I've become unhealthily interested in image compression on Pico8.
Hit the "z" key to see the next image.
What we have here is something that might resemble the JPEG compression scheme if you squint.
Compression goes like this:
--Discrete Cosine Transform + Quantatization
--Zigzag encoding of blocks
--Huffman encoding
--Run Length Encoding
--Conversion to a base 64 character set
--Overall compresses to around 1/5 of original 8-bit grayscale size.
What didn't work well:
Image quality is kinda low. There are only about 16 levels of gray that are discernible on pico8 with dithering. Not sure that it makes a lot of sense to encode 256 shades of gray in that case. Not to mention that the image quality at 64x64 is pretty lame to begin with.
This path may be a dead-end because I can only get about 12 64X64 images in here before hitting compressed code limit. If I drop the base 64 coding, I could fit maybe 10 images into the sprite area... Because Pico8 compresses data as well (definitely with better algorithms), my compression may be working at cross purposes to Zep's compression. I still need to do an experiment to see whether the base 64 encoding is any better than the base 16 coding.
I've got some glitching in the bottom right blocks.
Here's the compression script: https://github.com/electricgryphon/pico_8_jpg
Perhaps this will inspire someone to greater image compression heights :-D.
-Electric Gryphon