For JPEG and MPEG pictures are divided into 16 x 16 pixel macroblocks each comprising four 8 x 8 pixel blocks. For MPEG-1 used for Video CD, there are up to 396 macroblocks per frame or picture.
Both the H.261 and the MPEG-1 specifications define a unit within an image called a macroblock. This unit is a 16-by-16 block of pixels. Both H.261 and MPEG-1 encoders can switch from intraframe encoding to interframe encoding on a macroblock basis.
In a video compression scheme, a group of spacially adjacent pixels, usually forming a rectangular block, processed more or less together and somewhat separately from other pixels.