In recent years, as a television receiver or the like has increased in image quality and made multi-functional, an image signal has been frequently subjected to digital signal processing. For example, gray level correction has been made by the digital signal processing in order to increase the contrast of an image. When a quantized digital signal is subjected to such digital signal processing, an unnatural contour referred to as a false contour may, in some cases, be produced in an image displayed on a screen. To address this, a technique for removing a false contour produced by digital signal processing is disclosed in JP-A-6-62280, for example.
Referring now to the drawings, description is made of a conventional false contour correction circuit which is disclosed in JP-A-6-62280. FIG. 9 is a block diagram showing the configuration of the conventional false contour correction circuit. The false contour correction circuit comprises a random number generator 5, a judgment circuit 6, and an addition circuit 7, and receives an n-bit digital image signal A. A signal F composed of predetermined lower bits out of the n bits composing the inputted digital image signal A is fed to the judgment circuit 6. The random number generator 5 outputs a digital random number H having the same bit width as the bit width of the signal F. The judgment circuit 6 compares a value represented by the signal F composed of the predetermined lower bits of the digital image signal A with the digital random number H outputted from the random number generator 5, and outputs a signal representing “1” or “0” as a correction signal I depending on the results of the comparison. The addition circuit 7 is an adder having the same bit width as that of upper bits G of the digital image signal A, and adds the upper bits G of the digital image signal A and the correction signal I outputted from the judgment circuit 6, to generate a corrected output signal J.
According to the above-mentioned false contour correction circuit, the correction signal I having no regularity is added to the upper bits G of the digital image signal A. Therefore, the signal F composed of the lower bits which are inputted to the judgment circuit 6 out of the n bits composing the digital image signal A and the correction signal I outputted from the judgment circuit 6 are not correlated with each other within the precision of the random number generator 5. In a case where an image which hardly changes in brightness or hue is quantized by thus making a correction having no correlation with the image, the position on a screen where the quantization level changes is dispersed backward, forward, right, and left, so that an unnatural false contour is reduced. Consequently, such a digital image signal after the correction makes it possible to obtain an image whose image quality is prevented from being lowered by quantization whose level is low.
In the conventional false contour correction circuit, the digital image signal is subjected to a variation corresponding to a change in the most significant bit without being correlated with an image so that the position on the screen where the quantization level changes is not correlated with the image signal. Therefore, even when a signal representing an image whose brightness is constant is inputted, an image which contains noises roughing the display on the screen is obtained by the variation of the image signal corresponding to the change in the most significant bit.
Therefore, an object of the present invention is to provide a false contour correcting apparatus capable of reducing a false contour in an image based on a digital image signal while avoiding lowering the image quality by such a side effect or the like caused by false contour correction in which the above-mentioned noises occur.