Inverse Scan (Horizontal) in MPEG-4

I am unable to understand that why we go for horizontal or vertical scan at
all while decoding MPEG-4 bitstream? I guess Zig-Zag scan is the best then
why not only ZIG-ZIg. Is there any more compression by horizontal or
vertical scan? According to mpeg-4 standard zig-zag scan is done only when
ac pred flag is set, Why?


		
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Depends on how the coefficents appear, somtimes your matrix doesn't
turn out w/ zeros increasing as you get closer to one corner (where
ZigZag encoding would be most efficient)!