The magazine of the Melbourne PC User Group Colour Correction - for the bookshelf Major Keary |
Recently I was asked if I could recommend "any books about Photoshop that give clear information on the use of histograms and curves to edit colour".
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What are histograms and the curves used in colour editing? Histogram is defined as "A diagram consisting of a number of rectangles or lines drawn (usually upwards) from a base line, their heights representing frequencies of a series of values (or value-ranges) of a quantity" [Oxford English Dictionary]. Figure 1 is a histogram from a scanner. It is generated by the scanner software and allows for adjustment of `exposure' settings, usually of greyscale or black-and-white images. Adjustments are made by moving the three slide bars. |
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In Photoshop (and other high-end image manipulation tools) a series of curve diagrams are used for correction of colour (including grey and black-and-white). The number of curves varies from three to four, depending on whether RGB or CYMK is being used. Figure 2 is a curve 'dialogue' for a greyscale image using CYMK.
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Adjustments are made either by dragging the curve into a new shape, or by
changing values in the 'input' and 'output' boxes.
Playing around with curves in the hope of striking the right solution is like a
lottery in which a couple of million tickets have been sold, and you have just
one. To use curves effectively it is necessary to understand colour, colour
management systems, channels, the output medium, and screen calibration.
Channel has a number of meanings, but in colour-speak "a channel is one of
several portions of image, usually containing all the information pertaining to
a singe colour. Most images have a red channel, a green channel, and a blue
channel, as well as some number of alpha channels used for masking and other
purposes"
[Romano: Encyclopedia of Graphic Communications, Prentice Hall, 1998]. These
days a fourth channel, representing black, is also used. An alpha channel has
been described as a kind of wild card channel that can store user-selected image
data for manipulation purposes.
The medium on which an image is printed or displayed can affect the final
result. For example, newsprint not only requires allowance for dot gain, which
has implications for certain colours. Newsprint is usually not white and that
fact has to be allowed for in the adjustment of colour - especially skin tones.
Calibration of the computer screen is essential for colour correction where
output results are critical. For ordinary users it may not matter, especially
for Web publication where the end-viewers' screens may be doing all sorts of
strange things.
Professional Photoshop
The definitive text on curves is Professional Photoshop, which has the sub-title
The Classic Guide to Color Correction. It has an index entry: Curves, 1-402.
The book has 402 pages and all of them deal with curves and colour in one way or
another. It is presently in its fourth edition, published in 2002, and there has
been no word of a new edition, which is not surprising. In the introduction the
author says, "That so little about colour correction is specific to any version
of Photoshop means that there's no reason anybody should have to buy a new
edition ever time there's a new Photoshop".
What does change is technique and the development of better methods of colour
correction. Usually that has nothing to do with new versions of Photoshop, but
with the way professionals make use of long-standing tools.
The remarkable thing about this text is that a highly technical subject is dealt
with in plain language. It has been written for a professional audience, but
anyone with a serious interest in colour correction - and prepared to invest
time in studying the subject - will find it a valuable resource. The discussions
are remarkably lucid and span a wide range of colour-correction related topics.
Some of the topics make fascinating reading, such as the discussion of unsharp
masking that is illustrated by example images. This is a book that can be opened
anywhere for an engaging browse - uncommon for a technical text intended for
professionals, but the author is an uncommonly good technical communicator who
maintains touch with the real world, especially in the field of preparing images
for professional printing.
A companion CD contains many of the images used to illustrate the text and extra
instructional material. The images enable readers to replicate the effects of
the author's solutions, and to experiment for themselves.
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Dan Margulis: Professional Photoshop 4/e ISBN 0-7645-3695-8 Published by Wiley, 402 pp. + CD, RRP $82.95 incl. GST |
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Reprinted from the June 2005 issue of PC Update, the magazine of Melbourne PC User Group, Australia
