Building Frames

Go with lavfi

Generating a GIF in two steps – generating the palette and then applying it – is not elegant. Besides, it leaves behind a PNG file that is useless for anything else.

Thanks to the lavfi virtual device, you can do everything in one go (Listing 2), which has the advantage of making you look like a veritable FFmpeg wizard, without generating any PNG cruft to boot.

Listing 2

One-Step GIF Generation


At this stage, it should be clear what is going on, but for the sake of clarity:

  • The clip_gray.mp4 file is the grayscale and scaled-down video you want to convert.
  • The lavfi filter options in quotation marks generate the palette image and pipe it through [out] to the …
  • -filter_complex chain of filters.

Apart from using the palette you created with the lavfi virtual device, you reduce the frame rate to 10 frames per second and push the resulting frames out to clip_gray.gif.

The resulting GIF image is smooth, acceptably lightweight, and created in seconds.


Not surprisingly, FFmpeg is the back end for so many video editing applications: The number of things you can do with it is astounding. Although it is true that some of the command lines you have to generate can be eye-wateringly complex, it is often worth working directly with FFmpeg, because it gives you that extra control on the final result. Apart from lavfi, you can learn more about FFmpeg devices by taking a look at the "More Devices" box.

More Devices

Apart from lavfi, a few other FFmpeg devices are worth investigating, especially if you want a raw, but very reliable way of grabbing streams from your camera or desktop:

(1) video4linux2, or v4l2 [4] for short (you can use either), captures streams from your webcam. To work out what devices you have on which to capture streams, run ls /dev/video*. My laptop has, apart from its built-in webcam, an external webcam with its own microphone, so I see:

> ls /dev/video*
/dev/video0 /dev/video1

If you run ffplay -f v4l2 -i /dev/video1, a window pops up showing the feed from the external webcam.

(2) On a related note, alsa [5] and pulse [6] capture audio. To know which device to capture audio from, if you want to use alsa, enter arecord -l. Again, on my laptop, apart from the built-in microphone, I have the microphone attached to the external webcam and a microphone input on an external USB sound card, so I see:

> arecord -l
**** List of CAPTURE Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC3241 Analog [ALC3241 Analog] Subdevices: 1/1
 Subdevice #0: subdevice #0
card 1: Device [USB Sound Device], device 0: USB Audio [USB Audio] Subdevices: 1/1
 Subdevice #0: subdevice #0
card 2: H2300 [HP Webcam HD 2300], device 0: USB Audio [USB Audio] Subdevices: 1/1
 Subdevice #0: subdevice #0

If I want to record from the microphone on the webcam, I can see it is card 2 (HP Webcam HD 2300) connected to device 0. This means ffmpeg -f alsa -i hw:2,0 voice.mp3 records to a file called voice.mp3.

With pulse, things are a bit different. First, you need to find out the name of your devices:

> pactl list sources
Source #2
  Name: alsa_input.usb-Hewlett_Packard_HP_Webcam_HD_2300-02.analog-stereo Description: HP Webcam HD 2300 Analog Stereo
  Driver: module-alsa-card.c
  Sample Specification: s16le 2ch 44100Hz
  Channel Map: front-left,front-right
  Owner Module: 7
  Mute: no
  Volume: front-left: 48497 /  74% / -7.85 dB, front-right: 48497 /  74% / -7.85 dB  balance 0.00
  Base Volume: 41350 /  63% / -12.00 dB
  Monitor of Sink: n/a
  Latency: 0 usec, configured 0 usec

Once you have located the device from which you want to record, you can then use its name in FFmpeg:

ffmpeg -f pulse -i alsa_input.usb-Hewlett_Packard_HP_Webcam_HD_2300-02.analog-stereo voice.mp3

In this case, it captures from an external webcam microphone and records to voice.mp3.

(3) x11grab [7] is interesting if you want to make screencasts. You can grab the whole screen, just a rectangle, or a rectangle centered around the cursor. For example, the line

ffmpeg -f x11grab -follow_mouse centered -framerate 25 -video_size 640x480 -i :0.0 screncast.mp4

captures a 640x480 region centered around the mouse and records it to screencast.mp4.

You can mix and match all the above devices and apply all sorts of effects to the streams they produce. Altogether, they provide a very complete and complex screencasting solution.

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