How does Lightroom’s HSL panel divide and blend color ranges?
Asked 8/10/2018
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I’m building a small application that mimics Lightroom’s HSL panel and want the color grouping to behave similarly. My first attempt was to split hues into simple ranges, but the results were not very close to Lightroom, especially around purple/magenta and neighboring colors.
Are there any official descriptions or careful experiments showing how Lightroom assigns pixels to the HSL color bands? If not, what is a good way to improve the ranges so the adjustments feel more like Lightroom’s?
From testing, it seems Lightroom may not use hard boundaries only by hue: adjacent bands appear to overlap and blend, and in some cases lightness may also affect grouping.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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I did a fairly in depth analysis of the black and white conversion tool in Photoshop back when they released it in CS3. Its only six color bands, but it might give you some insight into how their suite thinks about color.
What I determined is that while the sliders are located at /6 increments around the polar grid, each one has an influence of /3. The red slider, for example, is centered on 0° but has a range from -60° to 60° with a decreasing influence over a pixel as its hue moves away from 0°.
For example, say a pixel's hue is located at 15°, which is 1/4 of the way between Red and Yellow. So it (essentially) calculates the pixel adjusted by the red settings, and the pixel adjusted at the yellow settings, then blends the results 75% red and 25% yellow.
I haven't done the leg work to figure out exactly how that blending is done for Hue and Saturation, but their exact algorithm is probably patented so you wouldn't be able to use it anyway.
Originally by user68136. Source · Licensed CC BY-SA 4.0
user68136
7y ago
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There doesn’t appear to be an official public spec in the answers, but the useful takeaway is that Lightroom-like tools likely do not use hard, non-overlapping hue buckets.
A good model is overlapping bands centered around the visible color labels, with influence falling off as hue moves away from each band’s center. In similar Adobe behavior, sliders are centered at equal hue intervals, but each slider affects a wider range, and pixels between two centers are blended proportionally between the neighboring sliders rather than assigned to just one.
So instead of saying “this hue is only yellow” or “only green,” use soft weighting across adjacent bands. That should help especially near boundaries such as yellow/green and purple/magenta.
The most practical way to match Lightroom is empirical testing: create a controlled hue wheel or patch chart, move one HSL slider at a time, export results, and measure which pixels changed and by how much. From that you can infer each band’s center, width, falloff curve, overlaps, and whether lightness also changes the weighting.
In short: use overlapping hue ranges with smooth interpolation, then refine them by test images against Lightroom output.
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