Request to add an option to the perspective filter to show through underlying content vs. extending the outside pixels

[jbmcgx]

The perspective filter appears to currently only have the option to extend the outside pixels instead of letting the area outside the desired coordinates show through.

With this command to overlay an image on another image with a perspective transform, you can see that the image underneath gets covered up:

ffmpeg -i "https://imposium-dev-uploads.s3.amazonaws.com/0ea8ab969efa8643b569db67be198db2/24IbheKbe5xbu0iehdv5Eb2ddd67Lci0W5G9/8.jpg" -i "https://imposium-dev-uploads.s3.amazonaws.com/0ea8ab969efa8643b569db67be198db2/4eea4565Rcf2S21051c6G8I6C060C018V1mc.jpg" -filter_complex "[1:v] scale=width=1920:height=1080:out_color_matrix=bt709:flags=lanczos,format=rgba,perspective=x0=534.49:y0=430.43:x1=867.1:y1=396.78:x2=526.01:y2=674.9:x3=887.03:y3=641.32:sense=destination [sized0]; [0:v][sized0] overlay=format=yuv444:x=0:y=0,format=yuv420p" -qscale:v 2 -frames:v 1 -pix_fmt yuvj420p "no_pad_extended_pixels.jpg"

I've discovered that you can get around this issue by adding a transparent border using the pad filter. This way the transparent pixels are extended instead of the edges of the overlaid image and the underlying image is shown. Here's that same command updated with that:

ffmpeg -i "https://imposium-dev-uploads.s3.amazonaws.com/0ea8ab969efa8643b569db67be198db2/24IbheKbe5xbu0iehdv5Eb2ddd67Lci0W5G9/8.jpg" -i "https://imposium-dev-uploads.s3.amazonaws.com/0ea8ab969efa8643b569db67be198db2/4eea4565Rcf2S21051c6G8I6C060C018V1mc.jpg" -filter_complex "[1:v] scale=width=1920:height=1080:out_color_matrix=bt709:flags=lanczos,pad=iw+4:ih+4:2:2:#00000000,format=rgba,perspective=x0=540.13:y0=430.7:x1=872.14:y1=397.37:x2=531.38:y2=675.01:x3=891.74:y3=641.77:sense=destination [sized0]; [0:v][sized0] overlay=format=yuv444:x=0:y=0,format=yuv420p" -qscale:v 2 -frames:v 1 -pix_fmt yuvj420p "with_pad_showing_underlying.jpg"

This works, however, it throws off the coordinates and it's not easy to calculate the resized border size and adjust accordingly.

It would greatly improve usability to add an option to set the outside pixels to a certain colour or even just something like outer_pixels set to either "extend" or "transparent".

[Michael Koch]

Duplicate of ticket 8124?
​https://trac.ffmpeg.org/ticket/8124