Bidirectinal bias correction for Gradient-Based Shift Estimation

This paper characterizes the bias of a gradient-based shift estimator (GBSE) and proposes a novel scheme to correct for the bias. The bias of GBSE comes from an inaccurate estimate of the image gradient energy, which is due to noise, aliasing and built-in low-pass filtering. For subpixel shift, the bias is linearly proportional to the true shift. The linear bias factor can be blindly estimated using two or more sub-pixel shift estimates of the same image pair, each estimate is performed on a slightly shifted version of the second image. Compared to the original GBSE, the new method reduces the bias by half at the same time improving the precision.