Dithering Revisited

Achieving 24-bit colour on a 15-bit device – 2021-12-31

[I found this unfinished post in a dusty corner of my drafts folder, and decided that tonight was the night to finish it!]

While I’m sitting through the all-too-familiar wait while Quartus builds a core, I wanted to write a few words about dithering and how I approached the problem of doing 24-bit colour video output on a platform which has only 15 bits of colour resolution on its VGA port.

The video DAC on the Turbo Chameleon 64 has just 5 bits per colour gun which means we can output 32 different levels each of red, green and blue for a total range of 32,768 colours. This is fine for the ECS Minimig core, since the original Amiga has only 4 bits per gun, for a total of 4,096 colours – but the AGA chipset doubles this colour depth to 8 bits per gun, full 24-bit output – so some compromises will be needed.

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What You Wanted, What You Got…

Sigma Delta without Math – Part 2 – 2021-12-04

Last time around I talked about strategies for halftoning graphics, and made the key point that “noise” and “objectionable noise” are not the same thing.

I haven’t yet mentioned, though, the class of dither patterns which has been used most commonly since the advent of the desktop printer – namely Error Diffusion.

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