This Tutorial published at http://www.retouchpro.com
"Every file has ten channels." Dan Margulis, Professional Photoshop.|
I remember, years ago, looking at the channels palette for the first time and asking myself, "What possible use would anyone have for this stuff?"
Today, I don't touch a single pixel without looking over the channels first.
Whether RGB, LAB, or CMYK, I do the channel tour; Cmd/Ctrl 1, 2, 3, and sometimes 4.
One idea that has transformed how I work in Photoshop is that every file has ten channels. RGBLABCMYK. That's what I call The Power of Ten.
Photographers, working with light, typically know their way around RGB and have trouble with CMYK. Printers, working with ink, deal with CMYK and usually shy away from RGB. Neither one tends to use LAB very often, assuming they even know what it is.
The purpose of this tutorial is to show how the ten channels are more alike than you may think. RGB is simply the flip side of CMYK, and passes through LAB to get there. This is actually how Photoshop converts from RGB to CMYK and vice versa. It uses LAB as the crossover from one to the other.
Attached is a chart which illustrates what I'm talking about.
The columns are the three color modes; RGB, LAB, CMYK.
The first three rows are the opponent color pairs; Red to Cyan, Green to Magenta, Blue to Yellow. The last row is the contrast channels; Lightness in LAB and blacK in CMYK. Although the opponent color channels have a very clear relationship to each other, the contrast channels don't. They're the odd man out and lend themselves to some very unique and powerful editing techniques.
Notice that LAB has only two color channels, the A & B. Even so, the three opponent pairs are still there. When the A & B channels are equal and positive, the color is Red. When they're equal and negative, the color is Cyan.
Here's a little exercise to help get acquainted with the idea of ten channels. Make three copies of any image. Convert one copy to RGB, one to LAB, and the last one to CMYK. Now open the channels palette and start comparing the various channels from the three images. In the discussion thread for this tutorial I've attached a Photoshop Action which will set this up for you.
You should notice right away that the channels which are opponents look almost the same. Red resembles Cyan, Green resembles Magenta, Blue resembles Yellow.
The LAB channels are a little tougher to comprehend, they just look like subtle gray blurs. The key to understanding what they represent is that warm colors (magenta, yellow, red) are light gray and cool colors (green, blue, cyan) are dark gray.
Contrast and Color.
Another thing to consider about the ten channels is the unique ways they handle contrast and color. In RGB, each channel is equally responsible for both. In CMYK, much of the responsibility for contrast gets shifted to the blacK channel. In LAB, contrast and color are almost completely separated into the L and A&B channels respectively.
This suggests certain strategies for image manipulation based on color mode, keeping in mind that there will always be exceptions.
As noted, in RGB each channel deals with contrast and color equally. In practice however, this is not always the case. It has been observed that certain "types" of contrast tend to gravitate towards specific channels. In general it breaks down like this: Red > Contrast, Green > Detail, Blue > Noise.
Doing a luminosity blend using the Red channel can greatly improve contrast in sky scenes. Since detail lives mostly in the Green channel, a very serviceable B&W can be extracted from many RGB composites by simply discarding the Red and Blue channels. To clean up noise, one can apply various noise reduction filters to the blue channel without the usual lose of detail one gets when targeting the composite image.
Since CMYK is the "flip side" of RGB, one will notice that the Magenta channel holds the most detail and noise tends to land in the Yellow channel. This means many of the same techniques one can use in RGB still work. But it's the blacK channel that really offers some very powerful moves which are extremely difficult to duplicate when working in RGB.
My favorite CMYK specific maneuvers deal with sharpening. One can use much higher settings and achieve quite wonderful results targeting only the blacK channel. For an extra sharpening boost I'll apply USM to the "unwanted" color as well. The "unwanted" color is the lightest of the CMYK channels. In a product shot featuring something which is predominantly Red, the Cyan channel will be the lightest. Applying the same USM settings to the Cyan and blacK channels will add some real snap to said image.
Enhancing shadows is one place where CMYK can't be beat. A curve to the K is often all it takes to really bring out the detail in the darker tones of an image.
LAB takes the separation of contrast and color a step further. The L channel has all the detail, while the A&B channels deal solely with color. But since there can also be contrast in color, and color can affect detail, we can't really say that contrast and color are completely separate.
The most fundamental LAB technique is the ability to increase color variation. The typical example takes a drab landscape shot and applies a curves adjustment to the A&B channels. The result is a much more vibrant, eye-catching image. This type of enhancement is almost impossible to duplicate in color modes where contrast and color live as one.
When faced with a particularly difficult need for cloning, try converting to LAB and cloning just the L channel first. Then follow-up with a simple patch to replace color. In addition, Sharpening and Shadow/Highlight are much more powerful when applied directly to the L channel.
A very common retouch is to colorize a B&W image. LAB outshines the other two color modes here as well. One can add color by painting directly into the A&B channels. Since all detail lives in the L channel, it's impossible to harm detail in any way.
Dealing with contrast and color separately can be extremely powerful, and these ideas are just a few of the ways in which they may be manipulated. As noted before, the more one thinks in ten channels, the more their treasures will be revealed.
Detailed descriptions of techniques using channels is beyond the scope of this tutorial. But if you read any of my posts you'll find examples of the ways in which I incorporate these concepts into my workflow.
I hope this post has provided a little food for thought. If you'd like to discuss any of these ideas further simply start a thread somewhere and I'm sure to jump in.
This Tutorial published at http://www.retouchpro.com