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Two days work code written for Speccy Party 2020.1 ZX81 Compo.
Machine: Sinclair ZX81 with 16k RAMpack
It's more a screen compression challenge/solution I wanted to try than the demo itself.

Due to very limited memory in ZX81 I wanted to find a way to use its semigfx to animate or fill the screen using as low memory as it's possible.

The semigfx encoding ZX81 ROM is pretty strange, but I figured out it. The code uses 384 bytes per screen (50% compression since text screen memory is 32x24 = 768 bytes), so half screen animation used in this demo is 192 bytes per frame as in the title.

The main procedure of decompressing the screen into a screen buffer is 122 bytes long. The rest of the file is BASIC loader (since there is no binary load in ZX81 BASIC) and raw encoded frame by frame data plus end screen text. I have used Photoshop to create 27 frames, saved into .pbm files and extracted into .asm code omitting first 9 bytes (header). I will describe it more on the blog.

The idea was to use semigfx (2x2 bits so 64x48 semigfx pixels) for frame by frame animation - due to my limited time it ended in 27 frames animation fit into 9 290 bytes .p file ready to use in a real computer (with ZX Pand or other allowing to load .p files) or emulator (remember to setup 16k memory).