Information electronization is progressing recently, and there is growing a demand for saving or transmitting electronic documents in place of paper documents. Especially, along with the development of low-cost storage media and the increase in communication band, documents to be electronized now include not only monochrome binary data but also full-color documents.
Document electronization here goes beyond simply photoelectrically converting a paper document into image data by an image input device such as a scanner. The electronization also includes recognizing the contents of image data and segmenting it into regions with different properties, e.g., texts, symbols, graphics, photos, and tables contained in the paper document. Data conversion to an optimum form is also part of the electronization. For example, a character part is converted into character code information, a graphic is converted into vector data, a photo is converted into image data, and a table is converted into structure data.
As the first step of the document electronization, a region segmentation process is executed. More specifically, the contents of a document image of one page are analyzed, and the image is segmented into partial elements with different properties such as characters, graphics, photos, and tables.
An implementation example of the region segmentation process is U.S. Pat. No. 5,680,478. In U.S. Pat. No. 5,680,478, sets of 8-connected outline blocks of black pixels and 4-connected outline blocks of white pixels in a document image are extracted. Characteristic regions such as characters, pictures, graphics, tables, frames, and lines in the document are extracted on the basis of the shape, size, and set state of each block.
However, the above-described region segmentation process assumes on its operation principle that the input document image is monochrome binary data. To use this technique for region segmentation of a color document, the document image must be binarized in advance. However, region information contained in the color image is lost upon binarization.
Binarization of a color image is done by calculating a luminance threshold value from the pixel luminance distribution and converting each pixel of the image into a white or black pixel in accordance with the luminance threshold value. For example, a method of defining one luminance threshold value for the entire image of a page aims at accurately binarizing a character part. This method cannot therefore reflect, on the binary image, the shape of, e.g., a photo region where the luminance difference at the outline boundary is indeterminable on the basis of the luminance threshold value.
There is also a method of segmenting an image into a tile-like sub-regions and setting different binary threshold values for the respective regions. In this case, a smaller luminance difference can be reflected on binarization. However, no satisfactory binarization result can be obtained always for objects with random positions and shapes in a color image because the process result depends on how to segment the sub-regions.
Consequently, in a halftone image such as a photo or natural picture, the boundary position may be lost upon binarization. In this case, the accurate coordinates of regions are hard to recognize.
In, e.g., Japanese Patent Laid-Open No. 2002-312794, an edge image is acquired by processing a color image by using a differential filter, and the region segmentation process is executed after the edge image and a simple binary image are ORed. However, if a halftone region to be extracted and another region are in close vicinity, the halftone region position may erroneously be extracted because the regions cannot be segmented due to the influence of image reduction which is executed before the region segmentation process to stabilize or speed up the process or the influence of blurring by the filter itself.
There is a need for a region segmentation processing method capable of accurately extracting a halftone region even when a color or gray multilevel document image is input and subjected to a reduction process as a pre-process for speedup.