lena.mp4
Convert any PNG into something a 1984 Thomson MO5 can actually display.
More details at the companion blog post
The MO5 has a fixed 16-color palette and a brutal constraint: each group of 8 horizontal pixels can only use 2 colors. That's it. No tricks, no workarounds, just 2 colors per block.
And the palette looks like this:
The converter optionally works in CIELAB perceptual color space — it brute-forces all 136 possible color pairs for every 8-pixel block, simulating Floyd-Steinberg error diffusion inside each candidate to find the pair that minimizes outgoing perceptual error. Then it commits the winner and diffuses the quantization error (damped ~90%) across the image. The result is surprisingly good for 16 KB of video RAM.
C++23. Single-file tools. No dependencies beyond stb headers and a ZX0 compressor.
| Original | MO5 (320×200, 16 colors, 2 per block) |
|---|---|
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Compressed sizes (ZX0, from 16 KB raw):
| Image | .mo5z size |
|---|---|
| lena | 10.3 KB |
| birds | 12.6 KB |
The .mo5z format splits color and pixel data into three separate ZX0-compressed streams — colors first, so a viewer on real hardware can progressively reveal the image during decompression.
| Tool | What it does |
|---|---|
| png2mo5 | Image -> MO5. Handles resize, palette quantization, dithering, outputs raw bins + preview PNG |
| mo5z | Compresses MO5 pixel+color banks into a single .mo5z file (three ZX0 streams) |
| mo5z2png | Decompresses a .mo5z back to a viewable PNG |
| k7tool | Creates and inspects .k7 cassette tape images for the MO5 |
| mo5zviewer | 6809 assembly viewer (309 bytes) — boots from tape, decompresses, displays. That's it |
The full pipeline: png2mo5 -> mo5z -> k7tool bundles the viewer + compressed image into a .k7 cassette file you can load in an emulator.
# 0. (optional) Preview the conversion before committing
# -> lena_preview.png (preview is the default, no flags needed)
png2mo5/png2mo5 lena.png
# 1. Convert PNG to MO5 raw banks (8000 bytes each)
# -> lena_pixels.bin, lena_colors.bin
png2mo5/png2mo5 lena.png --bin
# 2. Compress into a .mo5z (three ZX0 streams, ~10 KB)
# -> lena.mo5z
mo5z/mo5z lena_pixels.bin lena_colors.bin -o lena
# 3. Glue the viewer + compressed image into a single binary
# -> lena.bin (stanalone binary, loads at $2800, exec at $2800)
cat mo5zviewer/mo5zviewer.bin lena.mo5z > lena.bin
# 4. (optional) Wrap it in a .k7 cassette file for use in an emulator
k7tool/k7tool -o lena.k7 lena.bin:0x2800:0x2800All the code in this repo — including the 6809 assembly — was written by Claude Opus 4.6 under my supervision.