farbfeld

farbfeld is a lossless image format which is easy to parse, pipe and
compress.
It has the following format:

| Bytes | Description |
|--------|------------------------------------------------------------|
| 8 | "farbfeld" magic value |
| 4 | 32-Bit BE unsigned integer (width) |
| 4 | 32-Bit BE unsigned integer (height) |
| [2222] | 4⋅16-Bit BE unsigned integers [RGBA] / pixel, row-aligned |

http://tools.suckless.org/farbfeld/

>>25

Okay, here are the results

6.1M Jan 13 13:25 hurl_48bit.ff
3.1M Jan 13 13:25 hurl_48bit.ff.bz2

I cut out the Alpha value, so it only stored RGB-pixels. Now, as we can see, this gets close to 25% based on the original, which means that we properly assessed the test's circumstances.