ffmpeg libplacebo filter cropping doesn't work

Question

Cropping a video using libplacebo filter doesn't work as expected. I want to crop and scale a video, I am using the following filter:

-vf libplacebo=crop_x=0:crop_y=138:crop_w=1920:crop_h=804:w=1280:h=-2: \
               force_original_aspect_ratio=decrease:normalize_sar=true

Expected behavior: video is cropped from 1920x1080 to 1920x804, and then scaled to the width of 1280 with height determined automatically preserving the aspect ratio, which would result in a 1280x536 video.

My result: video is cropped, but then forcefully stretched to 1280x720, not preserving the aspect ratio.

It works when I am using ffmpeg's standard crop and then passing the cropped video to libplacebo filter:

-vf crop=1920:804:0:138,libplacebo=w=1280:h=-2:force_original_aspect_ratio=decrease:normalize_sar=true

But I'd like to understand how to crop using libplacebo native means and replicate the default crop behavior.

It works as expected using ffplay:

ffplay -vf "libplacebo=crop_x=0:crop_y=138:crop_w=1920:crop_h=804:w=1920:h=804" input.mkv

I am getting a properly cropped 1920x804 video that is not stretched to 1920x1080, with the correct aspect ratio. But I can't seem to get it working when trying to encode it into a new file with ffmpeg.

Answer 1

Change h=-2 to h=ow/(a*sar), normalize_sar to false.

And add setsar=1 filter after libplacebo.