Comment by steerablesafe
5 years ago
> I don't get what you mean, linearity (dL'star') being defined wrt perceptual uniformity, isn't CIELAB 76 linear by definition (to an approximation) ?? And color arithmetic pretty much by definition implies dealing with distances and angles in a perceptually uniform color space, doesn't it ??
No. Linearity is about physical light intensities relating to perceived color. Imagine having two light sources on the same spot that you can turn on or off separately. If you turn the first one on you perceive a certain color, if you turn the other on you perceive an other color and if you turn both on you perceive a third one. It turns out the perceivable colors (under normal viewing conditions) are representable in a three dimensional vector-space (for most people) so that the first two colors add up to the third color for every possible two light sources. Such a linear colorspace is XYZ for example [1].
This has nothing to do with perceptual uniformity. Perceptual uniformity is about the capability of distinguishing near colors. This defines a distance between colors, and there are three dimensional colorspace representations where the Euclidean distance approximate this perceptual distance well. cieLAB is such a colorspace, but AFAIK there are better state of the art colorspaces for the same purpose. I'm not very well versed in this, I learned from them from this video [2].
[1] https://en.wikipedia.org/wiki/CIE_1931_color_space
[2] https://www.youtube.com/watch?v=xAoljeRJ3lU
edit: gimp 2.10 now defaults to use a linear colorspace (not perceptually uniform!) for most if not all of its functionalities. This affects alpha-blending layers, the paintbrush, resizing, blur, and pretty much everything that involves adding/averaging colors. There is still a "legacy" option on these tools to turn back to the probably wrong sRGB methods, probably for compatibility with old gimp files. There is a dramatic difference when you use a soft green brush on a red background for example, it's worth to try out.
Ok, my bad, I should have re-read our lesson more carefully : we're actually supposed to do sRGB => XYZ > CIELAB (and later back).
And it looks like that you can either have an Euclidean vector (linear) space (linear sRGB, XYZ), or a perceptually uniform one (CIELAB), but not both !?
(I guess that I should have figured that out myself, sigh… this is why it isn't CIELAB that is used for monitor calibration, but CIELU'V' ? EDIT : Nope : "[CIELUV is] a simple-to-compute transformation of the 1931 CIE XYZ color space, but which attempted perceptual uniformity. It is extensively used for applications such as computer graphics which deal with colored lights. Although additive mixtures of different colored lights will fall on a line in CIELUV's uniform chromaticity diagram (dubbed the CIE 1976 UCS), such additive mixtures will not, contrary to popular belief, fall along a line in the CIELUV color space unless the mixtures are constant in lightness. ")
So you have to pick the best color space for the job, in the case of doing color averages that would be one of the linear ones (linear sRGB, XYZ), while if you are trying to design a perceptually uniform gradient for data visualization, you would better pick a perceptually uniform space (CIELAB, CIELUV) ?
See the recent Oklab post[0] for guidance on choosing a perceptually uniform color space for gradients. It's better than CIELab and CIELuv, both of which I would consider inferior to newer alternatives. In particular CIELab has particularly bad hue shifts in the blue range.
I'm also working on a blog post on this topic (there's an issue open in the repo for my blog, for the curious).
[0]: https://news.ycombinator.com/item?id=25525726