I was able to add a watermark to 2 position(top left & bottom right) of a video with scaling image height to tenth of the video height in one command
ffmpeg -hide_banner -i /path/to/input.mp4 -i /path/to/watermark.jpg -filter_complex "[1:v][0:v]scale2ref=oh*mdar:ih/10[logo-out][video-out];[video-out][logo-out]overlay=10:10[flag];[1:v][flag]scale2ref=oh*mdar:ih/10[logo-out2][video-out2];[video-out2][logo-out2]overlay=W-w-10:H-h-10" -c:a copy /path/to/output.mp4
But the above command is too redundant, so I remove the second scale2ref
ffmpeg -hide_banner -i /path/to/input.mp4 -i /path/to/watermark.jpg -filter_complex "[1:v][0:v]scale2ref=oh*mdar:ih/10[logo-out][video-out];[video-out][logo-out]overlay=10:10[flag];[flag][logo-out]overlay=W-w-10:H-h-10" -c:a copy /path/to/output.mp4
But sadly, error occurs
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fb195013c00] Invalid stream specifier: logo-out.
Last message repeated 1 times
Stream specifier 'logo-out' in filtergraph description [1:v][0:v]scale2ref=oh*mdar:ih/10[logo-out][video-out];[video-out][logo-out]overlay=10:10[flag];[flag][logo-out]overlay=W-w-10:H-h-10 matches no streams
I know error occurs because of the first overlay didn't set an image output specifier, but it seems we can't do this? I only know overlay can set a video stream specifier.
How can I use the [logo-out] specifier which output from scale2ref in the second overlay?
An output generated inside a filtergraph can only be consumed once. To reuse it, split it first.
ffmpeg -hide_banner -i /path/to/input.mp4 -i /path/to/watermark.jpg -filter_complex "[1:v][0:v]scale2ref=oh*mdar:ih/10[logo-out][video-out];[logo-out]split=2[logo-left][logo-right];[video-out][logo-left]overlay=10:10[flag];[flag][logo-right]overlay=W-w-10:H-h-10" -c:a copy /path/to/output.mp4