In an interferogram, the variation of intensity background and fringe amplitude displaces the fringe centerlines and gives rise to some systematic phase error if fringe tracking techniques are used. The behavior of this systematic phase error is analyzed in detail. This phase error oscillates from zero in the middle between bright and dark fringes to its extrema in both bright and dark fringe centerlines, but it is not accumulated over the whole image. A 2-D envelope transform with correct envelopes is proposed to remove the systematic phase error. To illustrate, some simulated fringe patterns are processed and analyzed.