Encoded orphan pixels for discriminating halftone data from text and

Abstract

Text and line art image data is distinguished from halftone image data for selectively enhanced rendering thereof. Specifically, a method of rendering a raster pixel image from a stored bitmap includes (i) determining whether an orphan pixel is detected within a bounded sampling window of the bitmap, and, (ii) processing at least one selected pixel of the bitmap within the sampling window relative to the determining of whether an orphan pixel is detected. In general, an orphan pixel is defined as an isolated white or black pixel in a halftone image. In one embodiment, an orphan pixel is recognized as having a predefined pixel value and as having no other pixels with that value adjacent thereto within the sampling window. In the event an orphan pixel is detected within the window, then the bitmap within the window is determined to be halftone image data and the selected pixel is processed accordingly. In the event an orphan pixel does not exist within the window, then the bitmap within the window is recognized as text or line art image data and processed respectively. In addition, a method of forming a dither matrix for a halftone image includes generating a pixel pattern such that an orphan pixel exists within the pattern of the dither matrix. The orphan pixel is designed into the halftone image. This measure increases the probability that orphan pixels will exist and be detected in the resultant raster image array. Thus, during subsequent rendering of the raster pixel image, selective application of resolution enhancement techniques may occur based on whether or not an orphan pixel is detected in the sampling window.