If using a value between 0 and 1 (or a pattern with middle gray values) you will notice noise by default since we use a stochastic sampling technique. However, integrators allow for smoother effects by specifying a depth in the Stoachastic Stochastic Opacity Depth parameter. Where each intersection counts as a depth limit. This will be explained below.

If rendering thick glass, presence is ignored as we need to know when we enter and exit a volume. A cutout here could cause issues with that condition.