Compositing Work with mattes

A matte is a grayscale image you use as either an alpha channel or a mask to isolate certain parts of the image.

For example, if you wanted to crop an image inside a hexagon shape, you would create a hexagonal matte and connect it to the node’s mask input .

Mattes can be simple (a shape or a gradient) or complex, based on the colors in the image you want to use the matte on.

To...	Do this
Generate mattes	See how to create shapes , gradients , and generate mattes based on image colors (keying) . You can also add noise to a matte to give it some variation.
Use a matte to isolate part of an image	Connect the matte to a node’s mask input . Connect the image to the first input of a Switch Alpha node, and connect the matte to the second input. The Switch Alpha operator copies the second input to the alpha channel of the first input. Turn on the Redo premultiply parameter to override any previous premultiplication. Use the Premultiply operator to use a matte stored in a plane of the image you want to isolate. Use the name of the matte plane in the Alpha parameter. Connect the image to the input of the node generating the matte. In the Image tab of the matte node’s parameter editor, change the Add image parameter to “C (RGB)” and “Multiply”.
Shrink or enlarge mattes	Use the Expand or Dilate Erode operator to shrink or enlarge mattes. The Dilate Erode operator is faster, but does not provide good quality for large changes. The Expand operator is slower, but produces higher quality.
Adjust edges and falloff of a matte	Because mattes are just grayscale images, you can use the full complement of image-manipulation tools in the compositor to adjust mattes. Use the Gamma operator to reduce falloff at the low end (gamma < 1) or high end (gamma > 1). Use an Expand operator with zero size and a small Dropoff to add a bit of falloff to a map. Use the Blur operator with a Gaussian blur to soften a matte. You can also use the Edge Blur operator, which blurs based on edge detection and so may preserve fine details of the map better.
Combine mattes	Use the compositing operators to combine mattes. To get the union of two mattes, use the Screen, Maximum, or Add operator. To take the union but cut out overlapping areas, use the XOR operator. To get the intersection of two mattes, use the Minimum operator. To cut one matte out of another, use the Subtract operator. To use one operator to modulate another, use the Multiply operator.