1. Field of the Invention
The present invention relates to a method for adjusting the brightness level of an image and an image processing apparatus and an image processing program for adjusting the brightness level of the image based on the method.
2. Description of the Related Art
Generally, the range of a brightness level which can be reproduced on photographic paper (photosensitive material) or on a monitor is narrower than that of a brightness level which can be obtained by an imaging element of a digital camera. Therefore, a function for adjusting the brightness level of the image is provided in an apparatus which reproduces a photographic image obtained by the digital camera on the photographic paper or the monitor.
For example, an apparatus and a method for outputting a print of an image after compressing the dynamic range of the brightness level of the image is disclosed in U.S. Pat. No. 6,122,076. In this apparatus, processing is performed so that a sharp image is output even if the dynamic range of the image is compressed. When the brightness level of the image is adjusted so that the difference in lightness/darkness is reduce, it is important that sufficient contrast is maintained so that a fine-detail structure in the image does not become unsharp.
Photography by an electronic endoscope is performed by emitting light which is guided by a light guide to a body cavity, which receives no natural light. Since the body cavity has a narrow complicated shape, light unevenly illuminates the body cavity. Therefore, the difference in lightness/darkness of an image obtained by endoscopy is much larger than that of a photograph of a person, which is taken against light. Further, the color of the body cavity is substantially the same through the entire region. Therefore, it is difficult to recognize a fine-detail structure in an image obtained by endoscopy compared with a photograph obtained by a digital camera or the like. Further, in endoscopy, it is necessary to reproduce a fine-detail structure in a region which is used for diagnosis. However, the region which is used for diagnosis is not always a dark region.
The method disclosed in U.S. Pat. No. 6,122,076 may be also applied to image processing of an electronic endoscope apparatus. However, the photograph obtained by the digital camera and the image obtained by endoscopy are different from each other, as described above. Therefore, a more sophisticated method for adjusting a brightness level, which is appropriate for image processing of the electronic endoscope apparatus, is desired.