1. Field of the Invention
The present invention relates to a color image reading and compensating device which reads images on original documents by using a color image sensor and compensating for differences in phase characteristics and resolution characteristics among resultant color image signals.
2. Description of the Related Art
Color image sensors for reading color images include dot sequential sensors, and linear sensors such as three-line sensors.
A dot sequential image sensor has three color filters, such as R (red), G (green), and B (blue) or Y (yellow), M (magenta), and C (cyan), which are sequentially arranged on a linear array of photosensitive elements, and can read one line of color information by the use of a single image sensor array. With the dot sequential sensor, however, three pixels of R, G, and B, which are placed apart from one another, are considered to form a color dot, and thus hue may vary if there is a brightness gradient in an image. This is referred to as a dot sequential error. This error is not a serious problem in the case of ordinary images. However, the error becomes a serious problem in the case where a high-definition image is read, a read image is enlarged, or a color decision is made for a read image.
To eliminate the dot sequential error, compensating methods using linear interpolation have been proposed and disclosed in Japanese Unexamined Patent Publication No. 61-154357 and U.S. Pat. No. 4,672,433. With this method, the position of an element of a reference color (for example, G) is taken as a reference position, and signals from two pixels of colors (for example, R and B) other than the reference color that sandwich the reference color element are used to estimate the reference color signal on the basis of linear interpolation. Thereby, a dot sequential error in a uniform brightness-gradient portion is corrected, thus eliminating variations in hue.
However, the above method suffers from a problem that lowpass filtering characteristics inherent in the linear interpolation processing will reduce the resolution of signals of two colors other than the reference color. For example, a sine wave response of linear interpolation at 1/4 of a sampling frequency is 56% in power, lowering frequency components material to the quality of high-definition images. In order to compensate for the reduced resolution resulting from the linear interpolation, the combined use of a high-frequency emphasizing filter and the linear interpolation might be adopted. The use of such a filter would greatly increase the magnitude of noise components on an image, decreasing the signal-to-noise ratio. With high-definition images, not only the dot sequential error but also a decrease in resolution and signal-to-noise ratio become serious problems.
On the other hand, a three-line color image sensor comprises three linear sensor arrays arranged apart from one another in the direction of sub-scan, i.e., in the direction perpendicular to the direction (main scanning direction) in which one line of an original document is scanned, and three color filters of R, G, and B are disposed on the respective line sensors. Each of the line sensors reads one line of a corresponding color component. The three-line image sensor suffers from a drawback in that it is somewhat complex in structure, that is, a buffer memory is required for compensating for differences in sub-scanning position among line sensors but has the following features. First, the three-line image sensor will not suffer from the above-described dot sequential error because information in the same position can be read for each of three color components. Second, the incident area, i.e., the aperture, in the direction of sub-scan can be chosen freely for each of color components. Thus, the aperture of the line sensor elements for blue (B) filter low in light transmittance can be broadened, thereby maintaining a balance among amounts of received red, green and blue (R, G and B) light rays. This can prevent the degradation of the signal-to-noise ratio of the B signal. Conversely, this will reduce the resolution of the B signal component.
As described above, the prior art techniques have the drawback of producing the degradation of phase and resolution characteristics in compensating color image signals having different phase and resolution characteristics due to the structure of a color image sensor used. In addition, the use of linear interpolation between signals of the same color and high-frequency emphasizing filters in compensating color image signals reduces the signal-to-noise ratio and the resolution.