Cat videos (part 2)

The previous post gave a simple tutorial on how to datamosh cat videos using ffgac and simple Unix commands. This post is a follow-up, with more ffgac usage, more Unix commands, and more cats.

The difference is that this post is more precise, using ffgac to split the videos one frame at a time.

0 - Choosing cat videos

Use the same cat videos from the previous post.

1 - Preparing cat videos

Prepare the cat videos in the same way as the previous post.

$ mkdir mpeg2
$ for I in *.mp4;
  do
    ffgac -i "$I" \
             -an -vcodec mpeg2video -f rawvideo \
             -mpv_flags +nopimb \
             -qscale:v 6 \
             -r 30 \
             -g 90 \
             -s 1280x720 \
             -y mpeg2/"${I/.mp4/.mpg}";
  done

(remember that -qscale, -r, -g, and -s are tweakable).

2 - Splitting the cats

Instead of splitting the cats using the split command, we will use ffgac to split the video in individual frames.

$ cd mpeg2
$ mkdir frames
$ let x=10
$ for I in *.mpg;
  do
    ffgac -i "$I" -vcodec copy frames/cat_${x}_%04d.raw;
    let x=x+1;
  done

Each file in the frames directory will contain one frame of an input video file. These may be I frames or P frames.

I decided to start the counter at 10 this time instead of 1 so that all video files are prefixed with two digits (assuming we have fewer than 90 input files). Otherwise we’d end up with cat_1_0001.raw between cat_10_0019.raw and cat_10_0020.raw and the output would be weird.

Change into the frames directory and see that you will have hundreds or possibly thousands of cat frames:

$ cd frames
$ ls -1
cat_10_0001.raw
cat_10_0002.raw
cat_10_0003.raw
[...]
cat_21_0702.raw
cat_21_0703.raw
cat_21_0704.raw

3 - Concatenating the cats

Now we will randomly concatenate the cat frames to create our final datamoshed cat video.

The difference from this post and the previous post is that in the previous post each 1 megabyte chunk would contain many frames, and in this post each frame file contains only one frame. So we have to be creative in choosing which files we will use to concatenate to create the datamoshed video.

For example:

$ cat cat_11_0001.raw $(ls | xargs -n 60 | sort --random-sort) > /tmp/concatenated_cats.mpg

This command will randomly sort the chunked cats, 60 frames at a time (by using the xargs Unix command), and concatenate them all into one cat video file at /tmp/concatenated_cats.mpg (you can repeat this command many times to get different random combinations of concatenated cats).

This file is not corrupted, because MPEG-2 is so awesome that you can just concatenate frames together and all will be fine, so there’s no need to bake the cats.

Since 60 frames were used for each chunk, the video is more evenly split (there are two seconds from each randomly selected chunk of video, not necessarily starting with an I frame).