Out of curiosity: why did they decide to go with a zig-zag pattern instead of grouping same frequencies of each block together? That way you'd end up with even

Out of curiosity: why did they decide to go with a zig-zag pattern instead of grouping same frequencies of each block together? That way you'd end up with even longer runs of zeros. edit: is that perhaps how progressive jpegs work and is that why they typically compress better?