Another Name for Zig-Zag Coding Is "Zonal Coding

Essay

Another name for zig-zag coding is "zonal coding". Suppose you invent a new zonal coding scheme for JPEG that simply discards anti-diagonals above the first few - i.e., we discard the higher frequency ones. Suppose we keep the first six zig-zag lines.
(a) How many coefficients are we keeping?
(b) How will we do, compared to keeping all the zig-zags (still using run-length encoding). Comment on both compression capability and image quality.