ffmpeg overlay watermark only to non-transparent pixels

Question

I have a .mov video with alpha channel and .png watermark. Is it possible to overlay watermark only over the non-transparent pixels of the video using ffmpeg?

Answer 1

Use

ffmpeg -i in.mp4 -i image \
       -filter_complex "[0]split[o][a];[a]alphaextract[a];\
                        [o][1]overlay=format=auto[oc];[oc][a]alphamerge" -c:a copy out.mov

Choose a codec that preserves alpha like -c:v png.