DESCRIPTION:

Add a given plane to all planes in a volume.
The name is somewhat misleading, as its main use is to subtract
a given plane to allow for digital subtraction angiography.
If planefact and/or bgfact are not given, they default to -1.0
and 1.0, thus yielding subtraction of the plane as standard
behaviour.
If offset is not given, it defaults to 0, if both factors are
positive and 1000 otherwise.