Channels and Masks in Photoshop

Using Channels
The wonderful thing about channels is that they offer you greater control and selectivity when doing those very things —blending, filtering, and retouching. Channels bring one more level of control when editing your images. You can use individual channels for layer blending options, filters, and as starting points for masks. Channels also come into play when saving selections for later use or for adding spot (custom) colors to your image. You can also use channels to turn color images into nicely contrasted grayscale images. And finally, you can play around with the colors in an image by mixing up the channels. So bear with me. The topic of channels may be a bit dry and technical, but in the end, they’ll enable you to hold the envious title of Master Editor.

Understanding Channels
When you look at a color image, you see one big, 24-bit, composite collection of colored pixels. Technically speaking, however, Photoshop doesn’t see that at all. Photoshop perceives a color image as individual bands of 8-bit, grayscale images. RGB images have three bands; CMYK images have four bands. I know it’s strange to think of a color image as being composed of several grayscale images, but it’s true. Each one of these bands, or grayscale images, is a channel. Specifically, they’re color channels. Another way of relating channels to the real world is in terms of hardware. Here’s how the most common hardware handles color:
- When you offset print a CMYK image, the process separates the colors into four colors — cyan, magenta, yellow, and black. Paper passes through four individual rollers on the printing press, each roller containing one of those four colored inks.
- Scanners scan in RGB via a pass of red, green, and blue sensors over your image.
- CRT Screens display images via red, green, and blue tubes.

A little bit about bit depth
When you’re standing around the water cooler or the color printer, and you hear people talking about a 1-bit or an 8-bit image, they’re referring to something called bit depth. Bit depth measures how much color information is available to display and print each pixel. A higher bit depth means the image can display more information — specifically, more colors. For example, a 1-bit image can display two color values — black and white. That’s why a purely black-and-white image is called a bitmap image. Likewise,
- An 8-bit image has 256 grayscale levels (28). Grayscale images are 8 bit (1 channel × 8 bits).
- A 24-bit image has about 16 million colors (224). RGB images are 24 bit (3 channels x 8 bits).
- CMYK images are 32 bit (4 channels x 8 bits). CMYK images, however, are limited to the number of colors that are physically reproducible on paper, which is around 55,000.

Bit depths typically range from 1 to 64 bits. Scanners often allow you to scan an image at 64 bits. At 64 bits, each channel contains 16 bits, for a total of 4,096 levels of gray. This added color information gives you more data to work with when you adjust levels, curves, hue/saturation, and color balance, and when you apply filters such as Unsharp Mask.

Previously, the tools and commands available for 16-bit images were somewhat limited. This new version of Photoshop, however, gives you expanded abilities to edit 16-bit images (especially helpful for filmmakers). If necessary, when your adjustments are made, you can still convert your 16-bit image to an 8-bit image. To do so, make sure you flatten your images if they have any layers. Then choose Image-->Mode-->8 bits/Channel.

Briefly, you can use alpha channels to create, store, and edit selections, defining them not by a selection outline, but by black, white, and varying shades of gray pixels — in other words, a grayscale image. Black pixels represent unselected areas of the image, while white pixels represent selected areas, and gray pixels represent partially selected pixels. You can create spot channels when you want to add a spot, or custom, color to your image. Spot colors are premixed inks often used in addition to or in lieu of CMYK colors. All images, no matter what their color mode, have at least one channel. Grayscale, Duotone, and Indexed Color (for GIF Web images) modes have only one channel. RGB and CMYK images have three and four channels, respectively. They also contain a composite channel, which reflects the combination of the individual color channels and gives you the full color display.