Blending channels is complex. It often produces additional unintended color effects that may require further correction, such as shifts in hue that aren’t uniform across the tonal scale. Blending channels is neither the simplest nor the most direct path to color adjustment, but in certain situations (files that are exceptionally problematic), it may be the best path. The resulting benefits can be dramatic.
There are several ways to blend channels: Channel Mixer, Apply Image, Calculations and using channels as layers. Let’s review the options in detail.
The Channel Mixer (Layer > New Adjustment Layer > Channel Mixer) blends percentages of channels within a single document. It can be applied as an adjustment layer, so corrections made this way can be changed or masked indefinitely. It can’t be used to blend channels from two documents. The Channel Mixer is an excellent choice for making global (the same percentage of channels for the whole image) color to black-and-white conversions. If you want to control black-and-white conversions locally (different percentages of channels for different image areas), use channels as layers instead.
The commands Calculations (Image > Calculations) and Apply Image (Image > Apply Image) can also be used to blend channels. With these two commands, you can combine any two channels, from different documents, from any layer, at any opacity, with most blend modes. With Apply Image, you target the channel you wish to change. With Calculations, you blend to create a new document, a new channel or a new selection. Neither Calculations nor Apply Image can be used as adjustment layers or layers; consequently, corrections you make with either of these features are made permanently to an image. With Apply Image and Calculations, you can take advantage of two less frequently used blending modes not found with other tools (Add and Subtract), but you can’t take advantage of four frequently used blending modes (Hue, Saturation, Color and Luminosity)—even if you use the Fade command.
For the greatest control and flexibility, use channels as layers. How do you do this? Copy any channel and paste it into any destination as a layer. (Target a channel by clicking on it; copy that channel (Select All > Edit > Copy); then target the master channel (RGB) and paste (Edit > Paste).) You can activate, deactivate, mask, change or replace this new layer indefinitely. Use Layer Styles (double-click on the Layer icon in the Layers palette) to determine Blend Mode, Opacity, Advanced Blending, to select which channel is affected, and Blend If options, to determine how This Layer affects the Underlying Layer or which values of the overlying layer affect the values of the underlying layer. What’s more, you get a dynamic preview of any changes you make while you make them. The adjustments you make are flexible, so you can remove them or fine-tune any of the settings in future editing sessions. You even can blend two or more channels first, as layers, and then use the resulting new layer to blend with the Background layer. By turning channels into layers, you can achieve everything that the other methods achieve and more.
One File, Many Channels
You may be surprised to find that every file has at least 10 channels to choose from. How do you get so many? Consider the file in different color spaces—RGB, CMYK and LAB. Convert a duplicate file into another color space, and you can use any and all of the resulting channels. In fact, you can choose between many, many more channels when you consider that when converting to CMYK, there are five different options for generating a Black plate (None, Light, Medium, Heavy and Maximum) with two styles for each with two Separation types (UCR and GCR). But for the vast majority of situations, I recommend you try to keep things as simple as possible and stick with the standard three.
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.
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.< br />
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.