You can make some parts of an actor opauqe and other parts transparent by specifying RGBA color+opacity for points or cells (either directly or via a lookup table).
All my tests leads to the impossibility to do that.
I you use point/cell data as RGBA & mix alpha values, as soon as at least one value has A < 1, then all the cell are drawn more or less transparent.
Please Andras, do you have a working POC of that ?