In an image obtained by picking up a dark object, gradation in a dark area is not sufficiently reproduced, and hence a blackened image is obtained. In contrast, in an image obtained by picking up a bright object, since more electric charges are formed in comparison with the charge storage amount of an imaging element, an image saturated in white is obtained. Both cases have a problem that images having inferior visibility are obtained since gradation characteristics of the images cannot be reproduced.
In order to solve this problem, a method of expanding contrast by generating a combined image from two images having different picking-up conditions is proposed in Japanese Application Kokai No. 2001-352486. In Japanese Application Kokai No. 2001-352486, a basic concept is to emphasize a certain space frequency component in an image, and high-frequency component of the image picked up with short exposure time is not emphasized, and the high-frequency component of the image picked up with long exposure time is emphasized for combining these images.
Further description will be made about characteristics of the image with low charge storage. Such an image has a large blackened area and has much noise in the dark area as described above, but the contrast in a bright area is high. Hereinafter, the image picked up with a low sensitivity as described above is referred to as “low-sensitivity image”.
Further description will now be made about characteristics of the image with high charge storage. Such an image has a large area saturated in white, but the contrast in the dark area is high as described above. Hereinafter, the image picked up with a high sensitivity as described above is referred to as “high-sensitivity image”.
As a method of picking up the low-sensitivity image, there are a method of reducing the exposure time, and a method of reducing the surface area of an aperture of a light-receiving element. On the other hand, as a method of picking up a high-sensitivity image, there are a method of increasing the exposure time and a method of increasing the surface area of the aperture of the light-receiving element.
Therefore, in Japanese Application Kokai No. 2001-352486, improvement in visibility of the image is aimed by emphasizing the high-frequency component of the high-sensitivity image while reducing noises by not emphasizing the high-frequency component of the low-sensitive image, and combining the respective images. However, the emphasized area is an edge of the dark area and low-frequency component in the bright area of the object, but no effect appears in the low-frequency component of the dark area and the edge component of the bright area. Therefore, the method in Japanese Application Kokai No. 2001-352486 does not correspond essentially to the method of emphasizing the contrast.
The method of emphasizing the contrast is disclosed in Japanese Application Kokai No. 2003-8935. In the method disclosed in Japanese Application Kokai No. 2003-8935, since the contrast with respect to a local area in a screen is emphasized, emphasizing of the contrast is achieved by calculating a superimposed value using pixel values in the periphery of a target pixel using a smoothing filter (epsilon filter) that stores an edge area of an input image, and determining the target pixel value is divided by the superimposed value as the contrast.
However, in Japanese Application Kokai No. 2003-8935, since there is no change between the pixel in question and the peripheral pixels in the blackened area or the white saturated area, and hence the contrast is 1, it is not suitable for emphasis. In other words, with the method disclosed in Japanese Application Kokai No. 2003-8935, since information is already lost, the contrast between the pixel in question and the peripheral pixels cannot be enhanced.
As described above, the contrast between the pixel in question and the peripheral pixels (hereinafter, referred to as “local contrast”) cannot be emphasized adequately from the dark area to the bright area by using the methods described above.
Accordingly, it is an object of the invention to provide an image processing apparatus that can generate an image with good visibility by emphasizing local contrast using at least two low-sensitivity image and high-sensitivity image and an imaging device therefor.