How to Use Paint and Edit Tools with Channels in Photoshop CS6
Sometimes, it’s better to edit individual channels rather than the composite image in Photoshop CS6. Mediocre flatbed scanners often reproduce, and digital cameras sometimes capture, an image that’s slightly soft or out of focus. You may want to counteract that effect by applying an Unsharp Mask or Smart Sharpen filter.
Before you do, you should examine each channel separately. You may find that the Blue channel contains a lot of garbage — artifacts, dithering, and other nasty crud.
Blue channels are notorious for acquiring this junk, so try to avoid sharpening this channel unless you really want to accentuate what’s already ugly.
Instead of applying the Unsharp Mask or Smart Sharpen filter on Blue channels, select the Red and Green channels in the Channels panel and then choose Filter→Sharpen→Unsharp Mask or Smart Sharpen. Similarly, you can apply a Gaussian Blur filter to a channel to soften the unsightly pattern (called a moiré pattern) caused by scanning a halftone.
Although Unsharp Mask, Smart Sharpen, and Gaussian Blur are a few corrective filters that you’ll use frequently, it is also useful to apply a special-effect filter to individual channels. Sometimes, applying a filter to the composite image produces an effect that’s, well, overdone.
Applying the filter to one or two channels can produce an effect that’s subtler and less in-your-face. For example, the image below has a filter applied to just the Blue channel for the flower on the top.
Using individual channels can also be useful for applying filters that produce monochromatic images, such as the Graphic Pen or Photocopy filters. (Be sure your foreground and background colors are black and white.) If you apply the filter to the entire image, you get a black-and-white image. If you apply it to an individual channel, you retain some color.
You can select a color channel and then edit that channel by using a painting or editing tool in the image. Keep these facts in mind:
Painting with white adds the color channel’s color at full intensity in the composite image.
Painting with black removes the color in the composite image.
Painting with a value of gray adds color at varying levels of intensity in the composite image.
For example, if you paint with white on the Blue channel in an existing image, Photoshop adds more blue to the color composite image. But if you paint with black, Photoshop adds yellow to the image because when you remove blue, you’re left with the opposite (or complementary) color — yellow.
To perform this channel magic, select the Brush tool and then select your desired brush size from the Options bar. Select your desired color in the Color panel. Select the channel you want to edit in the Channels panel and paint on the image. You can see the results by selecting the composite channel in the Channels panel.
The results are a little different if you try this technique on a blank CMYK canvas. For example, when you paint with black on the Cyan channel, your composite color image displays cyan. When you paint with white, you get no change.