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Comment by joosters | Hacker News Reader

Has anyone tried using the equivalent of animated GIFs to help with dithering? If you have a limited palette of colours, perhaps you could produce two dithered versions, with pixels sometimes having different colours in alternate frames. If the image is refreshed fast enough, two colours could blend into a third.

The most extreme example could be 'dithering' a grey square into 1) a black square and 2) a white square, and when they are rapidly switching between the two it might appear as a grey instead.

I guess that in practice, the refresh rates of monitors are too low to make it seem anything other than a terrible flickering image, but on old CRTs the effect might work a little better. It's also memory and CPU intensive, but I'd still be curious to see if it could be used successfully, and if it improved the quality compared to 'just' a single dithered image.

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You don't even need the animation speed for this. In the GIF format you can have a new color palette for every frame so paint the first 256 colors in the first frame. Add 256 more colors in the right spots in the second frame etc.

See http://notes.tweakblogs.net/blog/8712/high-color-gif-images.... for a nice animation

This was especially common on 8-bit and 16-bit computers to fake 'high colour' displays on machines that had only limited colour capability. They often took advantage of the fact that people were using their machines with TVs with relatively slow-changing phoshor screens, so the flicker was less evident than on a modern monitor.

e.g. Photochrome on the Atari ST, was especially impressive at the time: https://www.youtube.com/watch?v=vPsY4P8bnVw

The most extreme version I've seen of this was on the ZX Spectrum, which had not only a very limited 15 colour palette, but also limited to 2 colours within each 8x8 block of the screen. Some bright spark came up with the idea of flipping rapidly between R, G, and B frames to give (limited) per-pixel RGB. Unfortunately it did flicker quite badly because of the extreme changes in colour levels (only two levels of each channel), and the fact that it required 3 whole frames to make a single colour virtual frame.

Example here: (not suitable if you have photosenstive epilepsy!) https://en.wikipedia.org/wiki/File:Parrot_rgb3.gif

Wow, that example parrot image is pretty impressive, given that it is just three colours, and I'm viewing it on an LCD!

I never knew anyone had tried that before on a spectrum. At first I thought you were just talking about the other trick, getting more than 2 colours per 8x8 by changing the palette as the raster scanned down the screen. The multi-colour parrot is way more adventurous!

I'd love to see the parrot image on an old CRT to get a feeling for what the effect might look like with the phosphor afterglow. Leaving the spectrum behind and using a bigger palette range, like on the ST, the effect seems much less epileptic fit inducing, because you can pick closer colours to switch between.

The idea is not new, but the gif renderers in browsers have a minimum frame rate, they turn 0-delay frames something larger.[0]

So that doesn't really work for looped animations.

But for single pass you can incrementally build a higher-color image, similar to interlaced loading.[1]

[0] http://nullsleep.tumblr.com/post/16524517190/animated-gif-mi... [1] https://en.wikipedia.org/wiki/GIF#True_color

That first link is interesting, thanks.

> The most extreme example could be 'dithering' a grey square into 1) a black square and 2) a white square, and when they are rapidly switching between the two it might appear as a grey instead.

I remember seeing old STN grayscale LCDs that seemed to work like this (and I even implemented it myself on the TI-83 as a test)

edit: looks like is called "Frame Rate Control" and used to be common http://robotics.ee.uwa.edu.au/eyejr/lcd/FRC-information.pdf

Yep - (cheap) monitor manufactures do it quite effectively. Many LCDs have less than 8bit per color and do exactly what you suggested.

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