Converting from block numbers to matrix indices:
starting matrix position in a direction = (block number in that direction, minus 1) times (block size in that direction), then plus 1
ending matrix position in a direction = (block number in that direction) times (block size in that direction), but never more than matrix size in that direction (because the last block in the direction might be a partial block)
Converting from matrix indices to block number:
block number in a direction = floor( (matrix position in that direction) minus 1) divided by block size in that direction) plus 1
For example if the block size was 64, the matrix index 219 would be block number floor((219-1)/64) + 1 = 4 in that direction. The first block would be index 1 to 64, the second block would be 65 to 128, the third block would be 129 to 192, the fourth block would be 193 to 256, so block #4 in the direction is correct for matrix index 219.
