1. Field of the Invention
The present invention relates to a technology for reducing noise in image signals acquired by an image pickup device or the like.
2. Description of the Background Art
With the increasing number of pixels in image pickup devices used in digital cameras, digital movie cameras, and the like, the light receiving area per pixel is becoming smaller and smaller. The reduction in the light receiving area per pixel in an image pickup device causes a decrease in the light receiving sensitivity, and hence, in the signal-to-noise ratio of the captured pixel signal.
An image pickup device having a color filter array known as a single Bayer array outputs an image signal having only one color component per pixel. Therefore, a digital camera having such an image pickup device requires pixel interpolation in which each pixel is converted into a pixel having all color components of the color space.
Under such circumstances in which the light receiving sensitivity of the image pickup device is decreasing, it is essential to perform noise reduction so as to improve the signal-to-noise ratio of pixel signals. In order to eliminate the adverse effect of noise in image signals, it is effective to apply noise reduction to data not yet subjected to pixel interpolation, that is, RAW data.
As described above, it is effective to apply noise reduction to RAW data; however, when the noise reduction is too strong the RAW data loses its original information, thereby possibly causing a decrease in the resolution feeling or other adverse effects.
Such adverse effects are remarkable especially when noise reduction is performed with a well-known filter such as a median filter.
In Japanese Patent Application Laid Open Gazette No. 2006-238060, noise reduction is performed by determining RGB signal levels independently of each other. For example, in the case of R signals, the level difference between a pixel of interest and a neighboring pixel is calculated, and when the level difference exceeds a threshold, the pixel value of the pixel of interest is replaced by the pixel value of the neighboring pixel.
Even the method disclosed in Japanese Patent Application Laid Open Gazette No. 2006-238060 may erroneously replace the pixel value of a high luminance pixel by the pixel value of a neighboring pixel.
In Japanese Patent Application Laid Open Gazette No. 2001-144996, it is determined whether a pixel of interest is a contour component based on the color difference between the pixel of interest and a reference pixel. When the pixel of interest is determined to be a contour component, a median filter is used; otherwise, a smoothing filter is used.