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LAB – The Undiscovered Colour Space

By

Association Member James Reynard

Introduction

Everyone who uses photo-editing software to produce colour imagery is probably familiar with one of the many flavours of the RGB colour space. Even those who prefer monochrome imagery probably work digitally in RGB. Those who print their own work may also be accustomed to CMYK. In this article I would like to promote the use of a third colour space - LAB colour – arguably the most powerful space for processing colour images.

This is not a tutorial, so you will not see minute details of how to perform each step in Photoshop. There are web pages and the occasional book for that. The purpose here is to tweak your interest in a colour space that allows you to do things you could never do in RGB, a colour space that has slipped under the radar of many photographers and most Photoshop books.

My knowledge and understanding of LAB has been primarily gleaned from Dan Margulis’ excellent Photoshop LAB Color (subtitled The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace), published by Peachpit Press, and available on Amazon.

This book, like a Dostoevsky novel, is easy to read, but gets a bit heavy in places. Like the Dostoevsky, you will probably need to go back and read it again. This is not a criticism – LAB requires a subtle change of mindset, but once that has been achieved, you will wonder why LAB has not received more coverage in the plethora of Photoshop literature on the market.

LAB Structure

So, what is LAB colour? Think of it as the colour space that separates out brightness from colour.

RGB is divided between the Red, Green, and Blue channels. Any changes you make to contrast or overall brightness alters the colour too. Any changes you make to colours also cause a slight shift in brightness/contrast.

LAB is made up of the L (Lightness) channel and two colour channels, A and B. A contains colours primarily composed of greens and magentas, and B the blues and yellows. You can already guess the major benefit of this organisation – adjustments to contrast, brightness, and adjustments to colour are completely mutually exclusive.
Unlike RGB, where each channel has values between 0-255, with LAB the L channel has values of 0-100 (black to white) and the AB channels both contain values in the range –128 to +127. A positive value represents a warm colour (on the magenta or yellow side) and a negative number a cool colour (greens and blues). 0 represents a neutral tone – black, white, or grey (depending on the value in the L channel). All colours are formed from a combination of A and B, even if the value in one channel is 0.

Contrast and Brightness

In my colour workflow I frequently switch straight into LAB after making initial adjustments in the RAW processor. You can flip between RGB and LAB as many times as you like without damaging the integrity of your file, but I like to get heavy duty contrast and colour manipulation completed early on in the editing process.

The tool of choice in LAB is curves. Most of the other tools work there too, although some modes are not available (such as lighten and darken) – but much of your work in LAB can be achieved through curves.

As with RGB, each channel can be adjusted individually in curves, but unlike RGB, there is no master (LAB) channel, as this would be contrary to the nature of LAB, which separates brightness from colour.

The L channel curve acts just like the RGB curve, except it is better. Use it to adjust the contrast in your image as you normally would, to alter the black and white points, and to adjust overall lightness/darkness. It will not touch the colours (check the numbers in your info palette as you run the cursor over your image with the curves window open.

That is straight forward enough, but the real strength of LAB lies in the A and B channels. Applying simple curves to these can sort out colour casts and (de)saturate colours without you having to use the more limited hue/saturation tool and other colour correction tools.

Colour Correction

This topic can get complicated in any colour space, but the two cases presented here are kept simple to illustrate the basic methodology. One important point to note concerns the curves dialog box. My display reverses the default presentation, and the following descriptions are based on my display set up:

To remove a cast from the whole image (or to add one for creative reasons), use a (paradox defying) straight-line curve. Take, for example, an image that is slightly too warm. Open up curves, and select the B (yellow-blue) channel. You can push the curve to the right to warm the image up, or to the left to cool it down; do this by moving the top or bottom extremity of the curve respectively, so the curve no longer crosses the midpoint. In this example, move the top of the curve line to the left to cool the image down. You can check the results visually, or by looking at the numbers in the info palette whilst holding the cursor over parts of the image that should be colour neutral.

To remove a partial cast, you can use a curve that actually does what the name suggests, i.e. curve. Imagine a scene involving mixed lighting – say tungsten and flash – you forgot to pack your coloured gels, so you could only balance the image for one of the light sources, the flash lighting the main subject. The background, a white wall, was lit by tungsten lighting and consequently has a lovely yellow hue in the photograph.

Open up a curves window, and select the B (yellow-blue) channel. Click on the various areas of the not-so-white wall to establish the part of the curve that needs to be shifted, and then pin the rest of the curve down, so that you are isolating your changes to those areas that need them. Then drag the chosen part of the curve gently to the left to remove the cast. Again, use the numbers in the info palette to verify what your eyes are telling you.

Naturally, more complicated lighting scenarios may require more complicated solutions, but a simple curve will solve most problems. Also, it is important to recognise that the A and B channels are not mutually exclusive. A colour may be primarily dictated by the values in one channel, but the other channel affects its hue. So when you are altering the colours in one channel you will frequently need to make a more subtle change to the other channel to get things just right.

Saturation

In RGB the quickest and easiest way to alter colour saturation is with the hue/saturation window, but that has drawbacks:

Applying curves to the AB channels avoids the artefacts and brightness changes. It is also less of a blunt instrument than hue/saturation, giving you more control over the colour shifts that take place.

The example this time is a red telephone box in the middle of a green field. In true British tradition, it has a white ‘out-of-order’ sign on the door. You want the telephone box to really stand out from the grass around it, so a simple straight-line curve in the A channel is what’s called for. By dragging the top and bottom of the line inwards by the same amount you boost the reds and greens at the same time as driving them further apart from each other – the reds get redder, the greens greener and the colour contrast between them is greater.

By making sure the centre of your curve stays in the centre of the grid, you don’t cause any change to areas of the image that should be colour-neutral. The white ‘out-of-order’ sign hanging on the door of the telephone box is still white.

If you are boosting one colour channel, you’ll probably want to boost the other, so everything stays in proportion. But you may not want to boost the blue sky in the background quite so much, so hold back on the B curve a little bit.

If you then decide that actually you want the telephone box to really stand out, but the grass to look natural, or even dull, you can right click the layer you’re working on and use layer blending to isolate the colour boost in the A channel to colours that are more magenta than green. The telephone box is now highly saturated, and everything else is just as it was before you applied the curve.

Deft use of the blending sliders (which can break in two to feather the areas of transition) to select elements that are primarily magenta or green or blue or yellow can save a lot of time masking. Like most things, it’s worth checking the image at 100% magnification, because some areas (shadows, for example) can have much more of a particular colour than you expect, requiring the sliders be carefully placed – and maybe requiring a bit of layer masking on top. Needless to say, if you intend to use layer blending, you need to be working on either a duplicate layer or an adjustment layer.

In Conclusion

There is plenty more one could say about LAB, including its application in:

However, I hope this brief introduction to a couple of LAB’s major concepts will inspire you to learn more about a criminally under-used colour space.

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