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Comment by raphlinus | Hacker News Reader

I should have clarified that I meant lines with subpixel phase and possibly widths that are not integral numbers of pixels, ie the antialiased case.

Also I totally agree that we need to take into account the perceptual differences between black-on-white and white-on-black even assuming the display technology is perfect. That's one reason doing these studies is not trivial!

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The non-antialiased “case” is one spot among the continuum of antialiased “cases”; it doesn’t satisfy totally different perceptual rules. What gamma curve would you propose to correct for the perceptual equality of a 9.3px white-on-black line with a 11.6px black-on-white line? What I’m saying is that you’re presupposing the wrong layer to make the correction (antialiasing or no).

You are correct. Change my third question to, "after applying a correct gamma curve to achieve perceptually uniform line widths in the unipolar case, what is the correspondence of line widths when inverting the polarity to preserve the perception of width matching?" Then you want to break that down into the contribution assuming a perfect display and the effect from lower dpi.

This question is less important than the other two, and plausibly the best place to address the effect is in design, rather than rendering.

As a simple demonstration, here's an image of a 1px white-on-black line and a 1px black-on-white line: https://imgur.com/a/9d9Tu3x

It's axis-aligned and box-filtered so there are no shades of gray, which means gamma is irrelevant. The white-on-black line appears to have a greater thickness than the black-on-white line.

There's a lot of empirical research showing that reading performance is better with positive-polarity (black-on-white) text than with negative-polarity text [1, 2], probably because the higher overall luminance results in a smaller pupil size and thus a sharper retinal image [3]. So, white-on-black lines appear thicker than black-on-white lines because the eye doesn't focus as sharply on them. This is true regardless of which color space blending is performed in.

Given this fact, if one wants to achieve uniform perceptual line thickness for black-on-white and white-on-black text, a more principled approach than messing with color blending would be to vary line width or the footprint of the antialiasing filter based on the luminance of the text (and possibly the background). This is the approach Apple and Adobe have taken for years with stem darkening/font dilation.

[1]: http://jdobr.es/pdf/Dobres-etal-2016-Displays.pdf

[2]: https://www.uni-duesseldorf.de/home/fileadmin/redaktion/Oeff...

[3]: http://www.psychologie.hhu.de/fileadmin/redaktion/Oeffentlic...

One caveat, if viewing this image on a high-dpi display there will be blurring from image upsampling by the browser. My eyes have the same result as yours: the white line appears thicker than the black one.

Here's the interesting thing about your observation: doing alpha compositing of text in a perceptual space (as opposed to a linear space) results in a thickening of black-on-white and a thinning of white-on-black. So doing gamma "wrong" actually results in better perceptual matching.

Do you have evidence that either Apple or Adobe varies the amount of stem darkening based on text luminance? I've tested Apple (macOS 10.12 most carefully) and have not seen this.

I don't think they actually take luminance into account (although I personally like the idea); I just meant that they solve the problem of "black-on-white text looks too thin" by thickening the text, rather than messing up other parts of the pipeline.

Absolutely, and I think there's a very strong case to be made for that. One of the points I'm trying to make is that you have to solve it somewhere. People who focus narrowly on "correct gamma" often miss this, especially when "incorrect gamma" is also plausibly a workable solution.

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