Sub-pixel accuracy: psychophysical validation of an algorithm for fine positioning and movement of dots on visual displays

Many visual experiments call for visual displays in which dots are plotted with very fine positional accuracy. Spatial hyperacuities and motion displacement thresholds can be as low as 5 sec arc. On computer graphics displays small angular displacements of a pixel can be obtained only with long viewing distances which impose a small field of view. To overcome this problem, we describe a method for positioning the centroid of a quadrel (a 2 × 2 block of pixels) with very high accuracy, equivalent to 0.4% of a pixel width. This enables dot displays to be plotted with high positional accuracy at short viewing distances with larger fields of view. We show psychophysically that hyperacuities can be measured with sub-pixel accuracy in quadrel displays. Motion displacement thresholds of 16 sec arc were measured in multiple-dot and single-dot displays even though the pixel spacing was 1.2 min arc. Quadrel displays may be especially useful in studies of optic flow and structure-from-motion which demand a fairly large field of view along with fine positional accuracy.