DPP Home Technique (R)evolution Blending Channels

Tuesday, June 4, 2013

Blending Channels

In the first of two articles, we examine the fundamentals of this powerful Photoshop tool


This Article Features Photo Zoom

5) Calculations dialog box.
Be cautious with older files and lower-end scanners when blending with the blue channel, as it often contains significant amounts of noise. In fact, in some instances, blending channels can be used to replace some or all of the blue channel and thereby remove unwanted noise. Unlike blurring or despeckling, this method of removing noise won't compromise sharpness, but it may produce unwanted color shifts that will require subsequent correction.

A Good Preview
The possibilities are staggering. Is there anything that can help with the decision-making process? Yes. A good preview. You'll want to have multiple documents of the same image in different color modes (RGB, CMYK, LAB) visible at one time to simultaneously see the blended and the blendee. You may even want to make a side-by-side comparison of the component channels of a single document. To do this, use the Split Channels option in the Channels palette submenu. This command will break a single multichannel document into multiple single-channel documents. (If a file has layers, it must be flattened first to use Split Channels.) While doing this with several documents will quickly fill a screen, having the channels separated makes evaluating their relative merits infinitely easier.


6) Layer Style dialog box.
Classic Strategies
With so many possibilities, how do you choose one channel as the best candidate to blend with another, and how do you use it? First, identify the channel causing the problem. Then, find the channel with the best contrast in the areas you wish to enhance, at a low opacity, or replace, at 100% opacity. Stronger adjustments require higher opacities. Finally, deal with any unintended side effects.

Look to the Luminosity channel in LAB. Look to the Black plate in CMYK. Look to complementary colors. Complementary colors often contain the best possibilities for increasing contrast—Red and Cyan, Green and Magenta, Yellow and Blue—in highly saturated values.

What are you looking for? Better detail in shadows or highlights, better contrast and a similar tonal distribution. If you change the relative distribution of tones in a channel, you'll create a nonuniform color shift where some colors will shift more dramatically than others.


7) You can use these blend modes with Layer Styles
Blending channels can produce unintended side effects. There are times when it's better to achieve the necessary effect with this technique and accept its side effects, if the side effects are easier to cure than the initial problem. Typically, all that's required is a little dose of additional tonal enhancement, either to the master channel (tone and contrast) or a single channel (color). If you find this isn't the case, take this as a sign that this isn't the right technique for the problem you face.

Part two of this article will appear in the September/October issue of DPP. It will cover controlling the mix with blend modes, enhancing the blend and constraining the effect.

John Paul Caponigro, author of Adobe Photoshop Master Class and the DVD series R/Evolution, is an internationally renowned fine artist, an authority on digital printing, and a respected lecturer and workshop leader. Get access to a wealth of online resources with his free enews Insights at www.johnpaulcaponigro.com.

John Paul Caponigro, author of Adobe Photoshop Master Class and the DVD series R/Evolution, is an internationally renowned fine artist, an authority on digital printing, and a respected lecturer and workshop leader. Get access to a wealth of online resources with his free ennews Insights at www.johnpaulcaponigro.com.

 

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