1. A method of processing pixel values from a captured image, the method comprising: receiving an array of digital pixel values corresponding to a captured image; and computing a rolling sum of the array of pixel values using each digital pixel value of the array of digital pixel values, wherein computing a rolling sum comprises: selecting successive groupings of the pixel values, wherein each grouping is a consecutive portion of the array of digital pixel values and comprises N×M pixel values, wherein the center locations of successive groupings of pixel values are displaced by one pixel value in a first direction of the array, and wherein at least one of the pixels in each grouping is included in at least one other grouping; summing all pixel values in each of the successive groupings to form at least a first summed pixel value and a second summed pixel value; and forming an output image using at least the first summed pixel value and second summed pixel values.