 The block padding/border information prevents the macroblock from closely matching with any other area of the video, and so, significantly larger prediction error information must be encoded for every one of the several dozen partial macroblocks along the screen border.DCT encoding and quantization (see below) also isn't nearly as effective when there is large/sharp picture contrast in a block.An even more serious problem exists with macroblocks that contain significant, random, "edge noise", where the picture transitions to (typically) black.