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The External Matte node has evolved over the years. What was once purely a means for importing matte channels for defining opacity and limiting adjustments has expanded to become a way to import the RGB channels of a media file to be used for overlaying grain, texture, and stylized distress onto an image, and even as a way to use the channels of a clip itself as a matte.
Matte clips can be added to your project in one of two ways:
— You can add mattes using the Media page, either by attaching them to a clip so a particular matte is only available to a particular clip as part of a Clip grade, or you can add timeline mattes that stand alone in the Media Pool, which are then available to any Track grade.
— You can also add a matte to a clip using the Media Pool in the Color page, by dragging a clip from the Media Pool to the Node Editor. When you do so, that clip is turned into an external matte in the current grade, which you can use as a matte for secondary adjustments, or as a compositing layer (in conjunction with the Layer mixer) for mixing textures or images with your grade. That clip is also automatically attached to the Media Pool clip that corresponds to the clip you’re grading, as a clip matte, to help you keep track of which clips are using other clips as mattes.
For more information about adding matte clips in the Media page, see “Adding and Removing External Mattes” in Chapter 18, “Adding and Organizing Media with the Media Pool.”
Whether attached or unattached, mattes operate within a grade using EXT MATTE (external matte) nodes. EXT MATTE nodes have the following outputs:
An external matte connected to the first node of a grade
— EXT MATTE Outputs: Four blue square key outputs let you output the channels contained within the EXT MATTE node, but which channels are available affects what is output. If the EXT MATTE node’s source clip has RGBA channels, then these are available as Alpha, Red, Green, and Blue key outputs that you can attach to any other node’s key input. On the other hand, if the EXT
MATTE node’s source clip only has RGB channels, then the key outputs that are made available are Y (luma), Red, Green, and Blue, and a “Use Lum for Alpha Output” setting in the Node Editor contextual menu lets you use the Y channel as a matte.
An interesting aspect of these four outputs is that each one is dedicated to individual A, R, G, and B color channels. Ordinarily, External Matte clips are written with matte data written