Colour Correction - Curves and Adjustment Layers

Page 4

Real Images

There is a huge difference between performing (or watching) colour correction at a seminar and the real world of colour correction. Usually at a seminar the speaker has worked though the correction beforehand and has a reasonable handle on what is going to be needed to achieve a satisfactory result. The reality of facing an image for the first time is a little more complex and can require a couple of starts before the desired result is achieved.

The example we chose to show here is typical. Originally, artist Carol Tipping had created her image and printed it to her satisfaction at A4 on an Epson 1290. The print matched the screen and she liked the result. To your Editor’s shame he did not properly examine the image in Photoshop before committing to the printing of a 30” print onto the rather expensive Epson Somerset Enhanced, a 500gsm archival board material. The first print out was massively blocked up in the shadows with all the gorgeous brushwork completely hidden and not what the artist had intended at all. Suitably chastened we went back and followed the advice we always give out – look at the number values of the colours before you start.

The eyedropper was set to 5x5 average (ie 25 pixels per measurement) and run around, over the dark folds of the clothing. This revealed that none of the RGB values got above 30 points and many were down at 5 points. We knew from our profiling and ColourAudit data that such values printed as fully dense black and, had we had the foresight to measure first, we would have realised!

The trick is to make an Adjustment Layer for curves and lift the density of the darkest tones until they are no longer blocked and therefore start to print. If you have made and printed a grey scale as described in part 2 of this series you will have determined which RGB values (if any) fail to differentiate. This tells you which values to avoid in your image if shadow detailseparation is required. The problem is particularly acute with Somerset (and other) fine art papers, which tend to absorb ink deep into their structure (see the boxes). We limited the black output values to 21 RGB points by lifting the left-hand base of the curve upwards as shown in the screen grab. Carol did not want the face of the girl or the roses to be lightened and so the mask (which is provided by default with an Adjustment Layer) was painted with black and dark greys to protect those parts of the image from being lightened by the Adjustment Layer.

Gamut Warning

Photoshop provides a way of displaying the colours in an image, which will not print accurately because they are “outof- gamut”. By default the gamut warning displays the outof- gamut colours for the CMYK workspace set in preferences. However, if a Proof Colours profile is active, the gamut warning is specific to that profile. This enables the viewer to set and compare a number of profiles, which can assist in the selection of the print media or settings. In the screen grab, the two views show the gamut warning for Epson Somerset Enhanced and Epson Premium Luster, both with PhotoBlack inks in an Epson 7600. The Premium Luster delivers all colours in the image, the Somerset struggles throughout. It should be emphasised that this does not imply that the Somerset is an inferior product, the colour accuracy is actually superb; it is just that it does not hold the same depth of black as a semi-gloss paper. It should also be noted that the gamut warning is very conservative and that often the out-of-gamut colour will print very well. For colours which are out-of-gamut due to their saturation being too high, the use of Selective Colour and the Saturation slider will often assist in bringing the colours into gamut.

Rendering Intent

The rendering intent determines how any out-of-gamut colours are to be dealt with by a profile/driver. Perceptual intent shuffles all the colours, maintaining the difference between two out-of-gamut colours. Relative Colorimetric intent moves the out-of-gamut colour to the gamut boundary it there. Two out-of-gamut colours may thus be rendered identically using Relative Colorimetric intent. However for some images, RC produces a brighter result and can be favoured. The image creator is usually the best judge of which intent is rendering the most pleasing image. The gamut warning may be used to assist in the decision.

ADJUSTMENT LAYERS

Adjustment Layers are the method of choice for professional colour correction. They may be made with any of the colour adjustment methods in the drop down menu (Image>Adjust or Layers>Make Adjustment Layer). An Adjustment Layer sits above the image layer upon which it was made and affects all those layers below it by default unless the “Group with previous Layer” checkbox is ticked. Note that this option is not flagged if the icon at the base of the Layers Palette is used to make the Adjustment Layer initially. An Adjustment Layer may be turned on and off like a regular layer using the eyeball icon in the layers palette and is saved with a Photoshop file. An Adjustment Layer automatically has a mask attached to it and this mask may be painted upon like a layer mask, except that instead of hiding the image the masking will prevent or attenuate the effect of the colour adjustment.

An Adjustment Layer has the advantage that it effectively saves the colour adjustment changes but only enacts them when the image is printed or flattened. If you double click on the layer mask icon you retrieve the adjustment and can tweak it without applying two destructive pixel changes. Because colour adjustments are damaging to pixels and do so when a file is flattened for printing, the number used should be kept to a minimum ie do not apply two half strength adjustments – combine them in one Adjustment Layer. In general you should try to keep your adjustments to Levels and Curves and avoid the other methods (although you are at liberty to reject this advice if your adjustments are adequate for your own purposes). The Brightness and Contrast Adjustment, for example, is a linear transformation, affecting every pixel along a tone scale, some of which will be irretrievably clipped.

Above: The graph shows the deepest black that can be acheived with various combinations of paper and ink. The image on the right mimics the clogging up of the shadows. Printing a grey scale and then examining it by eye with reference back to the image will tell you which RGB values are too dark to be differentiated from their neighbours in the scale.

The Gamut Warning (Ctrl- Shift-Y) greys over those parts of the image which are out of gamut and should be studied in more depth before printing. They may or may not print to your satisfaction.

By default, all Adjustment Layers are made with a mask (red surround in diagram). Pressing the backslash key (\) reveals the mask in ‘frisket red’ by default. The mask prevents the Adjustment Layer changing the image in the masked areas or attenuates the effect if grey is painted on the mask. In this example, the lightening of the dress is confined to that part of the image only, leaving the face unaltered.

Curves are the kings of colour correction sitting at the top of the hierarchy. Their lead over their rivals, Levels, lies in the ability to bend the curve into an S-shape to locally control image contrast, something that levels cannot do. Here it is the curves function provided in Adobe RAW that has been adjusted into an S-shape to enhance the mid-range contrast while leaving the deepest blacks untouched and the sky detail at least partly protected. It does not make this dull shot perfect but it is certainly improved. In the final rendering (for an upcoming feature on filters) we abandoned the sky and found a new one – there is a limit even to curves adjustment.

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