Several effects rely on a control layer 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. For example, the Displacement Map effect uses the color values of a control layer to determine how far to shift pixels of the underlying layer, and in which direction. The compound effect ignores effects, masks, and transformations of a control layer, unless the layer has been precomposed. 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 effect layer’s dimensions. This provides a pixel in the control layer corresponding to each pixel in the effect layer. If you deselect this option, the compound effect’s calculations are performed as if the control layer is centered on the effect layer at its original size.

For many compound effects, neutral gray pixels in the control layer correspond to null operations. This makes a neutral gray solid layer a good starting point for creating a control layer. Apply the Fractal Noise effect to a layer and precompose it to create a good control layer for turbulent or atmospheric results.