1. Field of the Invention
The present invention relates to an image processing apparatus for reproducing a halftone image from a digital image such as a pseudo halftone image.
2. Description of the Related Art
In recent years, various types of digital document data processing apparatuses such as facsimile systems and wordprocessors have been developed and are often networked. However, when resolutions of document data to be processed by different apparatuses in the network are different from each other, the document data must be enlarged or reduced before transmission or after reception.
Known methods of enlarging/reducing document images before transmission or after reception are a selective processing conversion (SPC) method, a logical sum method (Journal of the Institute of Data Processing, PP. 920-925, September, 1985), a projection method (Journal of the Institute of Image Electronics, Vol. 11, No. 2, PP. 72-83, 1982), and the like. Image data subjected to these conventional enlargement/reduction processing methods are limited to bilevel image data, e.g., character and line draw data or halftone image data, e.g., photographic data having an analog density value.
Document data to be processed by the recent document data processing apparatuses, however, include pseudo halftone image data such as image data processed by a dither method (to be referred to as dither image data hereinafter) and image data processed by an error diffusion method (to be referred to as error diffusion image data hereinafter) in addition to bilevel image data. When the same enlargement/reduction processing as in the bilevel or halftone image is performed for these pseudo halftone image data, gradation and sharpness of the pseudo halftone image may be undesirably lost. In addition, a moire pattern appears in the dither image according to the enlargement/reduction coefficient, thus greatly degrading the image quality.
In order to prevent the above drawbacks, pseudo halftone image data must be converted into halftone image data prior to enlargement/reduction processing. An image processing method described in Published Unexamined Japanese Patent Application No. 62-114377 is available as a convention method of performing pseudo halftone image/halftone image conversion. In this method, the pseudo halftone image is restricted to an dither image. Image data is divided into pixel blocks each having a predetermined size, and the number of black pixels (or white pixels) within each block is counted, and the counted value is averaged by using the size of the block, thereby estimating a halftone density value of each pixel block.
According to this method, however, when the predetermined block size is small, halftone image data completely free from a frequency component of the dither image cannot be reproduced, and generation of a moire pattern due to the dither image pattern cannot be perfectly prevented. To the contrary, when the block size is too large, the cyclic image having a repetition cycle shorter than the dither matrix cycle cannot be reproduced, thereby reducing the sharpness of the original image.
Another conventional example is an image processing method described in Published Unexamined Japanese Patent Application No. 61-35070. In this method, the pseudo halftone image is also restricted to the dither image. Image data are averaged in units of blocks whose size is equal to that of the known dither matrix, thereby obtaining a halftone image. Although this method can eliminate the moire pattern, reproducibility of information of a cyclic pattern smaller than that of the dither matrix is poor. As a result, sharpness of the original image data is often lost.
The both prior arts described above are effective when images to be processed are restricted to only dither images. However, they are not effective for other pseudo halftone images such as an error diffusion image. Moreover, when bilevel image data such as character and line draw data and the pseudo halftone image data are mixed in digital image data, no effectiveness can be expected.