Colour Adjustment - Part 5

Paper lightness In our continuing series on colour correction we look at the problems associated with correction of real images, in which the adjustments have to be tuned to the known characteristics of the printer/paper/ink combination.

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. Colour adjustment

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 3 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 detail separation 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 and 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.

Colour adjusmtent Gamut Warning

Photoshop provides a way of displaying the colours in an image, which will not print accurately because they are "out of gamut". By default the gamut warning displays the out of 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 Lustre, both with PhotoBlack inks in an Epson 7600. The Premium Lustre 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 bring 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 and leaves 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 effects 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 Layer 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 and made when a file is flattened for printing the number used should be kept to a minimum i.e. 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 has a reputation for producing more data clipping than is ideal.

The SWPP 2008 Convention was an outstanding success,
we have 191 days to get ready for the 2009 convention - which starts on January 14, 2009

Photo Quote: No place is boring, if you've had a good night's sleep and have a pocket full of unexposed film. -Robert Adams