While 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.
The Custom Alpha feature generates an individual pure black&white Alpha mask per each individual object you choose, providing a perfect selection mask for that object.
To enable custom alphas for certain objects, enable the Custom Alpha channel checkbox in the Channels panel and open the Custom Alpha Channels Editor.
Enabling the export of Custom Alpha channels in Studio
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 groups.
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.
The masks will be exported as independent alpha images in the indicated format.
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