Compound effects and control layers

Several effects rely on a control layer (or layer map) as input. These compound effects use the pixel values of the control layer to determine how to affect the pixels of the layer that the effect is applied to (the destination layer). In some cases, the effect uses the brightness values of the pixels in the control layer; in some cases, the effect uses the individual channel values of the pixels in the control layer.

For example, the Displacement Map effect uses the brightness values of a control layer to determine how far to shift pixels of the underlying layer, and in which direction. The Shatter effect can use two control layers—one to customize the shapes of the shattered pieces and one to control when specific parts of the destination layer explode.

The compound effect ignores effects, masks, and transformations of a control layer. To use the results of effects, masks, and transformations on a layer, precompose the layer and use the precomposition layer as the control layer.

It is common to use a control layer that is not itself visible—that is, its Video switch is off.

Most compound effects include a Stretch Map To Fit option (or a similarly named option), which temporarily stretches or shrinks a control layer to the dimensions of the destination layer. This provides a pixel in the control layer corresponding to each pixel in the destination layer. If you deselect this option, the calculations for the compound effect are performed as if the control layer is centered on the destination layer at its original size.

You can create control layers by drawing or painting in an image-editing program, such as Adobe Photoshop.

Tips for creating control layers:

For many compound effects, neutral gray pixels in the control layer correspond to null operations. Therefore, a neutral gray solid layer is a good starting point for creating a control layer.
Apply the Turbulent Noise effect to a layer and precompose it to create a good control layer for turbulent or atmospheric results.
You can create a control layer by precomposing a white solid layer, a black solid layer, and a mask on the top layer that determines which areas are white and black. Increasing the feather of a mask softens the transition between black and white values.
The contrast between adjacent pixel values determines how smoothly the values change across the surface of the control layer. To create smooth changes, paint using a soft or anti-aliased brush, or apply gradients. To create abrupt changes, avoid intermediate shades, using a few widely spaced shades, such as 50% gray, black, and white.