1. Field of the Invention
The present invention relates to a color image signal processing method of reducing hue turn of a color image that is picked up by an imaging device such as a digital still camera, a digital video camera, or the like, a color image outputting device such as a color image printer, a color monitor, a digital color television set, or the like, and an imaging device such as a digital still camera, a digital video camera, or the like.
2. Description of the Related Art
In the solid-state imaging device such as CCD or the like used to pick up the color image by the digital still camera, the digital video camera, or the like, a color filter in any one color of three colors of red (R), green (G), and blue (B) is provided to each surface of each pixel (photodiode), and a signal charge that responds to an amount of incident light passed through each color filter is accumulated in each pixel. Then, the signal charge that responds to an amount of incident light in each color is read from each pixel. Thus, the color of the incident light on each pixel position can be reproduced from respective amounts of signal charges in three colors.
The solid-state imaging device reproduces the color of the incident light by classifying the incident light into three colors and then accumulating an amount of signal charge, which is in proportion to an amount of incident light in each color, in each pixel. In this case, there is an upper limit to an amount of signal charge that each pixel of the solid-state imaging device can accumulate. Thus, there is such a restriction that the signal charge that exceeds an amount of saturation charge of the photodiode constituting the pixel cannot be accumulated.
In other words, if an amount of signal charge that responds to an amount of incident red-color light, for example, out of the incident light exceeds an amount of saturation charge of the photodiode because of generation of the overexposure, the color that is reproduced by amounts of signal charges of the green color and the blue color, which do not reach an amount of saturation charge respectively, and an amount of signal charge of the red color (which reaches an amount of saturation charge) lacks a tinge of red. Therefore, the so-called “hue turn” is caused.
In the recent solid-state imaging device, the number of pixels is increased enormously with the progress of the miniaturization technology, and thus a resolution force has a capability that comes up to that of the silver photography. However, because the pixel is miniaturized, each pixel is reduced in size correspondingly and then an amount of saturation charge of each pixel becomes small. Thus, there is such a problem that the image picked up by the solid-state imaging device does not come to the silver photography in respect of the color reproducing characteristic. In particular, there is such a problem that, since the human eye is sensitive to the flesh color, yellowish change of the flesh color stands out clearly when such flesh color is picked up at the overexposure.
Another reason may be considered as the cause that brings about the hue turn. First, respective amounts of signal charges corresponding to R, G, B, which are input from respective pixels of the solid-state imaging device, are converted into, for example, 10-bit digital data by an A/D converter, and then converted into, for example, 8-bit digital data by the γ transformation based on the γ characteristic shown in FIG. 34. Then, such data are recorded as the final image via various signal processes. When the image is displayed on the standard CRT, the characteristic curve of the relationship between the output luminance and the luminance of the original scene is given by a straight line, where an ordinate denotes log [maximum output luminance/output luminance] and an abscissa denotes log [input luminance (luminance of the original scene)] (where a unit is arbitrary). However, as shown in FIG. 35, actually a gradient of the characteristic curve in the high luminance range is set gently such that the high-luminance image can also be expressed.
Ratios of the input signals R, G, B in any color (e.g., R/G, B/G) are constant how an amount of exposure should be changed. That is, intervals between R, G, B are maintained constant on the abscissa in FIG. 35. Therefore, ratios of the output signals Ro, Go, Bo of this color (e.g., Ro/Go, Bo/Go) are constant in the range in which the characteristic curve is given by the straight line, and thus the hue turn does not occur. In contrast, output ratios of R. G, B (e.g., Ro/Go, Bo/Go) are changed in the high luminance range, i.e., in the range in which the characteristic curve is given not by the straight line but by the curvature, and thus the hue turn is generated. The hue turn caused based on this characteristic curve can be eliminated by correcting the characteristic curve in FIG. 34 to the straight line up to the high luminance range, but the high-luminance image cannot be expressed in such case. As a result, the necessity for overcoming the problem of the hue turn by other approaches arises.