The requirement for printing quality becomes more demanding as the uses of printer and scanner become frequent. The printing operation is required to be both fast and clearly. The most frequently used halftone methods for conventional printer are as below.
1. Ordered Dither
The image in a specific region is encoded in dot format to simulate gray level visually, that is, increasing gray level at the expense of reducing resolution.
2. Error Diffusion
By using gray level error of pixels along a specific direction, the next pixel is determined to be black or white by accumulation of error of specific ratio.
However, the above-mentioned methods do not provide satisfactory printing quality for text even thought they are quite good for picture. To overcome this problem, much effort has been devoted to this study, for examples, U.S. Pat. Nos. 4,411,015, 4,503,556, 4,504,972, 4,513,442, 5,091,964, 5,280,367, 5,335,290, 5,375,197, and 5,381,241. In above patents, the division of textual and picture block is executed by combining and separating pixel according to the similarity or connection of gray level. The divided block is further judged to be text or picture by histogram or shape thereof. However, considerable data such as data of the whole page is required for correct division, thus being not suitable for FAX machine or copier which require real-time printing.
Moreover, the above problem can be overcome by setting the edge threshold according to the forepassed statistical data and forcing the division of edge, however, the printing result has blurred or discontinuous edge.
It is an object of the invention to provide a method for enhancing the printing quality or gray-level text or RGB color text while the printing quality of picture is not influenced.
It is another object of the invention to provide a method for enhancing printing quality with adaptive edge threshold, wherein the edge feature of the row of pixels being processed is used to modify the edge threshold of the next row of pixels, thus providing real time printing.
It is another object of the invention to provide an algorithm to choose a most preferred edge feature among several edge feature to remove direction difference from the operation of edge operator and edge roughness from error diffusion.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which: