Comment by jmwilson | Hacker News Reader

Comment by jmwilson | Hacker News Reader

> ACEP achieves a full color gamut, including all eight primary colors

Last I checked there are three primary colors, and what is a full color gamut? There are several standard choices of gamut.

What confuses me about color e-ink products is that it is a reflective display, yet none of them use a reflective color model (CMY) - all of them talk about RGB pigmented subpixels. Either what I learned about color reproduction for print media is wrong, or these devices are probably incapable of showing a large number of colors. Given the washed-out appearance of several photos of real products, I'm leaning toward the latter conclusion. I'd love to see magazine-quality color in a reflective display some day, but they don't seem to be starting from the right fundamentals. Has anyone benchmarked the color reproduction of these products?

In a subtractive colour model you typically have a fourth channel (CMYK) because simply mixing all three primary colours together still reflects too much light to get good contrast.

For E-ink the colours don't actually mix with each other, so some basic maths shows that you would have even worse contrast with only three colours in a subtractive model:

Each ink colour absorbs only a single RGB channel, eg. cyan absorbs red. This means that at each location on the display, 2/3 of the light is reflected even when CMY channels are all at their maximum.

I think E-ink is neither an additive nor a subtractive colour model: it can only produce colours that exist somewhere between the extreme primary colours, ie. it's more of an "interpolating" colour model.

The inks probably come in pairs: black/white, red/cyan, green/magenta, blue/yellow. This gives the expected eight primary colours. For each pair, you would be able to pick any mix, eg. 30% black, 70% white.

For the darkest black you would have 100% black, red, green and blue. For lightest white you would have 100% white, cyan, magenta and yellow.

(This would reflect (1/3) * 3/4 + 0/4 = 25%, and (2/3) * 3/4 + 1/4 = 75% of the light respectively assuming perfect inks and equal coverage for the channels, which is pretty decent contrast)

Right, but this situation also happens in traditional color printing as well. Ink can't be varied in intensity, so images are halftoned/dithered. Halftone screening is done at a different angle per channel to avoid moire, so there is a mix of true overprinting and places where dots of distinct color are next to each other.

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