< Previous | Contents | Next >
When using a MediaIn node, you can modify how an embedded alpha channel is interpreted using the Clip attributes window. The Clips Attributes window includes an Alpha Mode menu setting to choose if the alpha channel is ignored, treated as premultiplied, inverted, or treated as nonpremultiplied (straight).
Controlling Premultiplication in Color Correction Nodes
Most nodes that require you to explicitly deal with the state of premultiplication of an RGBA image have a “Pre-Divide, Post-Multiply” checkbox. This includes simple color correction nodes such as Brightness Contrast and Color Curves, as well as the Color Correct node, which has the “Pre-Divide/ Post-Multiply” checkbox in the Options panel of its Inspector settings.
The Pre-Divide/Post-Multiply checkbox of the Color Curves node, seen in the Inspector
This checkbox allows you to connect an RGBA premultiplied image to the node and perform a color correction operation. It takes the RGBA image input, performs a divide operation to remove the semi- transparency and then performs a multiplication operation before outputting the color corrected image. This way, the color correction is done using a nonpremultiplied image but the resulting output is a Merge-friendly premultiplied image.
Controlling Premultiplication With Alpha Divide and Alpha Multiply
The Alpha Divide and Alpha Multiply nodes, found in the Matte category of the Effects Library, are available when multiple operations in a row expect a “straight” alpha channel. Instead of performing repetitive Pre-Divide/Post Multiply operations on each node, you can use these two nodes to book- end the other nodes. Simply add the Alpha Divide node when you want the RGBA image data not to be premultiplied, and add the Alpha Multiply node when you want the image data to be premultiplied again. For example, if you’re using third-party OFX nodes that make color adjustments, you may need to control premultiplication before and after such an adjustment manually.
A node tree with explicit Alpha Divide and Alpha Multiply nodes