Over, Atop, Inside, and Outside Nodes

Unlike the Mcomp and Zcomp nodes, these four composite nodes take only two inputs each, which must be four channels (RGBA). The exception is the Over node, for which the background input does not require an alpha channel.

The Over and Atop nodes require you to specify the premultiplied status of both inputs; for the Inside and Outside nodes, only the premultiplied status of the top (A) image input is relevant as the RGB data of the bottom (B) input does not contribute to the result. For more information, refer to About Premultiplication at the beginning of this chapter.

Note:
For the Over node, you can also input an Ultimatte image. Refer to Premultiplication with Ultimatte at the beginning of this chapter if you need more information.

Over Node

The Over node places a foreground image over a background image in an "A over B" composite. This is one of the most commonly used composites. The A image is placed over the B image wherever the A image has alpha coverage. The Over node accepts two inputs (only the A image input requires an alpha channel) and performs the following computation:

Rrgba = Argba + Brgba * (1 - Aa)

Click and hold the Input Image A or Input Image B button in the Over parameter tab to display a popup menu that enables you to specify whether the image is premultiplied, not premultiplied, or Ultimatte.

Tip:
To do an "under" operation, simply reverse the inputs to the Over node.

Atop Node

The Atop node places the A image over the B image wherever the A image has alpha coverage, but only where the A image falls inside the alpha coverage of the B image. The Atop node accepts two RGBA inputs and performs the following computation:

Rrgba = Argba * Ba + Brgba * (1 - Aa)

The Atop parameter tab provides checkboxes that enable you to specify whether or not Image A and Image B are premultiplied.

Inside Node

The Inside node places the A image wherever the A image falls inside the alpha coverage of the B image. However, none of the B image is visible in the result (even where the A image has no alpha coverage). Effectively, the B image acts as a matte, and the Inside and Outside nodes can be used to create complementary mattes from the same B image.

The Inside node accepts two inputs (note that only the B image requires an alpha channel) and performs the following computation:

Rrgba = Argba * Ba

The Inside parameter tab provides a checkbox that enables you to specify whether or not Image A is premultiplied.

Outside Node

The Outside node places the A image wherever the A image falls outside the alpha coverage of the B image. However, none of the B image is visible in the result, even where the A image has no alpha coverage. Effectively, the B image acts as a matte, and the Outside and Inside nodes can be used to create complementary mattes from the same B image.

The Outside node accepts two inputs (note that only the B image requires an alpha channel) and performs the following computation:

Rrgba = Argba * (1 - Ba)

The Outside parameter tab provides a checkbox that enables you to specify whether or not Image A is premultiplied .




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