Ffmpeg Filters Explained

Ffmpeg is a powerful tool for working with multimedia files. I’ve put an introduction here. This article talks about how filters work in ffmpeg in more detail.

Ffmpeg ships with many audio and video filters.

Examples of FFmpeg filters

You can use the -vf option to apply a filter to a video or the -af option to apply a filter to an audio file.

For example you can use the following command to apply the boxblur filter to a video with radius 10.

Box blur filter

ffmpeg -i input.mp4 -vf "boxblur=10" output.mp4

High and low pass filters

The highpass filter is used to remove low frequency components from the audio or video. However, it is more useful for audio. For example, you can use the following command to remove low frequency components below 100Hz:

ffmpeg -i input.mp4 -af "highpass=f=1000" output.mp4

Here f is the cutoff frequency.

Using a similar command you can remove high frequencies from your audio. This is usually useful to remove noise that is above a certain frequency.

ffmpeg -i input.mp4 -af "lowpass=f=1000" output.mp4

Filter Graphs

If you want to combine multiple filters, you can use the -filter_complex option. This is a more advanced option that is used to apply multiple filters to a video stream and connect them together in a complex filtergraph. With filter_complex, you can apply filters such as scaling, cropping, blending, and overlaying to a video stream, and connect them together in any way you like. For example, imagine you have the following video:

If you want to add a watermark to the video, you can use the overlay filter as follows:

ffmpeg -i scrat.mp4 -i logo.png -filter_complex "[0:v][1:v]overlay=10:10" scrat_watermark.mp4

This command the uses the filter_complex option to apply the overlay filter to the video stream of the input video file and the logo image. The [0:v] and [1:v] parts of the command refer to the video streams of the input video and the overlay image respectively. The overlay=10:10 part of the command specifies the position of the overlay image. The resulting video will look something similar to the following.

Anatomy of a filter graph

Filtergraph is a term used to describe a collection of filters connected together. The syntax goes as follows:

[inputs]filter=param1=value1:param2:value2[outputs];...

To explain filter graph syntax it’s best to illustrate them with some examples.

Consider the following command:

ffmpeg -i scrat.mp4 -i logo.png -filter_complex "[0]vflip[f];[0][f]vstack[vs];[vs]split[l][r];[l][r]hstack[h];[h][1]overlay=10:10" scrat_stacked.mp4

This command will result in a video similar to the following:

The following graph illustrates the filter graph used in the above command

Here’s what the command does:

1. Pass the first input [0], scrat.mp4 in this case as input to the vflip filter and assign the output to the variable [f].
2. Pass the [0] input and the [f] output from previous step as inputs to the vstack filter, which stacks the two inputs vertically and assign the output to [vs].
3. split the output of the vstack filter into two streams and assign the left stream to [l] and the right stream to [r].
4. Pass the [l] and [r] streams as inputs to the hstack filter, which stacks the two inputs horizontally and assign the output to [h].
5. Pass the [h] output from the previous step and the logo.png input, [1] as inputs to the overlay filter which overlays the logo on top of the output from previous step at position (10px, 10px).
6. The output from the overlay filter is saved to the output file scrat_stacked.mp4.

Read more about vflip, vstack, split, hstack, and overlay filters.

If this example wasn’t super clear, consider the following one.

Enabling and Disabling filters

You can enable and disable filters by using the enable parameter, for example:

ffmpeg -i testsrc.mp4 -vf "boxblur=enable='lt(t,4)'" output.mp4
ffmpeg -i testsrc.mp4 -vf "boxblur=enable='between(t,0,4)'" output.mp4

The two commands above, when applied to a video generated using the testsrc input device, will enable the boxblur filter for the first 4 seconds of the video, producing a video like the following one.

To read more about timeline editing, check the docs.