image the center pixels ofthe images correspond and have the same x y array coordinates the pixels around the center pixels also correspond corresponding pixels can be identified moving outward from the center of each image until corresponding pixels are identified for all the pixels in the smaller image there are no corresponding pixels for the remaining pixels in the larger image results of comparisons for those pixels are undefined and are not used in further processing in many instances groups of pixels in an image are selected for simultaneous consideration one such selection technique is called a 34 local window 34 for example if a 3 u00d7 3 subarray of pixels is to be considered that group is said to be in a 3 u00d7 3 local window the local window is centered over the pixel in the center of the window it isoften necessary to process every such group of pixels which can be formed from an image in those instances the local window is thought of as 34 sliding 34 across the image because the local window is centered above one pixel then moves and is centered above another pixel and then another and so on it should be noted though that if the local window is centered above a pixel along the side of the array a subarray of pixels cannot be formed around that pixel because no pixels exist to one side of that pixel generally when a local window is slid across an image it is not centered above the pixels along the sides of the array because certainpixels under the window would have undefined values.