Apply an image mask to a layer
Assigning an image mask is a two-part process. First, you create a blank image mask underneath a layer. Then you assign the image you want to use to create transparency.
Add an image mask to a layer
Select a layer to mask.
Choose Object > Add Image Mask (or press Shift-Command-M).
The image mask appears underneath the layer in the Layers list and Timeline.
With the image mask selected, do one of the following:
Open the Image Mask Inspector, then drag a layer to use as the mask from the Layers list to the Mask Source well.
Open the HUD (if the HUD is not visible, press D or F7), then drag the layer to use as the mask from the Layers list into the Mask Source well.
Important: Click and drag in one movement to successfully drop a layer in an image well. If you select the layer to use as the source, then release the mouse button, you lose the selection.
In this example, text is used as the image mask.
In the Mask Inspector, click the Source Channel pop-up menu and choose the color channel (red, green, blue, alpha, and so on) you want to use to create transparency.
For more information about choosing a color channel, see “Select the image mask’s color channel,” below.
Select the image mask’s color channel
When you use a layer as an image mask, you can choose which of the layer’s color channels to apply to create transparency.
Apply an image mask and assign a mask source, as described in the previous task.
In the Image Mask Inspector, click the Source Channel pop-up menu, then choose a color channel (Red, Green, Blue, Alpha, or Luminance).
Because alpha channels are basically 8-bit grayscale images, you can use any single color channel as an image mask. Luminance lets you use the aggregate luminance from the red, green, and blue channels of an image to create transparency.
For more information about the Source Channel pop-up menu, see Image mask controls in the Inspector.