Masks

Various reconstruction algorithms support projection data and reconstruction
volume masks. These behave as follows.

The projection data elements corresponding to locations with SinogramMask
value 0.0 will be ignored during the reconstruction. Similarly,
the reconstruction data elements corresponding to locations with
ReconstructionMask value 0.0 will be ignored during the reconstruction, and
their values will be preserved. (Mostly, see note on constraints below.)

The algorithm will behave as if the rows and columns corresponding to the
masked voxels and projection data elements have been removed from the
projection matrix entirely. In other words, it will iteratively try
to match the projection of the non-masked voxels to the non-masked projection
data elements.

NB: MinConstraint/MaxConstraint will affect even masked voxels.