I hadn't thought of that, but this is hilarious, and illustrates my point perfectly.
If your compression algorithm isn't aware of the exact dither you're using, the decompressor can't reproduce the dither on the other end using only rules and image data. The compressor needs to encode every single dither pixel as an expensive "Hey decompressor, you're never going to be able to guess this pixel value, so here's the whole thing" residual value.
This is also why old image compression algorithms that were aware of simple dithers (i.e. a handful of fixed grid patterns) could produce small-ish images that looked slightly better than un-dithered, but still kind of bad. But then as soon as you customized the dither to use a more random looking pixel arrangement that looked significantly better, the filesize would explode -- because the compressor was blissfully unaware of the more complicated dither and had no choice but to encode all of the seemingly random pixels directly.
If your compression algorithm isn't aware of the exact dither you're using, the decompressor can't reproduce the dither on the other end using only rules and image data. The compressor needs to encode every single dither pixel as an expensive "Hey decompressor, you're never going to be able to guess this pixel value, so here's the whole thing" residual value.
This is also why old image compression algorithms that were aware of simple dithers (i.e. a handful of fixed grid patterns) could produce small-ish images that looked slightly better than un-dithered, but still kind of bad. But then as soon as you customized the dither to use a more random looking pixel arrangement that looked significantly better, the filesize would explode -- because the compressor was blissfully unaware of the more complicated dither and had no choice but to encode all of the seemingly random pixels directly.