Histogram of RMS contrast of small areas: we divided the full area of the intensity image into a grid of squares, each of which had a size of 5 pixels × 5 pixels. The grid is indicated as orange dots on the intensity image shown in the left panel. A histogram of the RMS contrasts computed at every square in the grid is shown in the right panel, for the three computation methods: LTE (black), 1D NLTE (red), and 3D NLTE (blue). The x axis has been clipped to show only bins with contrasts ≥10%.