Skin Tune

The favourite way to quickly correct an image using RAW files (or, now, JPEG files using Lightroom) is to ensure that you have something neutral grey present in the image and to use this to set the colour balance in Photoshop. This tactic is the most successful method but you do not always have the luxury of a true grey in your scene. However, one of the most likely tones to be present is skin. What if you could perform some magic in Photoshop whereby the tone of the skin were treated like a grey card? At first glance this seems like a tall order but SkinTune exploits the fact that the range of colours found across all the human races is relatively small (although not as small as they claim, their maths seems flawed to the writer). The human race comes in all sorts of shades from African, pale Nordics, and yellow Far Easterns. However, the range is governed more by lightness value with the hue and saturation being more limited. In general, complexions become paler the further you move from the equator because the strength of the ultraviolet declines making it less necessary to have a protective black skin. There is reasonably strong evidence that males prefer lighter females as mates but the evidence suggesting that women tend to be lighter than men is a little more flaky (end of anthropology lesson, if you are burning to know more visit http://dienekes.blogspot.com and look for sexual dimorphism).

When testing a product such as SkinTune it is necessary to include some basic tests of an everyday nature but then to push it to the limits and compare it to more sophisticated methods.

Test 1 – incorrect white balance

In the composite image shown, the as-shot rendering is shown in the top left. The camera was set to Auto White Balance and has been completely fooled, by the expanse of green, into over-correcting towards the complementary colour – magenta. The other five shots are the renderings produced by different correction methods and the table plots the error in the colour components of hue and saturation, compared to a reference colour of the Macbeth Light Skin Swatch. This is slightly flawed because our model, Jon, might not be true Macbeth Skin coloured but he has a reasonably ‘average’ sort of skin complexion. In a nutshell, the error, to start with, was 25.5 points (Lab Delta E) and we halved it using a single click of the mouse in SkinTune. However, it was not as accurate as using either default ‘daylight’ or ‘cloudy’ white balances in Adobe Camera RAW, and the best result was still a visually adjusted rendering on a calibrated monitor. Click balancing on the subject’s cap did a reasonable job, but the cap was not a neutral grey so it left some residual bias. The graph plot shows where the skin tones ended up on our own plot of commonly found skin tones. Most of the methods left the skin slightly red and slightly desaturated.

Test 2 – baby skin too blue

This is a problem we have discussed before in Professional Imagemaker. Babies are inherently pinker than adults as the melanin has not had a chance to acquire a ‘tan’ from the sun. However, because the baby skin is so translucent, and the paler melanin is not blocking out the underlying skin structure, the blue, venous blood can present the little darlings with rather ugly, blue arms. In the baby image shown, by member Martin Sellars, the facial tones are correct, the arms blue. Correcting the entire image using SkinTune left the baby’s face too yellow (the complementary of blue) and the quilt had taken on an ugly cast. The solution, shown in the lower right, was to use SkinTune on a duplicate layer (to correct the arm) and then mask out the over-yellow part of the image. The result compares quite well to a handcorrected attempt shown in the upper right, which also took some of the pink out of the baby. There is a common tendency for digital cameras to emphasise skin tones a bit on the red side.

Photo Quote: A mad, keen photographer needs to get out into the world and work and make mistakes. - Sam Abell