CODEWITHSUNDEEP
ffmpeg
HJ Kim
HJ Kim

Reputation: 55

FFmpeg: About filter_complex command

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

Upvotes: 3

Views: 5997

Answers (2)

Gyan
Gyan

Reputation: 93319

What scale2ref=(iw/ih)*ih/8/sar:ih/8 does is scale the image height to 1/8th the video's height, and scale the image width to, well, a weird value. If the scale is meant to preserve the image's aspect ratio, use

scale2ref=oh*mdar:ih/8

(FFmpeg version 4.0 to 7.1)

Edit: Since FFmpeg 7.1, scale2ref has been merged into existing scale filter with secondary reference input:

scale=-1:rh/8

Note that this filter outputs single stream of the modified one instead of two video streams.

Upvotes: 1

Roland Puntaier
Roland Puntaier

Reputation: 3521

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.

Upvotes: 7

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