1. An apparatus for compressing an m.times.n array of delta coded points, the location of each point being defined as an ordered pair (i,j) for 1.ltoreq.i.ltoreq.m and 1.ltoreq.j.ltoreq.n, comprising: means (1,3) for generating delta coded values x of the array in row major order; an exhaustive non-linear predictor stage (14) comprising a predictor (15-17) and a correlator (25), said predictor (15-17) being responsive to the delta coded values x from the generating means for predicting a value x at the array location (i,j) based upon previous values x.sub.1,x.sub.2,x.sub.3 at corresponding locations (i-1,j-1), (i-1,j),(i,j-1) according to the relation: EQU X = x.sub.3 x.sub.2 +x.sub.3 x.sub.1 +x.sub.1 x.sub.2; the correlator generating a value y correlating the acutal x and predicted values x according to the relation y=x.sym.x; and means (29) for variable length encoding runs of like consecutive values of y from the correlator.