Abstract: A system is described for reformatting halftone data for compression, wherein an original bilevel image is reformatted to produce another bilevel image that allows vertical correlations to be recognized by the compression technique, thus improving compressibility dramatically, with particular suitability for facsimile transmissions. In reformatting it is assumed that a selected halftone frequency H will satisfactorily describe an entire document, and each of successive sets of H consecutive lines are concatenated to form respective single lines. The thus reformatted lines have a clearer halftone periodicity offering greater correlation and permit more efficient coding by well-known standard bilevel compression algorithms (e.g., CCITT G3 (MR) or GF4 (MMR)). For an image with unknown pattern frequency, a technique for readily estimating the frequency for use in reformating the image is described.

Abstract: A method for rotation of a binary image by 180 degrees, includes the steps of: positioning first and second address pointers at the center of at the beginning and end respectively of an image to be rotated; indexing the first pointer in a first direction through said image; indexing the second pointer in a second direction through said image; exchanging, for each index step, a first image word at the first pointer with a second image word, at the second pointer if the first image word and the second image word have any two bits with different values; reversing all exchanged words; storing the reversed, exchanged words at said pointer locations; repeating the above steps until the first pointer and the second pointer have been indexed through all words in the image at which point the image has been rotated by 180 degrees.