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Abstract

A tile-oriented technique and associated apparatus for manipulating a continuous tone (contone) image through image rotation, anamorphic scaling and digital halftone screening for use in illustratively implementing a page description language. Specifically, an incoming contone image is first partitioned into aligned non-abutting tiles. Overlapping blocks are then defined which will hold output data for corresponding tiles. To effect rotation and anamorphic scaling of the contone image, two-dimensional sampling increments, in fast and slow scan directions, are defined to relate movement between successive pixels in an output block to movement between corresponding pixels in the contone image. Similar, though independent, sampling increments, based in part upon screen angle and screen ruling, are defined for movement between successive pixels in a halftone reference cell. Incremental sampling occurs in the contone image to yield a corresponding sampled contone value. This value, in conjunction with incremental halftone sampling addresses, then defines a sampling location that is read in a halftone reference plane, the resulting output of which is single bit halftone data that defines a writing spot. Each tile in the contone image is successively processed with resulting output data for that tile being written into appropriate pixel locations in a corresponding block in the output image. Clipping variables, incrementally varying in two-dimensional fashion and in unison with the contone pixel sampling location, define valid output data for a contone tile that is to be written into a corresponding block.

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