FFmpeg: About filter_complex command [closed]

Question

I use this command.

ffmpeg -i Input.mp4 -i logo.png -c:v h264_nvenc -filter_complex "[0:v]scale=-1:720[video];[1:v][video]scale2ref=(iw/ih)*ih/8/sar:ih/8[wm][base];[base][wm]overlay=10:10" output.mp4

But what does this mean?

scale2ref=(iw/ih)*ih/8/sar:ih/8

Answer 1

To understand the -filter_complex language, it is essential to read this small chapter of the ffmpeg docs.

You can put the filter description also into a file and use -filter_complex_script <file>.

Compare the filter to a function. The parameters are separated by :. By-position just value, else dictionary-style name=value. To start the parameter list, you use =, too.

The actual data goes via labels [<inlabel>]<filter>[<outlabel>]. Within a chain of filters (separated by ,), labels are not necessary. Chains are separated by ;.

ffmpeg -filters | grep scale2ref

would give you the in and out channels for the scale2ref filter (VV->VV), i.e. 2 videos, in and out.

ffmpeg -help filter=scale2ref gives you info on the parameters of the filter. The order of the parameters is not so obvious, as in this case w is followed by width. But w and width are the same. So the actual order is width,height of output.

(iw/ih)*ih/8/sar:ih/8 is thus width=(iw/ih)*ih/8/sar:height=ih/8.

To know what variables are predefined/preset for a filter, you need to look into the docs or even better into the source code.