Caveats

The Chromophile color maps are imperfect. This section lists some known deficiencies.

One aspect of optimization that could be improved is the sRGB gamut constraints. The optimization would be faster if the gradient of the constraint function was available. It might also be more reliable; it is possible to find choices of parameters where the way in which NumPy was compiled affects the optimization result. But evaluating the constraint function requires converting from CAM16-UCS to sRGB, and this is complicated. We relied on the Colour package [MMP+22] to do this conversion, but it does not include the gradient of the conversion (it is unlikely anyone has wanted it before).

The optimization is intended to make the color maps better, but attempting to optimize a color map does not necessarily improve it [LH92]. The objective function did not account for some physical phenomena, such as chromatic abberation in the lens of the eye. It put very little weight on chromatic variation because, even though it is known to have an effect on the smallest features that an observer can resolve [WTB+19], we did not know how to quantify the trade-off between chromatic variation and lightness variation. A better objective function would, of course, produce better color maps.

The Chromophile color maps have few highly chromatic colors, and this makes them less aesthetically pleasing than some alternatives. Highly chromatic colors in sRGB are limited to a narrow range of lightnesses. Because of this, the only way for a constant chroma color map to have a large lightness difference is to avoid highly chromatic colors.

Data with naturally or conventionally associated colors should be mapped using those colors [TGH+16], but it is not always possible to do this and satisfy the uniformity constraints of the Chromophile color maps. For example, one popular type of color map simulates the appearance of black body radiation at increasing temperatures by starting at black and progressing through red, orange, yellow, and finally white. Just like an actual black body, color maps of this type become more colorful during the transition from black to red and less colorful during the transition from yellow to white. No Chromophile color map has a similar sequence of colors. The closest, cp_seq_red_yellow_ccw, has a very different appearance.

The Chromophile color maps were designed for typical computer displays, not for print. In print, the available gamut depends on the inks, the paper, and the printing equipment, and it can be very different from a computer screen. There are print setups that easily exceed the sRGB gamut. Others have a smaller gamut than, or a quite different gamut from, sRGB. If you must be certain that a Chromophile color map will print accurately, consult a print professional.

The Chromophile color maps do not take advantage of the gamuts available in Ultra HD televisions and in movie projectors. These gamuts allow colors with greater chroma, a larger lightness difference between the endpoints, or both. They also have a larger bit depth, so they would also provide smoother gradients. As of this writing, most computers are not equipped with such displays, so designing such color maps did not seem worthwhile. This may change some day.