ID Mask Node
The ID Mask Node can be used to access an alpha mask per object or per material.
The ID Mask node is superseded by the Cryptomatte Node. Cryptomatte is more feature complete and supported by Cycles and EEVEE. It is recommended to use this feature moving forward.
Selection of the previously specified index.
This post-processing filter smooths the mask edges. See Anti-Aliasing.
The mask is white where the object is and black where it is not. If the object is transparent, the alpha mask represent that with gray values.
An index can be specify for any object or material in the scene. The Object Index can be set inand for the Material Index. To be accessible after rendering, the Render Engine must be Cycles, and Object Index or Material Index render pass has to be enabled.
In this example, the left rear red cube is assigned Pass Index 1, and the right cube Pass Index 2. Where the two cubes intersect, there is going to be noticeable pixelation because they come together at a sharp angle and are different colors. Using the mask from object 1, which is smoothed (anti-aliased) at the edges, we use a Mix Node set on Multiply to multiply the smoothed edges of the image, thus removing those nasty lines, thus, being smoothed out.
Volume Objects are not supported.