no such filter, when using filter complex with pre-scale

Question

the desired output is combining 3 vids, and using scale or pad (do some processing on the input).

ffmpeg -i in.mp4 -i in2.mkv -i in3.mkv \
-filter_complex "[0:v][v0];[1:v]scale=-1:1440[v1];[2:v][v2],[v0][v1][v2]hstack=inputs=3"
 -map "[v]" -map 1:a output.mp4

give:

[AVFilterGraph @ 0xec0a80] No such filter: '' Error initializing complex filters. Invalid argument

Answer 1

[0:v][v0] and [2:v][v2] don't contain any filter declaration, and thus ffmpeg complains. Insert whichever filter is desired in between the input and output pads.

I suppose you meant to scale the heights of the 3 inputs to match. In which case,

ffmpeg -i in.mp4 -i in2.mkv -i in3.mkv \ -filter_complex "[0:v]scale=-1:1440[v0];[1:v]scale=-1:1440[v1];[2:v]scale=-1:1440[v2];[v0][v1][v2]hstack=inputs=3" -map "[v]" -map 1:a output.mp4

on input processing with -filter_complex, you use this pattern:

[input_number:v]operation[tmp_output_name]