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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. To solve this problem, the Custom Alpha channel feature lets you create as many extra black & white alpha channels as you need, and each extra channel will contain only the objects that you choose, 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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The Custom Alpha channel feature allows you to get independent masks for the selected group of objects. Here the car body, the wheels and the whole car
Custom
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Alpha based on materials
New in Maxwell Render 3.1 is the ability to create alphas also based on materials. The procedure is exactly the same as adding object alphas, except that you choose a material (or several ones) that should belong to a certain custom alpha channel.
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