The present invention relates to a printing apparatus and method suitable for printing a digitized image at an altered resolution without loss of image quality.
Improvements in image-processing technology have enabled facsimile machines, personal computers, workstations, and similar devices to exchange and process a wide variety of images. For example, an image received by a facsimile machine or television receiver may be captured into the memory of a personal computer, processed in the computer, then printed on a printer connected to the computer.
A problem that arises is that different devices generate digitized images at different resolutions, and printers also have different resolutions. Thus a personal computer may receive an image having a resolution of six hundred dots per inch (600 DPI), and have to print the image on an electrophotographic printer with a resolution of 300 DPI. The simple solution of converting the resolution by skipping alternate dots and lines leads to serious image degradation.
Japanese Patent Kokai Publication No. 71940/1994 discloses a printer that solves this problem by doubling the vertical resolution while halving the horizontal resolution. Each raster line in the original 600-DPI image yields two 300-DPI printed lines, with dots combined by AND logic in one line and exclusive-OR logic in the other line. This solution works well when the resolution ratio is 2:1, but does not apply to other resolution ratios.
Non-integer resolution ratios are particularly troublesome. When a personal computer's facsimile application software receives image data having a resolution of 200 DPI, for example, and prints the image on a 300-DPI printer, the resolution must be increased by a factor of 1.5. Conventionally, this is done by repeating every second dot and every second line, but that leads to uneven printing of fine detail, such as the fine lines that form characters in text. Moreover, if the image contains bit-mapped data generated from a natural image by dithering, the repeating of every second line and dot can generate moire patterns that greatly impair the appearance of the image.
Another known technique accordingly divides an image into a text part and a non-text part, converts the resolution of the text part by a mapping method that takes line thicknesses into account, and converts the resolution of the non-text part by a method such as error diffusion that avoids moire patterns. Text and non-text parts must be accurately identified, however, because if the wrong method is applied, the appearance is the printed output is considerably worsened. Complex and expensive processing circuits are required to recognize text and non-text parts of images with reasonable accuracy, and even these circuits become unreliable when text is superimposed on a natural image.
The problems described above occur not only with electrophotographic printers, but also with thermal printers and ink-jet printers which, like electrophotographic printers, form images from dots.