Phase of the cross-spectrum of an image observed in the continuum and a numerically displaced image with a contrast inversion at high spatial frequencies. The red linear curve shows the phase corresponding to the imposed displacement of 0.05 pixel. The spatial frequency zero is at pixel 129. The cyan, blue, and magenta lines show how the measured slope would vary according to the width of the linear fitting interval at the origin, namely ±5 pixels, ±15 pixels and ±25 pixels, respectively.