Interlacing usually happens in two-dimensions in image formats, if I understand it correctly (ie it first sends one pixel out of each 2x2 block).
The only thing is I think that is one of the actual pixels, rather than an average. In an ideal world you could send the average color of each 2x2 block, then send the 2nd, 3rd, and 4th pixels and use that data to reconstruct the 1st pixel.
