[FFmpeg-devel] [PATCH 2/2] lavc/vaapi_encode_h264: correct bit_rate_scale setting.

[Jun Zhao]

On 2017/10/31 18:26, Mark Thompson wrote:
> On 31/10/17 02:37, Jun Zhao wrote:
>> From d1e105057e93e7c2788d6d684292db9008fbf3ac Mon Sep 17 00:00:00 2001
>> From: Jun Zhao <jun.zhao at intel.com>
>> Date: Tue, 31 Oct 2017 10:19:08 +0800
>> Subject: [PATCH 2/2] lavc/vaapi_encode_h264: correct bit_rate_scale setting.
>>
>> As H264 Spec 2012 E.2.2, bit_rate_scale means the max input bit rate.
>>
>> Signed-off-by: Jun Zhao <jun.zhao at intel.com>
>> Signed-off-by: Wang, Yi A <yi.a.wang at intel.com>
>> ---
>>  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 1d43e934ef..27a810c64e 100644
>> --- a/libavcodec/vaapi_encode_h264.c
>> +++ b/libavcodec/vaapi_encode_h264.c
>> @@ -406,7 +406,7 @@ static int vaapi_encode_h264_init_sequence_params(AVCodecContext *avctx)
>>          // Try to scale these to a sensible range so that the
>>          // golomb encode of the value is not overlong.
>>          hrd->bit_rate_scale =
>> -            av_clip_uintp2(av_log2(avctx->bit_rate) - 15 - 6, 4);
>> +            av_clip_uintp2(av_log2(FFMAX(avctx->bit_rate, avctx->rc_max_rate)) - 15 - 6, 4);
>>          hrd->bit_rate_value_minus1[0] =
>>              (avctx->bit_rate >> hrd->bit_rate_scale + 6) - 1;

avctx->bit_rate will change with FFMAX(avctx->bit_rate, avctx->rc_max_rate) when setting bit_rate_value_minus1.

>>  
>> -- 
>> 2.14.1
>>
> Can you offer some more justification for that?  I think the change is wrong: consider under what circumstances overflow/underflow occurs.
In VBR/CBR mode, I think max bit rate = FFMAX(bit_rate, rc_max_rate) for
bit_rate_scale/bit_rate_value_minus1.
>
> - Mark
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