmail.com wrote:
> 
> On Fri, Sep 1, 2023 at 2:13 AM Zhao Zhili <quinkblack at foxmail.com> wrote:
>> 
>> 
>>> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of Jun Zhao
>>> Sent: 2023年8月31日 22:50
>>> To: ffmpeg-devel at ffmpeg.org
>>> Cc: Jun Zhao <barryjzhao at tencent.com>; Jun Zhao <mypopydev at gmail.com>
>>> Subject: [FFmpeg-devel] [PATCH 2/2] lavc/videotoolboxenc: enable low-latency mode for HEVC
>>> 
>>> Enable the HEVC encoder that supports low-latency mode.
>>> 
>>> Signed-off-by: Jun Zhao <barryjzhao at tencent.com>
>>> ---
>>> libavcodec/videotoolboxenc.c | 3 ++-
>>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>> 
>>> diff --git a/libavcodec/videotoolboxenc.c b/libavcodec/videotoolboxenc.c
>>> index 8e493c4f7a..bfc03787a0 100644
>>> --- a/libavcodec/videotoolboxenc.c
>>> +++ b/libavcodec/videotoolboxenc.c
>>> @@ -1568,7 +1568,8 @@ static int vtenc_configure_encoder(AVCodecContext *avctx)
>>> #endif
>>> 
>>>     // low-latency mode: eliminate frame reordering, follow a one-in-one-out encoding mode
>>> -    if ((avctx->flags & AV_CODEC_FLAG_LOW_DELAY) && avctx->codec_id == AV_CODEC_ID_H264) {
>>> +    if ((avctx->flags & AV_CODEC_FLAG_LOW_DELAY) &&
>>> +        (avctx->codec_id == AV_CODEC_ID_H264 || avctx->codec_id == AV_CODEC_ID_HEVC)) {
>> 
>> How about just remove the codec_id check?
>> Is there any unwelcome side effects for encoders other than H264/HEVC?
> 
> 1. One reason is that there are now other encoders available, such as
> prores_videotoolbox.
> 
> 2. It may be more appropriate to use a strictly qualified check
> instead of an open condition.

Both HEVC and ProRes failed with low_latency on my machine (with or without -allow_sw 1)

[hevc_videotoolbox @ 0x61900002a380] Error: cannot create compression session: -12902
[prores_videotoolbox @ 0x61900002a380] Error: cannot create compression session: -12902

	kVTParameterErr							= -12902,

1. FFmpeg users should have an idea whether or not to set AV_CODEC_FLAG_LOW_DELAY.
AV_CODEC_FLAG_LOW_DELAY doesn’t make sense for ProRes, but if user wants to try, let he/she
do.

2. I can’t find API to query the capability. Only VideoToolBox framework itself know how
to handle the argument. There isn’t much protect to provide for users.

Even with H.264, the results is unexpected: the input framerate is 25 but the output
framerate is 5, it dropped 80 percent frames silently.

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