Exploring the LCH and HSV blend modes

I write articles and tutorials about color management and radiometrically correct color mixing, topics that make most people’s eyes glaze over as they automatically turn around and head for the exit. Here is one such tutorial, that systematically compares the LCH and HSV layer blend modes:

Exploring the LCH and HSV blend modes using a Wikipedia photograph of Kenyon Cox’s portrait of Saint-Gaudens (http://ninedegreesbelow.com/photography/determine-image-tonality-and-palette-part-1.html).

If anyone actually reads and follows along in the tutorial, I’d love to hear what you think about the LCH blend modes compared to the HSV blend modes. Those artifacts you often see when using the HSV blend modes? That’s because unlike LCH, HSV doesn’t cleanly separate color and tonality.

A legitimate question (that I’ve been asked more than once) is “Does understanding all this color management/color science mumbo jumbo help me make better images?” Another question (that I’ve also been asked more than once) is “Do I need to know any of this stuff to make good paintings and photographs?”

My answers, respectively, are “Maybe” and “No”. Which raises the very legitimate question of “Then what’s the point?”

My own interest in all this “mumbo jumbo” is grounded in finding practical answers to real digital darkroom problems. For example, I coded up an LCH color picker for GIMP because I wanted to modify a blue sky color to make it darker and more saturated. I was using Krita to paint colors into a black and white photograph, and I ended up spending a huge amount of time chasing that blue sky color around using Krita’s many HS"X" color modifying tools. Eventually I gave up, sampled the desired color from a night sky photograph, and used the ArgyllCMS command line “xicclu” utility to systematically modify the LCH values, retrieve the equivalent RGB values, dial the RGB values into Krita’s advanced color selector, and then paint using the resulting colors.

Unfortunately I never did get the sky color exactly where I wanted it, and then my computer died, taking most of the Krita kra files along with it.

So after building a new computer I retrieved the remaining Krita kra files, exported the painted image as a tiff, and modified the colors in GIMP-CCE using the LCH color picker, Hue-Chroma tool, and LCH blend modes. Here’s a link showing the “before” and “after” images: B2. Using color blend layers and a brush to recolor a photograph: http://ninedegreesbelow.com/photography/combining-painting-and-photography.html#recolor

If you would like to try using LCH to pick and modify colors, the LCH color pickers and Hue-Chroma tool are available in my patched “GIMP-CCE”, but not yet in GIMP-2.9 (Bug 749902 – Add Hue-Chroma operation/tool and LCH color selector). The LCH layer blend modes are in both versions of GIMP.

Oh, why did LCH enable me to quickly dial in the desired blue sky color when HS"X" did not? Because LCH is based on how people actually perceive color differences. The HS"X" color spaces are very poor substitute for LCH, from a time when computers were far too slow to calculate LCH values in anything like real time (HSL and HSV - Wikipedia).


Nice read; being a filter guy, I will have to agree with your words (a bit over my pedigree that is; lol). I do like to use Convolution techniques to amplify colors. Sometimes, I would assign aRGB to an image, convert it back to sRGB (this intensifies the color) and then create a convolution layer and apply that to the original to blend to taste (easy with layer properties to create convolution layers. Just copy the original image on top of your changed image and set the top layer to grain extract and merge down). :slight_smile: