1. A reference image generator, comprising: a memory for storing a second image that is acquired during a second exposure period and a reference image; and an image processor, adapted to: (i) define a first image as a reference image, wherein the first image is acquired during a first exposure period that precedes the second exposure period; (ii) define multiple reference regions of interest within the reference image, wherein an aggregate number of pixels within the multiple reference regions of interest is substantially smaller than a number of pixels of the first image; and (iii) update the reference image in response to content of the reference image, content of the second image and a first type of spatial relationship defined between the reference image and the second image; wherein a shift is defined in response to multiple spatial relationships between the multiple reference regions of interest and between multiple regions of interest of the second image.