[FFmpeg-devel] [PATCH] nvenc: Compensate for hardware trying to mess with aspect ratio of DVD content.

[Reimar Döffinger]

On 21.01.2015, at 07:17, Agatha Hu <ahu at nvidia.com> wrote:
> On 2015/1/18 4:01, Philip Langdale wrote:
>> There is a long sad story behind all this, but it's somewhat ambiguous as to
>> whether DVD content should be treated as 720 pixels wide or 704 pixels, with
>> 16 pixels cut off. If you decide is should be 704 pixels wide, you need to
>> adjust the sample aspect ratio to keep the final display aspect ratio correct.
>> 
>> For reasons we are not privy too, nvidia decided that the nvenc encoder should
>> apply this aspect correction, whether you want it to or not. (I guess there
>> might be a flag for it, but if there is it's not documented). So, if you want
>> to transcode DVD content at the original size, you need to adjust the aspect
>> ratio information you pass to the encoder to compensate for their 'correction'.
>> 
>> This 'correction' is only applied to 720x480 and 720x576 content - and it does
>> so regardless of the input aspect ratio.
>> ---
>>  libavcodec/nvenc.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>> 
>> diff --git a/libavcodec/nvenc.c b/libavcodec/nvenc.c
>> index efa3f04..8a0d584 100644
>> --- a/libavcodec/nvenc.c
>> +++ b/libavcodec/nvenc.c
>> @@ -587,6 +587,18 @@ static av_cold int nvenc_encode_init(AVCodecContext *avctx)
>>          ctx->init_encode_params.darWidth = avctx->width;
>>      }
>> 
>> +    // De-compensate for hardware, dubiously, trying to compensate for
>> +    // playback at 704 pixel width.
>> +    if (avctx->width == 720 &&
>> +        (avctx->height == 480 || avctx->height == 576)) {
>> +        av_reduce(&dw, &dh,
>> +                  ctx->init_encode_params.darWidth * 44,
>> +                  ctx->init_encode_params.darHeight * 45,
>> +                  1024 * 1204);
>> +        ctx->init_encode_params.darHeight = dh;
>> +        ctx->init_encode_params.darWidth = dw;
>> +    }
>> +
>>      ctx->init_encode_params.frameRateNum = avctx->time_base.den;
>>      ctx->init_encode_params.frameRateDen = avctx->time_base.num * avctx->ticks_per_frame;
>> 
>> 
> 
> Here's the reply from NVENC engineers
> It is not same thing as SAR. It is the display aspect ratio i.e width/height = DAR/SAR
> 
> The calculation below should be like
> 
> If (avctx->sample_aspect_ratio.num > 0  && avctx->sample_aspect_ratio.den > 0 )
>    av_reduce(&dw, &dh, avctx->sample_aspect_ratio.num * avctx->width, avctx->sample_aspect_ratio.den * avctx->height, INT_MAX);

That doesn't answer anything.
Doing that, to keep the problem description short, creates different results from libx264 and any other encoder we have for DVD resolution which is highly undesirable for consistency and compatibility reasons if nothing else.