1. A method for rotating a binary image in situ by 180 degrees, comprising the steps of: segmenting an image to be rotated into units of bits of a size exchangeable with each other; determining first and second halves of said image to be rotated; positioning first and second index pointers at first and second locations on the first and second halves of said image to be rotated; indexing said first pointer in a first direction through said first half of said image; indexing said second pointer in a second direction through said second half of said image; exchanging, for each index step, a first image unit addressed by said first pointer with a second image unit addressed by said second pointer; reversing the order of the bits in each said exchanged unit; storing said reversed, exchanged units at said pointer locations; and repeating said above steps of indexing, exchanging, reversing, and storing until said first pointer and said second pointer have been indexed through all units in said first and second values, respectively, of said image, whereby said image is rotated by 180 degrees.