Colour Adjustment - Part 6

Tucked away in the top right corner of the colour picker in Photoshop are some radio buttons marked L, a, and b. These belong to a colour mode known as Lab and are often the last stopping place for a Photoshop user on their journey to expert status. However when used correctly (and often sparingly) Lab can provide some spectacular results for the power user. If you want to know more, read on.

What is Lab In an RGB Photoshop image you will find four channels, the Composite RGB, a Red, a Green and a Blue. Between them they define the actual colour and density of a pixel and they are inter-related. A neutral pixel will have RGB values that are all equal for example 121R 121G 121B is a mid tone 18% grey (in a system with a gamma of 2.2). If you try to remove a red cast using the Red curve by moving the curve you also change the tone density. You are continually forced to consider whether to correct the red by removing red or adding blue and green. Both will correct the colour cast but one will make the tone lighter, the other move will make the tone darker.

This is the key difference with Lab - the colour and tone density are divorced from each other and can be independently adjusted without any cross-over effects. The channels in Lab are the L-Channel, the lightness or Luminosity a-channel the greenness to redness (-128 is green +128 is red)* b-channel the yellowness to blueness (-128 is blue +128 is yellow)

*The green is actually close to an aqua and the red is closer to a magenta. When the Lab colour was defined (also known as L*A*B* or L*a*b* or CEI Lab) they chose the colour pairs on the grounds that we do not perceive any "greenyreds" or "yellowy-blues" - they are essentially opponent colours.

Practical Importance The practical importance of Lab is that colour and detail are not linked. A colour with aand b values of zero is always neutral, regardless of how light or dark it is. Therefore if you have an image that has neutral colour balance you can steepen the L-channel and add sparkle to the result by increasing the contrast OR you can steepen both the aand b channels whilst keeping their curves passing through the mid points without affecting the colour balance but increasing their saturation.

In the example landscape shown, the image is very slightly blue but overall the early morning light has not produced much colour saturation. Increasing the contrast was not appropriate as it would have veiled the highlights and clogged the shadows- the Lightness channel was therefore left untouched. More sparkle was however provide by steepening both the a and b channels by rotating the curves around the centre point of the curves graph. Note in the composite screen grab how flat looking the channels are and that the Lightness channel is a moderately good monochrome image containing all the tone density and details.

LAB

LEFT: The magenta-green and yellow-blue opponent colours show on the axes of the
Lab £D colour space, the lightness rises on
the vertical plane. The shape of a gamut is
shon on the right. Only those colours which lie
within this shape can be accurately printed by the
device, other colours will be
adjusted.

When to use Lab
There are three circumstances when Lab will score heavily over other methods of colour adjustment

1. When a lot of sharpening has to be applied to recover an image.
2. To apply a lot of blurring to control digital noise.
3. When drastic colour moves are required such as total colour
changes or massive colour bias corrections.

Smart Sharpening
When Unsharp Masking is applied Photoshop seeks out lines that seem to represent changes in the image and then increases the colour values to the dark side of that line and decreases the colour values to the lighter side. The "Amount" in USM is the amount of lightening or darkening applied, the "Radius" is the thickness (in pixels) over which that colour change is made. (for the record, the Threshold is the difference level that Photoshop deems to be an edge - zero it has a go at everything, at about 3 it leaves 35mm film grain alone, by 255 it does nothing at all). The problem is that it sometimes gets carried away and produces sharpening artefacts - halos of colour fringing around sharpened edges in the image.

To overcome this effect you can convert the file to Lab mode, activate only the Lightness Channel and just apply USM to that. No colour fringing can be created because there is no colour in the L-Channel - magic!

Smart Blurring The converse of the above is that burring either or both of the a and b channels may be carried out while leaving the detail untouched. This is useful for getting rid of digital noise as a lot of blur can be applied without much noticeable effect on the image - the detailed L-Channel being left unscathed. You can obtain slightly similar results from a digital camera image by blurring just the blue channel (which is usually the one containing the most noise). However any image structural detail in the blue will be blurred.

Drastic Colour Moves

Drastic colour moves are such things as changing the colour of a red dress to green (or red car, red flower, red bicycle - delete as appropriate), or huge colour bias in an image. Note though that a colour bias must be uniform along the tone scale from light to dark. This method will not rescue you from crossed curves.

out of gamut

Riding Cars

Photoshop DIGITAL GARDENING

The example shown on the previous page was relatively simple to mask but what if you need to change the colours of a bunch of flowers to send to the nice lady at the DVLA? The technique shown in the box opposite is essentially the same as for the car change except that the sophisticated blending of layers is exploited to control just which flowers change and which bits of the picture are left unchanged. The basis of the method is to perform the Lab achannel inversion to turn the red flowers green. Then activate the Layer Blending Options by double clicking the Layer in the Layer Palette and adjust the blending parameters until the required effect is achieved. Note that the sliders are split by Alt-Clicking on them when they may then be moved independently. To assist control still further we also applied a mask to the adjustment layer to perform the final tidying up of the colour change.

photoshop

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