RGB vs Lab colour – What it is and how you can use it when processing your images

Most of us probably never leave the realm of editing our images in a regular RGB colour space. But many applications also allow you the option to edit your images using Lab colour. What is Lab colour? How do we use it? and how does it benefit us?

Aurélien Pierre, developer of filmic module for the open-source editing software Darktable, recently put together a massive article (English version) covering everything you need to know about working in Lab colour. And while some of the steps might be specific to Darktable, the principles can be applied to many different editing applications.

The color space CIE Lab was published in 1976 by the International Commission on Illumination (CIE), in an attempt to mathematically describe the color perception of the average human being. Lab space aims to decouple the brightness information (L channel) from the chroma information (channels a and b) and takes into account the non-linear corrections that the human brain makes to the linear signal it receives from the retina.

In the article, Pierre covers many factors of working with Lab colour, and particularly about its downfalls. Lab isn’t some perfect way of working with colour, and there are times when you won’t want to use it (particularly for high dynamic range images). Understanding how to, though may change the way you think and process your own images.

The big disadvantages of Lab are:

1. It doesn’t work well for strong contrast (> 7 EV), and especially outside the range [1:100] Cd/m²,
2. It is not linear in hue, i.e. if one fixes a pixel’s a and b chromaticity components and changes only its brightness L, the same hue would be expected at a different brightness (this was the design purpose of the Lab space), however there is a slight shift in the hue, more or less marked depending on the original color of the pixel.

Unlike RGB where you start off with blackness and add varying levels of red, green and blue brightnesses to form your final pixel colour and brightness value, LAB uses the three channels representing “L” (lightness), “a” (Red/Green) and “b” (Blue/Yellow). It sounds like a strange way of working at first, but when you get used to it, it can be quite valuable for correcting or altering colour.

As I said, there are times when you won’t want to use it, which is probably most of the time, to be honest. Not just for the reasons mentioned above, either. It’s just more difficult to work with than RGB for many tasks. But there are also times when Lab colour allows you to do things that RGB simply can’t – especially for colour correction and toning.

Pierre covers how Darktable handles Lab as well as how developments made in the filmic module for Darktable have helped to overcome some of the limitations. Darktable’s worth checking out if you haven’t seen it already, so head on over to their website to find out more. It’s open-source and completely free, so you have nothing to lose by doing so.

If you speak French, you can read the original article here. If you don’t, there’s an English translation here. There’s a lot of information in there both on Darktable as well as general colour and workflow theory that can be applied to many other software and workflows.