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

[Nicolas George]

L'octidi 28 nivôse, an CCXXIII, Philip Langdale a écrit :
> 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.

Thanks for testing.

> ---
>  libavcodec/nvenc.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/libavcodec/nvenc.c b/libavcodec/nvenc.c
> index efa3f04..a51ada2 100644
> --- a/libavcodec/nvenc.c
> +++ b/libavcodec/nvenc.c
> @@ -587,6 +587,13 @@ static av_cold int nvenc_encode_init(AVCodecContext *avctx)
>          ctx->init_encode_params.darWidth = avctx->width;
>      }
>  

> +    // Compensate for hardware assuming playback will be at 704 pixel width.

Nit: de-compensate for buggy encoders that compensate for...

> +    if (avctx->width == 720 &&
> +        (avctx->height == 480 || avctx->height == 576)) {
> +      ctx->init_encode_params.darWidth *= 44;
> +      ctx->init_encode_params.darHeight *= 45;
> +    }

Here, and also in Timo's patch, I would advice to use lavu's rational
operators: they will handle overflow better and reduce the result.

> +
>      ctx->init_encode_params.frameRateNum = avctx->time_base.den;
>      ctx->init_encode_params.frameRateDen = avctx->time_base.num * avctx->ticks_per_frame;

Regards,

-- 
  Nicolas George
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