Generally, in image processing, in order to enhance the sharpness of an image, an edge that is the boundary between image parts located within the image (the edge of the image part) is detected, and an edge enhancement process is performed in accordance with the shape of the edge (e.g., a stepped edge type, a protrusive edge type, a straight edge type, and the like).
For example, as in Japanese Laid-open Patent Publication No. 2000-295496, it is known that the secondary differential value of the pixel value of an edge enhancement process target pixel is calculated and added to the pixel value of the edge enhancement process target pixel, thereby enhancing the sharpness of an image. In this existing art, at the edge part, the luminance of the pixel changes greatly. Thus, in the edge, the secondary differential value is low at the center of the edge, but the secondary differential value is great at an end of the edge (a top or a bottom of a protrusive edge).
Further, for example, as in Japanese Laid-open Patent Publication No. 2004-172815, it is known that a pixel value is obtained by: calculating a secondary differential value from the pixel values of pixels that are before and after an edge enhancement process target pixel and are distant from the edge enhancement process target pixel by two pixels; adding the secondary differential value to the pixel value of the target pixel, and a median filter process is performed on the three pixels including the two pixels before and after the edge enhancement process target pixel.
Further, for example, as in Japanese Laid-open Patent Publication No. 2008-047950, it is known that, in order to suppress overshoot at an end of an edge, the maximum value and the minimum value of the values of pixels in filter taps are calculated, and control is performed such that a pixel value obtained by subtracting a secondary differential value from the pixel value of an edge enhancement process target pixel does not exceed the above maximum value and does not fall below the above minimum value. It is noted that the number of the taps is the number of multipliers each of which multiplies the number of pixels that are to be processed by a filter, by a filter coefficient.
Further, for example, as in Japanese Laid-open Patent Publication No. 2003-32514, it is known that the primary differential value of the pixel value of an edge enhancement process target pixel is compared to a threshold to recognize a part where a pixel value changes like a mountain or a valley, and a pixel at the top and a pixel at the bottom are detected. Thus, Edge enhancement is performed by adding a correction value to the pixel value of the peak pixel detected, and subtracting a correction value from the pixel value of the bottom pixel detected.