Yes, the N+1 shift registers (where N is the size of the cube in one dimension) are daisy chained, one for the anode planes and N for the cathode columns. The first shift register on the line controls the anode planes and the rest of N shift registers control the cathode columns.
Indeed, this only writes out one byte:
digitalWrite (PIN_SS, LOW); //Start transferring data
SPI.transfer (0xFF); //Turn off the current plane (in fact all planes)(all 1s)
digitalWrite (PIN_SS, HIGH); //Done transferring data
That should be enough to turn everything off, because the value (all 1s) gets into the first shift register and turns off all the anode planes, so it does not matter what is left in the next N shift registers, which control the cathode columns. Or at least, that's how I imagine it...
cubeData is a matrix consisting of NxN bytes. The first index is the plane index, the second index is the row index inside the plane. Each element of the matrix contains one byte, which holds the data for one column inside the horizontal plane. In other words cubeData has NxN elements and each element contains N bits (obviously N cannot exceed 8 ).
Did I understand correctly that when you have X pieces of shift registers daisy chained, the first byte sent out via SPI.tansfer() goes into the first shift register, then the second SPI.transfer() pushes it into the second shift register, and so on?