How-To: February 2008 Archives

Unfortunately my Linux based non-linear editing tool of choice, Open Movie Editor, doesn't currently support directly altering video playback speed. For example, if you wanted a portion of your new compilation to run at 200% of original recorded speed, it can't be done within OME. This exact functionality was something I needed for an existing editing project.

After some thought and investigation, such changes can be achieved through using a combination of FFmpeg and yuvfps, which is part of mjpeg tools, to alter the framerate of the desired footage. If your original file is PAL based, with a framerate of 25fps, changing the framerate to 50fps will result in the video running twice as fast, for half as long.

I didn't initially have mjpegtools installed, but on my Debian based system this was easy enough with

sudo apt-get install mjpegtools

Next, the input video needs to be converted to yuv4mpegpipe format, passed through yuvfps and output to a new avi file. Here's the command line I used to create a clip at 50fps:

ffmpeg -i input.dv -f yuv4mpegpipe - | yuvfps -s 50:1
-r 50:1  | ffmpeg -f yuv4mpegpipe -i - -b 28800k -y output.avi

Change the 50:1 ratios to whatever framerate you require. e.g. 100:1 for 100fps. Be sure to set the output file bitrate to a relevant quality level. Omitting this flag will result in a poor quality AVI output file by default.

The resulting AVI file was easily played back with Totem, and handled on the timeline admirably by OME.

Thanks to Victor Paesa on the FFmpeg mailing list for pointing me in the right direction.

Some other options to investigate include the new Libavfilter for FFmpeg and converting the original footage to a raw data file, which will lost the audio.

Extracting all frames from a video file is easily achieved with FFmpeg.

Here's a simple command line that will create 25 PNG images from every second of footage in the input DV file. The images will be saved in the current directory.

ffmpeg -i input.dv -r 25 -f image2 images%05d.png

The newly created files will all start with the word "images" and be numbered consecutively, including five pre-appended zeros. e.g. images000001.png.

From a video that was 104 seconds long, for a random example, this command would create 2600 PNG files! Quite messy in the current directory, so instead use this command to save the files in a sub-directory called extracted_images:

ffmpeg -i input.dv -r 25 -f image2 extracted_images/images%05d.png

Moving on, let's say you just wanted 25 frames from the first 1 second, then this line will work:

ffmpeg -i input.dv -r 25 -t 00:00:01 -f image2 images%05d.png

The -t flag in FFmpeg specifies the length of time to transcode. This can either be in whole seconds or hh:mm:ss format.

Making things a little more complex we can create images from all frames, beginning at the tenth second, and continuing for 5 seconds, with this line:

ffmpeg -i input.dv -r 25 -ss 00:00:10 -t 00:00:05 -f image2 images%05d.png

The -ss flag is used to denote start position, again in whole seconds or hh:mm:ss format.

Maybe extracting an image from every single frame in a video, resulting in a large number of output files, is not what you need. Here's how to create a single indicative poster frame, of the video clip, from the first second of footage:

ffmpeg -i input.dv -r 1  -t 00:00:01 -f image2 images%05d.png

Notice that the -r flag is now set to 1.

If you want the poster frame from a different part of the clip, then specify which second to take it from using the -ss tag, in conjunction with the line above.

Lastly, if you wanted to create a thumbnail story board, showing action throughout the entire length of the video clip, you'll need to specify the output image dimensions. Use the following line:

ffmpeg -i input.dv -r 1 -f image2 -s 120x96 images%05d.png

My original file was 720x576, so the image dimensions are a whole division of this.