= Work with mattes =

A _matte_ is a grayscale image you use as either an alpha channel or a
[mask|/composite/masks]  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|/composite/masks] .

[Image:images/matte_1.png]

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.

tasks>>
    Generate mattes:
        * See  [how to create shapes|shapes] ,
          [gradients|fills] , and  [generate mattes based on image
          colors (keying)|keying] .

          You can also  [add noise|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|/composite/masks] .

        * Connect the image to the first input of a [Switch Alpha
          node|Node:cop2/switchalpha], 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|Node:cop2/premultiply] 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|Node:cop2/expand] or [Dilate
          Erode|Node:cop2/dilateerode] 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|Node:cop2/gamma] to reduce falloff at
          the low end (gamma < 1) or high end (gamma > 1).

        * Use an [Expand operator|Node:cop2/expand] with zero size and a
          small __Dropoff__ to add a bit of falloff to a map.

        * Use the [Blur operator|Node:cop2/blur] with a Gaussian blur to
          soften a matte. You can also use the [Edge Blur
          operator|Node:cop2/edgeblur], which blurs based on edge
          detection and so may preserve fine details of the map better.

    Combine mattes:
        * Use the  [compositing operators|/composite/layers]  to combine
          mattes.

          [Image:images/matte_combine.png]

          To get the union of two mattes, use the
          [Screen|Node:cop2/screen], [Maximum|Node:cop2/max], or
          [Add|Node:cop2/add] operator. To take the union but cut out
          overlapping areas, use the [XOR operator|Node:cop2/xor].

          To get the intersection of two mattes, use the [Minimum
          operator|Node:cop2/min]. To cut one matte out of another, use
          the [Subtract operator|Node:cop2/subtract].

          To use one operator to modulate another, use the [Multiply
          operator|Node:cop2/multiply].