In order to improve the quality of images picked up by a scanner or a digital camera, image data on these images are subjected to retouching. More specifically, digital data are put into a computer, on which a photo-retouching application is actuated to emphasize desired color contents and/or contrast. Or prescribed color contents are removed to weaken their influence. These are some of the various retouching options that are available.
For document images of which the data to be retouched mainly consist of characters or photographic images mainly composed of photographic elements, it is sufficient to retouch the image data constituting their principal parts, and the required processing is relatively simple. On the other hand, as DTP techniques have become highly developed to make possible ready preparation of presentation documents, in which characters and color graphics are integrated, or album images, in which characters and photographs are similarly arranged, there emerge frequent needs for so-called mixed images, in which characters and photographic elements are mixed.
Character elements of image data in such mixed images tend to have intermediate tones on their borders with the background, and consequently the characters often become blurred and poor in readability. On the other hand, photographic elements are subject to periodic noise attributable to high-frequency components, and this noise sometimes invites visually perceivable deterioration of picture quality.
Accordingly, when image data in mixed images are to be retouched, the contrast on borders between characters and their background is emphasized to clear the blurring of character elements. This can serve to enhance the readability of the characters. To do this, prescribed sharpening to emphasize contrast is executed on each picture element on the borders.
On the other hand for photographic elements, as the aforementioned noise consists of high-frequency components, each picture element with noise is subjected to a prescribed smoothing procedure to reduce high-frequency components.
In this context, border zones of character elements are generally referred to as edge picture elements, and the periodic noise attributable high-frequency components arising in photographic elements, as moiré picture elements.
Image data retouching applicable to such mixed images will be outlined below. First, image data of images picked up by a scanner or a digital camera are acquired as image data which express picture elements in a dot matrix form in multiple tones, and picture elements constituting these image data are successively scanned to compute differences of tone levels from surrounding picture elements in a prescribed range with reference to a criterion picture element. If there are many picture elements with large differences of tone levels, such elements are extracted as edge picture elements. With the positions of these extracted edge picture elements being taken into consideration, the range of edge picture elements to be sharpened are demarcated and determined.
Then, the picture elements constituting the image data are successively scanned again to compute differences of tone levels of picture elements in the prescribed range from the surrounding picture elements with reference to a criterion picture element, and if there are many picture elements whose differences of tone levels are relatively small, they are extracted as moiré picture elements. With the positions of these extracted moiré picture elements being taken into consideration, the range of moiré picture elements to be smoothed are demarcated and determined. Once the range of edge picture elements and that of moiré picture elements are determined in this manner, sharpening and smoothing are executed. In this case, the sequence is that sharpening is executed first, followed by smoothing.
At the sharpening step, the range of edge picture elements is extracted and sharpened, and the sharpened range of edge picture elements is held.
At the smoothing step, the range of moiré picture elements is extracted and smoothed, and the smoothed range of moiré picture elements is held. Thus, ranges to be sharpened and ranges to be smoothed, both distinguished element by element, are extracted individually, and processed individually.
And image data in each range having been appropriately processed and held are relatively positioned for integration, and retouched image data are thereby generated.