Comment by 20k | Hacker News Reader

Comment by 20k | Hacker News Reader

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>When antialiasing, I think you do want to model light: a fully black object occluding 40% of a pixel should cause it to emit 40% less light. Linear intensity representations of colour should therefore be used.

The antialiasing value you get represents how much the pixel is covered by the glyph in question, and directly represents a desired change in perceptual brightness. There's no physical underlying lighting process, so it doesn't make sense to use physical light units

Blending in linear RGB models different strength lights being mixed together. This is why you do want to blend images together in linear RGB, but not fonts - because its not an underlying light based transport process

To take a direct example: Imagine two cases

1. We blend a white font on a black background

2. We blend a black font on a white background

Using perceptual blending, the antialiasing will be exactly the same efficacy in both cases. Using blending in linear space, these two test cases will look very different and render incorrectly!

> There's no physical underlying lighting process, so it doesn't make sense to use physical light units.

I disagree with you here. Text rendering specifically is incredibly complicated, but for antialiasing in other contexts, the problem can be seen as trying to approximate what would be seen if the display had higher resolution and the viewer has blurry eyesight. In this model, a linear color space makes sense - if 60% of the pixels within a region on a higher dpi display would be lit, then that is best approximated* by a single pixel emitting the same number of photons as those pixels would (which is 60% as many photons as there should be in the situation where all pixels on the higher dpi display would be lit).

See https://en.wikipedia.org/wiki/Spatial_anti-aliasing#Anti-ali... .

*there are better filters if you're looking at more than one pixel at once

> Using perceptual blending, the antialiasing will be exactly the same efficacy in both cases. Using blending in linear space, these two test cases will look very different and render incorrectly!

Whatever color space you do your blending in, 40% black onto white should look the same as 60% white onto black.

>if 60% of the pixels within a region on a higher dpi display would be lit, then that is best approximated* by a single pixel emitting the same number of photons as those pixels would (which is 60% as many photons as there should be in the situation where all pixels on the higher dpi display would be lit).

This assumes that the output from the coverage process represents a semi transparent line with a light shining through it, which isn't what font rendering outputs. It outputs a perceptual brightness, because if a cell is 50% covered, we want it to be 50% dark. Not emitting 50% of the photons

>Whatever color space you do your blending in, 40% black onto white should look the same as 60% white onto black.

What you want is 40% black onto white to have a similar difference in intensity as 40% white onto black, otherwise your darkmode font will look significantly different at the same intensity as your lightmode font. This is why it doesn't make sense to do it in a linear colourspace

Note that the wikipedia article is wrong, given that photoshop uses a nontrivial gamma exponent

> What you want is 40% black onto white to have a similar difference in intensity as 40% white onto black, otherwise your darkmode font will look significantly different at the same intensity as your lightmode font. This is why it doesn't make sense to do it in a linear colourspace

Thanks for putting this clearly. I had not given this argument enough thought and respect previously. Would you agree if I said this is about maximizing the amount of useful information given to the reader (even if it deviates from approximating a printed page) and a perceptual colour space is the way to measure that information?

I should mention that I can find plenty of resources that suggest you should use a different font for dark-on-light vs light-on-dark (although I'm aware I'm not a good judge of the quality of said resources). This is not necessarily opposed to your point, since your reasoning can be extended to conclude that identically shaped printed text subject to blurring in linear colour space would be perceived differently depending on whether it's light-on-dark or dark-on-light (including when it's naturally blurred due to imperfect eyesight).

> Note that the Wikipedia article is wrong, given that photoshop uses a nontrivial gamma exponent

If we set text rendering aside, and consider something like games which prioritize photorealism rather than legibility, would you agree that linear colour space is the sensible one to do antialiasing in? This is for essentially the same reason you should do image resizing in linear color space, for which Wikipedia's citation [6] provides a convincing demonstration.

[6] https://www.ericbrasseur.org/gamma.html?i=1

Also consider blending different levels of grey. Blending 20% to 0% versus 40% to 60% versus 80% to 100% will look wildly different.

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