FFmpeg - Basic Filter Graphs

FFmpeg is considered the Swiss Army knife of video transcoding/streaming. Let’s start with some very basic examples today of what we can do with FFmpeg.

Format conversion

ffmpeg -y -i input.mkv output.mp4

In this simplest example, FFmpeg produces MP4 output from MKV input. -y denotes that we want to overwrite output.mp4 if it already exists. -i marks the input. In addition to these two, FFmpeg supports many other popular multimedia file formats, including MXF, AVI, WAV, M4A, JPG, PNG etc.

Codec conversion

ffmpeg -y -i input.mkv -vcodec libx264 -acodec flac output.mkv

It is common to use FFmpeg to transcode from one codec to another. Here, we specify the video and audio codecs with -vcodec and -acodec.

Take only video or audio

ffmpeg -y -i input.mkv -map 0:v output.mp4

-map 0:v selects the video streams from the first input (we have only one input file here anyway, marked by 0), while 0:a means the audio streams.

Resize the video

ffmpeg -y -i input.mkv -filter_complex scale=720:480 output.mp4

-filter_complex can be used to combine many filters together. Filters sit between the input and the output and make some change to the media flowing through them. Here we are using the scale filter and specifying the output width and height.

Clipping

ffmpeg -y -i input.mkv -ss 30.4 -to 40.15 output.mp4

Here, -ss and -to are used to specify the start and end times in seconds.

Snapshot at time

ffmpeg -y -i input.mkv -ss 2.598 -vframes 1 output.jpg

We can give a filename with .jpg extension, and -vframes 1 will tell FFmpeg to produce a single image at the specified time instance.

Thumbnails at interval

ffmpeg -y -i input.mp4 -filter_complex "fps=1/5,scale=320:180" thumbnail-%03d.jpg

The fps filter is used here to say that we need 1 frame every 5 seconds. We scale it to required dimensions and give a pattern for the thumbnail filename to be numbered as we like.

Extract audio channels

ffmpeg -y -i input.mxf \
-filter_complex "[0:a]asplit=2[a1][a2];[a1]pan=mono|c0=c0[ch1];[a2]pan=mono|c0=c1[ch2]" \
-map [ch1] channel_1.wav \
-map [ch2] channel_2.wav

Getting slightly complicated now, but the graph image above should make it easier to understand. We first fork the input audio with the asplit filter into two identical branches labeled a1 and a2. Then we connect each of these to pan filters that are each set up to produce one mono channel. The first pan filter will select the first channel (c0) from a1 and the second one will select the second channel (c1)from a2, and we label the outputs from these pan filters ch1 and ch2 respectively. Lastly, we map ch1 and ch2 to produce two separate WAV files.

Complex graphs with shared filters

In this final example, we try to reuse common filters and produce simultaneous outputs in parallel.

ffmpeg -y -i input.mkv \
-filter_complex "[0:v]format=yuv420p,yadif,split=3[in1][in2][in3];[in1]scale=1920:1080[hd];[in2]scale=720:576,hflip[sd];[in3]fps=1/5,scale=320:180[thumbnails];[0:a]aresample=48000,asplit=2[a1][a2]" \
-map [hd] -map [a1] hd.mov \
-map [sd] -map [a2] sd-flipped.mp4 \
-map [thumbnails] thumbnail-%03d.jpg

As shown above, we apply the common operations early in the graph and try to reuse the filters whenever possible. For example, we reuse format, yadifand aresample filters for the HD and SD outputs, but do the scaling for them separately since they require different dimensions. In addition, we do a horizontal flip with hflip for the SD output only, just for fun! On the other hand, the thumbnails share some common filters (deinterlacing with yadif) with the video filters, but then needs fps and scale to adjust the frame rate and size specific to thumbnails only.

Conclusion

FFmpeg supports a vast collection of many other filters which can be combined to implement a lot of advanced requirements. But I hope the above examples are helpful to understand the basics and get started with this versatile and extremely powerful tool.

If you are interested to learn more, here is a short FFmpeg course on Udemy, with concepts explained with easy diagrams and step-by-step hands-on demo videos with lots of practical use cases and examples.

Happy transcoding!