The Knockout blending options control how layers within a layer set blend with the layers beneath them. Shallow knocks out to the bottom of a layer set, and Deep knocks out all the way to the background or just below the lowest layer in the stack. Let's take this feature for a test drive and see how it works:
Make the car layer the active layer. If you still have some leftover channel blending from the previous step, go in and turn on all the channel check boxes. Notice that this layer is contained within a layer set. When a layer's blend mode is set to Normal, you won't notice any difference between the two knockout settings, so let's change the car's blend mode to Overlay (see Figure 7). This gives a cool effect that makes it look as if the car is flying through the clouds. The overlay blend mode is controlling how the car layer blends with all the layers below it: the clouds, the green oval, and the black, gray, and white background.
Figure 7 The blend mode for the car layer set to Overlay.
Before you start tinkering with the knockout options, click on the Car & Title layer set to make it active. Notice that the blend mode for the entire layer set is Pass Through. This is a special blend mode that appears only with layer sets. With this mode, any layer blending within the set "passes through" the bottom of the set to continue blending with layers below the set. This is why you see the car layer blending with the green oval and the black, gray, and white layer, even though both of those are not in the same layer set. Change the layer set's blend mode to Normal and see how it affects the image. Now the overlay blending of the car layer stops at the clouds and doesn't continue (pass through) to the layers outside of the set. (See Figure 8.)
Figure 8 The differences between the Pass Through and Normal blend modes when used with a layer set.
Set the layer set blend mode back to Pass Through, and then double-click the car layer to summon the Layer Style dialog box. Open the Knockout menu and select Shallow.
The Shallow knockout setting means that the car layer is now "knocking out" any layers below it, or punching a hole through to the bottom of its layer set so that it is blending only with layers that are below it and outside the set. The overlay blend mode on the car layer is now affecting how the car blends with the green oval and the bottom layer. Because the Shallow setting knocks out to the bottom of the layer set, the clouds are no longer blended with the car layer. (See Figure 9.)
Figure 9 The Shallow knockout creates a knockout effect to the bottom of a layer set.
Now change the knockout setting to Deep. The car layer creates a knockout all the way through to the bottom of the image, which, in this case, means all the way through to below the bottom black-gray-white layer. Because there is nothing beneath this bottom layer, the overlay blend mode has no effect and the car layer appears normal. (See Figure 10.)
Figure 10 When the bottom-most element in a layer stack is a layer, the Deep Knockout option creates a knockout effect through the entire layer stack.
So far, you've tested this on a file that has a regular layer as its bottom-most element. One thing to be aware of is that the Deep knockout setting behaves differently if the file has a Background layer. To check this out, leave the car knockout set to Deep and click OK to dismiss the Layer Style dialog box. Make the lowest black-gray-white layer active; then, from the main menu, choose Layer, New, Background from Layer. This turns the layer into a background and changes how the Deep knockout interacts with it. Now the car layer knocks out, or punches a hole through, all the layers but does not knock out the background. The overlay blend mode for the car layer is now interacting with the new background. (See Figure 11.)
Figure 11 When the bottom-most element in a layer stack is a background, the Deep Knockout option creates a knockout effect to just above the background layer.
In the layers palette, return to the car layer, double-click it, and set the knockout back to None.