Spatial-filtering unit for performing adaptive edge-enhancement process

Claim 2

A spatial-filtering unit for spatially-filtering image data representing tone levels of an image, said spatial-filtering unit comprising: first filtering means for performing on the image data in the form of a reflectance-linear signal a smoothing process by which tone oscillations formed by the tone levels are suppressed, the first filtering means having an output providing smoothed image data, the reflectance-linear signal representing tone levels linear to reflectance of light reflected by an original image; second filtering means for performing on the smoothed image data which has previously undergone said smoothing process an adaptive edge-enhancement process which enhances sharpness of edges constituting parts having steep tone gradients in the image, said adaptive edge-enhancement process depending on a combination of a resolution of input image data and a coefficient of said second filtering means so that said second filtering means may selectively enhance portions of said image having steep tone gradients on a half-tone background; and data-form converting means connected between said first filtering means and said second filtering means for converting a first signal in the image data in the form of the reflectance-linear signal to a density linear signal prior to said adaptive edge-enhancement process, said density linear signal representing tone levels linear to densities of the original image.

Claim 3

The spatial-filtering unit according to claim 1, further comprising image-region classification means for classifying the image into character regions and other regions, the character regions being regions comprising characters in a white background, and said smoothing process and said adaptive edge-enhancement process being performed on said other regions.

Claim 4

A spatial-filtering unit for spatially-filtering image data representing tone levels of an image, said spatial-filtering unit comprising: first filtering means for smoothing the image data in the form of a reflectance-linear signal so as to substantially suppress tone oscillations formed by the tone levels, the first filtering means having an output providing smoothed image data; an edge determining unit for determining edge amounts of a character based upon a predetermined spatial frequency below a spatial frequency of a half-tone background; second filtering means for performing, on the smoothed image data which has previously undergone said smoothing process, an adaptive edge-enhancement process which enhances sharpness of edges constituting parts having steep tone gradients in the image, said adaptive edge-enhancement process depending on a combination of the edge amounts and a predetermined spatial frequency filtering, said edge amounts indicating steepness of the tone gradients of tone levels on the half-tone background being selectively enhanced by said second filtering means; and data-form converting means connected between said first filtering means and said second filtering means for converting a first signal in the image data in the form of the reflectance-linear signal to a density linear signal prior to said adaptive edge-enhancement process, said density linear signal representing tone levels linear to densities of the original image.

Claim 5

A spatial-filtering unit for spatially-filtering image data representing tone levels of an image, said spatial-filtering unit comprising: first filtering means for smoothing the image data in the form of a reflectance-linear signal so as to substantially suppress tone oscillations formed by the tone levels, the first filtering means having an output providing smoothed image data; an edge determining unit for determining edge amounts of a character based upon a predetermined spatial frequency below a spatial frequency of a half-tone background; second filtering means for performing, on the smoothed image data which has previously undergone said smoothing process, an adaptive edge-enhancement process which enhances sharpness of edges constituting parts having steep tone gradients in the image, said adaptive edge-enhancement process depending on a combination of the edge amounts and a predetermined spatial frequency filtering, said edge amounts indicating steepness of the tone gradients of tone levels on the half-tone background being selectively enhanced by said second filtering means; and data-form converting means connected between said first filtering means and said second filtering means for converting a first signal in the image data in the form of the reflectance-linear signal to a density linear signal prior to said adaptive edge-enhancement process, said density linear signal representing tone levels linear to densities of the original image.

Claim 6

The spatial-filtering unit according to claim 1, wherein: said first filtering means removes tone oscillations due to halftone dots present in the image; and said second filtering means enhances edges other than edges due to the halftone dots.

Claim 7

The spatial-filtering unit according to claim 1, wherein: said second filtering means comprises: edge-detecting means for detecting edges and boundaries of characters present in the image; and edge-enhancement means for enhancing the edges detected by said edge-detecting means.

Claim 8

The spatial-filtering unit according to claim 5, wherein said edge-detecting means comprises a spatial-frequency band-pass filter for detecting edges of the image having spatial frequencies corresponding to those of ordinary characters.

Claim 9

The spatial-filtering unit according to claim 5, wherein said edge-enhancement means enhances high spatial frequency edges.

Claim 10

The spatial-filtering unit according to claim 1, wherein: said second filtering means comprises: an edge-amount calculating filter for calculating the edge amounts; and an edge-enhancement filter for enhancing the edges in a manner depending on the edge amounts calculated by said edge-amount calculating filters, said edge-amount calculating filter having large output values for image parts of the image having low spatial frequencies, while said edge-enhancement filter has large output values for image parts of the image having high spatial frequencies higher than said low spatial frequencies.

Claim 11

The spatial-filtering unit according to claim 8, wherein said edge-amount calculating filter obtains each of the edge amounts as a result of obtaining a difference between the maximum tone level and the minimum tone level of tone levels of a relevant image part of the image.

Claim 12

The spatial-filtering unit according to claim 8, wherein said edge-amount calculating filter obtains a first derivative of the image data so as to obtain the edge amounts.

Claim 13

The spatial-filtering unit according to claim 8, wherein said edge-enhancement filter comprises a Laplacian for obtaining a second derivative of the image data.

Claim 14

The spatial-filtering unit according to claim 8, further comprising fine-adjustment means for fine adjustment of said coefficient as to the edge amounts to be taken into account by said edge-enhancement filter.