1. Field of the Invention
The present invention relates to a scan/print imaging system and, more specifically, to a method and apparatus of processing a series of image signals obtained by photoelectrically scanning a document.
2. Discussion of Related Art
As discussed in U.S. Pat. No. 4,742,400, a digital copying machine contains a printing unit in the form of a laser printer capable of printing two gray value levels at most. A scanning unit delivers image signals corresponding to image dots and capable of representing 64 gray value levels. To obtain binary image signals suitable for the laser printer, a conversion operation is carried out in which multi-value image signals are converted to binary image signals so as to produce a print corresponding to the original gray value as far as the eye is concerned.
The known instant apparatus comprises an image processing unit which generates a first group of binary image signals which are obtained after a conversion operation in which a number of image dots corresponding to the required gray value are arranged to a fixed pattern in a matrix of 8.times.8 image dots. This process is frequently termed dithering. A disadvantage of dithering, however, is that sharp transitions in a scanned image, for example in the case of letters or lines, are unsharp in the finished print due to the resolution-lowering effect of the matrix used. With such image information it is preferable to use a conversion operation based on thresholding per image dot and not per matrix of image dots. However, if letters or lines can assume different gray values, as is the case, for example, with colored texts, or if the background is colored, dithering with a matrix with fewer image dots may be an acceptable compromise. In the known apparatus, the image processing unit accordingly generates a second group of binary image signals obtained by a conversion operation in accordance with this principle. For this purpose, the image processing unit comprises an averaging unit which averages image dots over a submatrix of 2.times.2 and a submatrix unit which again subjects these average values to a dithering operation with an 8.times.8 matrix. In this way, reasonable edge sharpness is maintained while at the same time no gray value is completely lost.
Another disadvantage of dithering is the interference by the dither matrix with raster patterns resulting in moire effects in the print. In such cases it is better to use a conversion operation based on the known error diffusion algorithm. In this case thresholding is carried out per image dot and the rounding-off error made is passed to one or more neighboring image dots.
Where more than one conversion operation is used to convert multi-value signals to binary signals, as is the case in the known apparatus, it is necessary to select the image signals as to information content such as text or photographs. On the basis of such a selection the image signals can be selectively converted by one of the conversion operations or the image signals generated by the different conversion operations can be chosen selectively, depending on the image information represented by the image signals. In the known image processing unit, the selection means makes a selection of image signals from one of the two groups of binary image signals, the selection being determined, per image area, by the size of edge transitions in the image area. The selected binary image signals are then fed via the switching means to the laser printer.
However, an error may occur during the selection process in selecting a non-optimal group of binary signals so that the final print may, undoubtedly locally, be of a poorer quality than the original. The occurrence of an error in the selection may result, for example, in the case of weak information, as is the case, for example, with low-contrast letters or lines. Also, in an image area with image information in which the selection takes place right against a selection threshold, minor variations in image information may result in highly varying selections. A uniform character of an image area may be disturbed as a result. Also, image areas with image information increasing or decreasing uniformly with respect to spatial frequency, for example, may have an abrupt transition due to another selection occurring at a specific frequency.