[FFmpeg-devel] [PATCH 2/2] doc/faq: explain DAR/SAR preserving.

[Clément Bœsch]

On Mon, Feb 16, 2015 at 07:34:52PM +0100, Nicolas George wrote:
> Le primidi 21 pluviôse, an CCXXIII, Kieran Kunhya a écrit :
> > The DAR for analogue compatible media (e.g. SD DVB) comes from the
> > active samples and not from the "full image".
> 
> The exact meaning of DAR in this context is entirely for us to decide. And
> that means collectively, not you alone. And a simple unrelated FAQ entry is
> not the place to discuss it anyway.
> 
> New patch attached with an example, and the CGA changed to EGA.
> 
> Regards,
> 
> -- 
>   Nicolas George

> From 2b238e7df5bd1c00a0c3c6c8b4070f4b6b0fa744 Mon Sep 17 00:00:00 2001
> From: Nicolas George <george at nsup.org>
> Date: Mon, 9 Feb 2015 16:17:22 +0100
> Subject: [PATCH] doc/faq: explain DAR/SAR preserving.
> 
> Signed-off-by: Nicolas George <george at nsup.org>
> ---
>  doc/faq.texi | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/doc/faq.texi b/doc/faq.texi
> index 494da86..c15fcf5 100644
> --- a/doc/faq.texi
> +++ b/doc/faq.texi
> @@ -467,6 +467,38 @@ point acceptable for your tastes. The most common options to do that are
>  @option{-qscale} and @option{-qmax}, but you should peruse the documentation
>  of the encoder you chose.
>  
> + at section I have a stretched video, why does scaling does not fix it?
> +
> +A lot of video codecs and formats can store the @emph{aspect ratio} of the
> +video: this is the ratio between the width and the height of either the full
> +image (DAR, display aspect ratio) or individual pixels (SAR, sample aspect
> +ratio). For example, EGA screens at resolution 640×350 had 4:3 DAR and 35:48
> +SAR.
> +
> +Most still image processing work with square pixels, i.e. 1:1 SAR, but a lot
> +of video standards, especially from the analogic-numeric transition era, use
> +non-square pixels.
> +
> +Most processing filters in FFmpeg handle the aspect ratio to avoid
> +stretching the image: cropping adjusts the DAR to keep the SAR constant,
> +scaling adjusts the SAR to keep the DAR constant.
> +
> +If you want to stretch, or “unstretch”, the image, you need to override the
> +information with the
> + at url{http://ffmpeg.org/ffmpeg-filters.html#setdar_002c-setsar, @code{setdar or setsar filters}}.
> +
> +Do not forget to examine carefully the original video to check whether the
> +stretching comes from the image or from the aspect ratio information.
> +
> +For example, to fix a badly encoded EGA capture, use the following commands,
> +either the first one to upscale to square pixels or the second one to set
> +the correct aspect ratio:
> +
> + at example
> +ffmpeg -i ega_screen.nut -vf scale=640:480,setsar=1 ega_screen_scaled.nut
> +ffmpeg -i ega_screen.nut -vf setdar=4/3 ega_screen_anamorphic.nut
> + at end example
> +

A lot of users will want to know how to avoid the transcode.

-- 
Clément B.
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