Earlier this month, the Turbo rainbow colormap was released and publicized on the Google AI Blog. This colormap attempts to mitigate the banding issues in the existing Jet rainbow colormap, while retaining the advantages of its high contrast; note that Turbo is not perceptually uniform, so care should be used where high accuracy is required, particularly for local differences. What particularly caught my attention was the fact that the author attempted to address the color vision deficiency-related shortcomings of Jet. I am of opinion that the creation of a colorblind-friendly rainbow colormap probably isn’t possible, since the confusion axes of color vision deficiencies become problematic once hue become the primary discriminator in a colormap instead of lightness;1 this made me a bit suspicious of the claim and prompted further investigation on my part. While the author’s attempt to consider color vision deficiencies in the creation of the colormap is laudable, it was unfortunately based on what I feel is a flawed analysis. Depth images visualized using the colormap were fed into an online color vision deficiency simulator, and the results were evaluated qualitatively by individuals with normal color vision; however, this particular simulator is, best I can tell, based on an outdated technique from a 1988 paper2 instead of the more recent and accurate approach of Machado et al. (2009).3 Below, I attempt what I feel to be a more accurate and quantitative analysis, which shows that Turbo isn’t really colorblind-friendly, despite the attempt to make it so.
Since rainbow colormaps are best suited for quickly judging values, their most important property is that colors in non-adjacent sections of the colormap are not confused.4 To evaluate this quantitatively, I devised the following metric. For each color in the colormap, the perceptual distance in CAM02-UCS5 is calculated for every additional color in the colormap. The weighted average of the perceptual distances is then taken, with the squares of the color location distances in the colormap used as weights. For color vision deficiencies, the method of Machado et al. (2009) is used to adjust the colors before the perceptual distance is calculated, as I did for randomly generating color sets and as was done in the development of Cividis;6 a severity of 100 was used, indicating deuteranopia, protanopia, and tritanopia. Thus, similar colors in distant locations in the colormap are penalized.
We will start with rainbow colormaps for our evaluation of colormaps by this metric, first considering Jet, the new Turbo colormap, and Matplotlib’s existing Rainbow colormap, which also attempts to address some of Jet’s shortcomings. In the plot legends, the abbreviations “Norm,” “Deut,” “Prot,” and “Trit” are used for normal color vision, deuteranopia, protanopia, and tritanopia, respectively. Higher perceptual distance, ΔE, is better, as are smoother and more consistent discernibility lines.
The discernibility lines for Turbo and Rainbow are much smoother than those for Jet, since both mitigate Jet’s significant banding issues. Although Jet’s banding issues are generally considered problematic, I, as a colorblind individual, find the banding to sometimes be a redeeming quality, since it makes it easier for me to match part of an image to the colorbar or other parts of the image. For normal color vision, Turbo’s discernibility line is smooth and fairly flat, a significant improvement over Jet, and a minor improvement over Rainbow, although Turbo arguably looks better. However, the discernibility lines for various color vision deficiencies are not nearly as uniform, for either Turbo or Rainbow. This means that for colorblind individuals some parts of the colormaps are considerably more difficult to discern than others, making data plotted with them liable to misinterpretation. Thus, while Turbo and Rainbow improve upon some of Jet’s shortcomings, neither is colorblind-friendly.
Next, we will consider cyclic rainbow colormaps. The classic, and severely flawed, version is the HSV colormap, and the improved version is Sinebow; in regards to non-cyclic rainbow colormaps, these are analogous to Jet and Turbo, respectively. Twilight, a perceptually uniform cyclic colormap, is also considered.
In evaluating the metric for these colormaps, their cyclic nature was taken into consideration in the colormap location distance calculation. Sinebow’s discernibility lines are much smoother than HSV’s, but neither does well for color vision deficiencies. Twilight is much more consistent and colorblind-friendly, although at the expense of average discernibility.
Now, we will consider two perceptually uniform linear colormaps, Viridis, the Matplotlib default, and Cividis a derivative designed with color vision deficiencies in mind.
The “V” shape of the metric for these colormaps is expected, since for a linear colormap, the center is closest to the greatest number of other colors. Note that the discernibility of Cividis, which was optimized with color vision deficiencies in mind, is the most consistent between normal color vision and various color vision deficiencies, although Viridis is also okay in this regard, and both are considerably better than any of the rainbow colormaps previously presented.
Finally, diverging colormaps will be evaluated. Here, we consider Matplotlib’s Coolwarm colormap and Peter Kovesi’s Blue-Gray-Yellow colormap.
These show a “V” shape, similar to linear colormaps, although this is less pronounced in Coolwarm. The Blue-Gray-Yellow colormap is linearly increasing in lightness and perceptually uniform, so its discernibility profile is much closer to that of perceptually uniform linear colormaps.
In summary, while Turbo does ameliorate many of the issues with Jet, neither Turbo nor any of the other rainbow colormaps evaluated here are colorblind-friendly, at least per the metric evaluated. It is likely that it is not possible to construct a rainbow colormap with such a property, unlike for linear, diverging, and cyclic colormaps. The Jupyter notebook used to evaluate the colormaps and produce the plots is available.
It probably is possible to create a colorblind-friendly rainbow colormap for a particular type of color vision deficiency. However, creating such a colormap that simultaneously works for multiple types of color vision deficiencies as well as for normal color vision is what is likely impossible. ↩
G. M. Machado, M. M. Oliveira, and L. A. F. Fernandes, “A Physiologically-based Model for Simulation of Color Vision Deficiency,” in IEEE Transactions on Visualization and Computer Graphics, vol. 15, no. 6, pp. 1291-1298, Nov.-Dec. 2009. doi:10.1109/TVCG.2009.113 ↩
When differences between adjacent colors are important, a perceptually uniform colormap should be used. ↩
J. R. Nuñez, C. R. Anderton, and R. S. Renslow. “Optimizing colormaps with consideration for color vision deficiency to enable accurate interpretation of scientific data,” in PLoS ONE vol. 13, no. 7, pp. e0199239, Aug. 2018. doi:10.1371/journal.pone.0199239 ↩