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/

>>13

In information theory, you assume an "ideal" compressor which basically goes as far as entropy goes.
Okay, the test case you are giving is problematic, because removing the alpha channel from an example image will yield to new neighbouring pixel.
If there's any correlation between the neighbouring B and R values, the image will further shrink down.

Did you know that A.I.-development and compression-algorithm are basically the same field? I'm sure that we'll all smile about it in 10 years when smart compressors really go to the next level.