While the 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, ; non-pure red, green, blue or white can cause halos in the selection edges when doing the chroma color extraction.
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In this editor, you can define the different alpha channels you want to get (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 groupsgroup.
Creating different alpha groups for the car body, the wheels and the whole car object
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As with the regular Alpha channel, you can render a completely white alpha also for transparent objects by enabling the Opaque option , useful - 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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