[FFmpeg-devel] [PATCH 1/3] libavfilter/scale2ref: Add constants for the non-ref input

Michael Niedermayer michael at niedermayer.cc
Sun May 28 03:29:41 EEST 2017

On Sat, May 27, 2017 at 10:10:44AM -0400, Kevin Mark wrote:
> Variables pertaining to the non-reference video are now available when
> using the scale2ref filter. This allows scaling a video with another
> as a reference point while maintaining the original aspect ratio of
> the non-reference video.
> Consider the following graph: scale2ref=iw/6:-1 [scaled][ref]
> This will scale [scaled] to 1/6 the width of [ref] while maintaining
> the aspect ratio. This works well when the AR of [ref] is equal to
> the AR of [scaled] only. What the above filter really does is
> maintain the AR of [ref] when scaling [scaled]. So in all non-same-AR
> situations [scaled] will appear stretched or compressed to conform to
> the same AR of the reference video. Without doing this calculation
> externally there is no way to scale in reference to another input
> while maintaining AR in libavfilter.
> To make this possible, we introduce seven new constants to be used
> in the w and h expressions only in the scale2ref filter:
>  * other_w/other_h: width/height of the input video being scaled
>  * other_a: aspect ratio of the input video being scaled
>  * other_sar: sample aspect ratio of the input video being scaled
>  * other_dar: display aspect ratio of the input video being scaled
>  * other_hsub/other_vsub: horiz/vert chroma subsample vals of input
> Of course the only new constants needed for maintaining the AR is
> other_w/other_h or other_a but adding additional constants in line of
> what is available for in/out allows for other scaling possibilities I
> have not imagined.
> So to now scale a video to 1/6 the size of another video using the
> width and maintaining its own aspect ratio you can do this:
> scale2ref=iw/6:iw/(6*other_a) [scaled][ref]
> This is ideal for picture-in-picture configurations where you could
> have a square or 4:3 video overlaid on a corner of a larger 16:9
> feed all while keeping the scaled video in the corner at its correct
> aspect ratio and always the same size relative to the larger video.

> I am not fully sold on the other_ prefix. It works but I'm open to
> suggestions on something that is more concise or clear. Unfortunately
> in_ is most clear but that is taken by the reference input, which in
> my opinion, should've been labeled as such (not as in_ but perhaps
> ref_). What is the best way to refer to this? The other input? The
> scaled input? The non-reference input?


are maybe prefixes that would work without conflicting with the
existing ones

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

it is not once nor twice but times without number that the same ideas make
their appearance in the world. -- Aristotle
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