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The MaterialID and ObjectID channels provide a multicolored material / object silhouette to be used as selection mask in a 2D image editor for further adjustments; but non-pure red, green, blue or white can cause halos in the selection edges when doing the chroma color extraction. The To solve this problem, the Custom Alpha feature generates an individual pure channel feature lets you create as many extra black & white Alpha mask per each individual object alpha channels as you need, and each extra channel will contain only the objects that you choose, providing a perfect selection mask for that object. letting you make perfect selections easily.
Enabling Custom Alphas
To enable custom alphas for certain objects, enable the Custom Alpha channel checkbox in the Channels panel and open the Custom Alpha Channels Editor.
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In this editor, you can define the different alpha channels you want to get have (left column), set a name to identify each channel, and assigning the object or group of objects to be included in the channel (dragging the objects into the right column). This assignment is not exclusive, so any object can be in more than one alpha groupchannel.
Creating different alpha groups for the car body, the wheels and the whole car object
As with the regular Alpha channel, you can render a completely white alpha also for transparent objects by enabling the Opaque option - useful if you want to maintain the full look of the transparency from the render instead of blending it with any other images in your composite, or leave it unchecked so the alpha grey value shows the transparency of the material.
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