Mcomp Node

The Mcomp node enables you to perform multilayer compositing on a virtually unlimited number of image inputs. You simply connect imagery from upstream nodes to the input connectors of the Mcomp node. Each time you add a new input to the Mcomp node, another input connector is automatically created.

Input Layers

You can connect imagery from upstream nodes in any order, and you can easily change the order of the inputs in the Layers tab of the node panel without reconnecting them. You can also temporarily disable layers to see the composite without the contribution of some inputs.

Note:
Because the Mcomp node is not limited to a specific number of inputs, and you can add and delete inputs to a particular composite, the top input to the Mcomp node is always the background image, while each successive input is layered over the previous one in a series of "over" operations. (Refer to the Over Node description if you need more information about this operation.)

Input Methods

The Mcomp node does not require an alpha channel for the background image (the top input in the Mcomp parameter list). However, an alpha channel is required for all other layers in the composite, either as part of a single input (RGBA, for example) or as the second of a pair of inputs (an 3-channel RGB input followed by a 1-channel Alpha input).

You can use either or both input styles within a single Mcomp node; that is, one layer can be a single input, such as RGBA, and another layer can consist of two separate inputs, such as an RGB image followed by an alpha input.

If the background layer is an RGB image (no alpha), when computing the composite Chalice treats the input as if there were an alpha value of 1 for every pixel.

Mcomp Parameter Tab

When you create a new Mcomp node, the Mcomp parameter tab is initially blank. A new layer entry is created for each input you connect to the Mcomp node. The controls provided for each layer are described below.

Transparency Tuner

The Transparency parameter enables you apply a multiplier value to proportionally increase or decrease the opacity of the matte as a whole. The alpha channel value of each pixel is multiplied by the value set in this parameter. Therefore, the default value of 1 specifies that the input values of the alpha channel will be used for the matte.

Values greater than one will increase the opacity of the matte; values less than 1 will decrease the opacity. For example, a value of 0.5 would reduce the opacity of the matte by 50 percent, and a value of 0 would create a black (transparent) alpha channel.

Premultiplication Status

Click and hold the Premultiplied button to display a popup menu that enables you to specify whether the input image for that layer is premultiplied (CGI), not premultiplied (live action), or Ultimatte (bluescreen). For more information, see About Premultiplication .

Tip:
Use the Data Format Preferences panel to set default state for this parameter to match the type of data you use most. This panel is described in chapter 11, "Setting Preferences ."

Layer Checkbox

This feature lets you turn an individual input layer off and on without disconnecting and reconnecting the associated input image from the Mcomp node. Click the Layer icon to toggle it to the opposite state. When this button is checked, the layer is enabled. When unchecked, the layer is disabled, and the associated Transparency parameter will be grayed out.

The background layer (top input) must be enabled; therefore, the Layer button for the background is grayed out.

Up Arrow

You can quickly and easily reorder each input layer in the composite. Pressing the Up Arrow icon for a particular layer moves the layer one step closer to the top of the list. The top entry in the Mcomp list of inputs is actually the bottom, or background, layer of the composite.

Note:
If you use a three-channel image for the background, it must remain the bottom layer of the composite, since the over operation used by the Mcomp node is based on the alpha coverage of the foreground image.



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