opBlockOp Blockwise application of operator on matrices.
B = opBlockOp(M,N,OPIN,BR1,BC1,BR2,BC2) creates an operator that
applies the given OPIN operator on two-dimensional data in a
blockwise fashion. In the forward mode this means that the input
vector is reshaped into an M-by-N matrix, which is then divided
into blocks of size BR1-by-BC1. Next, we apply OPIN to each
(vectorized) block and reshape the output to BR2-by-BC2
blocks. These blocks are gathered in a matrix which is vectorized
to give the final output. In transpose mode, the input vector is
reshaped into a matrix with M/BR1-by-N/BC1 blocks of size
BR2-by-BC2, and the conjugate transpose of OPIN is applied to each
block as described above to give BR1-by-BC1 blocks. These form an
M-by-N matrix which is vectorized for output. When omitted, BR2
and BC2 are respectively set to BR1 and BC1 by default.