The present invention relates to a method and apparatus for converting continuous-tone images to bi-level images.
A known method of effecting this conversion simulates the gray scale of the continuous-tone image with local variations in the proportion of black and white picture elements or pixels in the bi-level image. This is typically done by using a threshold matrix, such as a dither matrix or a density-pattern matrix, to generate blocks of bi-level pixels. A dither matrix is a matrix of threshold values that are compared one-to-one with a block of continuous-tone pixels to generate a block of bi-level pixels. A density-pattern matrix is a matrix of threshold values that are compared with a single continuous-tone pixel to generate a block of bi-level pixels. Dither matrices are used in printers, copiers, facsimile machines, and other devices to convert continuous-tone images having comparatively high resolution. Density-pattern matrices are used are used to convert continuous-tone images having lower resolution.
Use of these methods alone, however, tends to produce a bi-level image that appears too dark. The reason is a non-linear relationship, known as a gamma curve, between perceived darkness and the actual density of black pixels in the bi-level image. Because of this relationship, the perceived darkness saturates, so that densities intended to represent different shades of dark gray look indistinguishably black.
The overly dark appearance and saturation can be avoided by adjusting the gray levels in the continuous-tone image according to the mathematical inverse of the gamma curve. Conventional image conversion is therefore a two-step process, comprising a gamma correction applied to the continuous-tone image, followed by the actual conversion from continuous-tone to bi-level form by dithering, density pattern generation, or a similar method.
A disadvantage of the conventional two-step conversion process is that it takes extra time and requires extra computing resources.