[FFmpeg-devel] Overlay documentation does not mention pixel aspect ratio.

[Zach Swena]

Hi,

I would like to propose a change to the documentation for the overlay
filter.  The overlay filter does not detect pixel aspect ratio.  This
causes improper display resolution when overlaying smaller images on a
larger image.  A common example of this would be overlaying an SD source on
an HD one.  The documentation does not mention the fact that overlay
ignores pixel aspect ratio because it is a pixel by pixel operation.  It
would be nice to remind people that they need to overlay pictures with the
same pixel aspect ratio or perform a scaling operation.  One common example
of this would be overlaying SD video on a HD background.  I tried to make a
patch, but I couldn't find the right file.

It would be nice to add something like this at the end of the 9.83 section
just prior to section 9.83.1 in what becomes ffmpeg-filters.html.  Please
let me know if I need to submit this in some other form.

Please note that overlay is a pixel per pixel operation so a scaling
operation will be required to properly combine sources with different
source aspect ratios such as SD and HD video.