Advanced Channel Blending
This next feature can be really baffling, especially if you are just getting comfortable with the idea that an RGB Photoshop image is separated into three main color channels of red, green, and blue. Even if that concept is old hat to you, channel blending can still be challenging to understand.
To explore this feature, you're going to use the top layer in the stack, called spectrum bars. This layer contains the three color-spectrum gradients that are arranged over the black (level 0), 50% gray (level 128), and white (level 255) areas of the bottom layer. Before getting into the actual channel blending, keep in mind that neutral colors (black, gray, and white) represent equal values of all three color channels. So, the 128 gray area is actually R=128, G=128, and B=128. The white area is made up of maximum intensity (255) of all three colors, and the black represents the total absence (0) of all three colors.
Make sure that you have a clear view of the info palette. If you're working on the downloaded file, notice that there are three color sample points in the bottom of the info palette. These are placed on the right side of each of the spectrum gradients. If you can't see the actual points in the image, select the eyedropper tool. If they still don't show up, go to the pop-out menu on the info palette and choose Show Color Samplers. With the spectrum bars layer active, double-click it to bring up the Layer Style dialog box again.
Just under the Fill Opacity slider, click inside the R check box to turn off the red channel for the spectrum bars layer. In Figure 4, notice that the spectrum bars have each changed in different ways, depending on the background color beneath them. See Figure 4.
Figure 4 The spectrum bar layer with its red color information turned off. All red values in the spectrum bar are now coming from the underlying layers.
This blending control turns off the visibility of the red channel data for the active layer. The original red channel data for that layer is then replaced by the red values present in the underlying layers. If you look at the color sampler numbers in the info palette, you'll see that the red values for each are the same as the underlying tones: 0, 128, and 255. For the black area on the left, there is no red in black (level 0).
Next, turn the red back on and try the same thing, first with the green and then with the blue (see Figure 5). Check out the values in the info palette as you do this. You can also move your cursor over different areas of the spectrum bars and look in the info palette to monitor values not shown by the color sampler points.
Figure 5 Examples of the advanced channel-blending controls using all three color channels.
Turn all of the channel check boxes back on and exit the dialog box. Next, you'll see how this looks when applied to an image layer. Make the car layer active and then double-click it to bring back the Layer Style dialog box. Go through the same procedure again with the channel check boxes, and see how this changes the appearance of the car layer. Notice that when a color is turned off, not only do certain areas of the car layer become less visible (the red garage, for example, when red is deactivated), but areas of the clouds layer below become visible as the individual channel tones in the clouds replace the tones that are turned off in the car layer. Because the clouds are mostly light areas, they contain high amounts of all three colors, so they show up quite well no matter which color is turned off in the car layer. (See Figure 6.)
Figure 6 Previewing the channel-blending effects using an image layer.
This is pretty complicated stuff and gets down to the basic math that's under the hood of all digital images. Don't feel bad if you're not getting it right away and it still seems like mumbo jumbo. When I was first learning Photoshop, features like this just left me shaking my head, thinking, "Well, I don't get it now but maybe some day I will." Keep playing around with these concepts, and always pay attention to the color numbers in the info palette when you're working with images. It'll eventually begin to sink in and not seem so strange.