The present invention relates to an edge crispening circuit and, more particularly, to a method of performing edge crispening by an out-of-green system in a color image sensor and a circuit for carrying out the same.
Heretofore, color cameras equipped with edge crispening devices have been proposed. Where an image signal from a color camera is processed by an edge crispening method in the prior art edge crispening circuitry, edge crispening circuits are provided for green (G), blue (B), and red (R) signals, respectively. These edge crispening circuits create edge crispening signals for the edges or contours of an image. These edge crispening signals are superimposed on the R,. G,and B signals, thus obtaining an edge-enhanced color image.
This conventional method needs three edge crispening circuits. Hence, large-sized circuitry is necessary. Also, the cost is increased. Therefore, this method has been mainly used for high-grade cameras such as those used in broadcasting stations.
In recent years, domestic video movies have become widespread. Therefore, there is a need for more compact low-cost color cameras. Also, an edge crispening circuit of a smaller size has been required.
An out-of-green edge crispening circuit is available as a means for accomplishing an edge crispening circuit of a smaller size, and is described in the "Prior Art Technique" column of Japanese Patent Laid-Open No. 285778/1990. In this prior art out-of-green edge crispening device, an edge crispening signal is created from the image signal from the green color image sensor device and superimposed on blue and red signals. In this way, edge crispening is performed for each color signal.
However, the prior art edge crispening device utilizing the out-of-green method has the following problems. Since the edge crispening signal is created from the green signal, the level of the edge crispening signal relative to all the R, G, and B signals is determined by the ratio of the green signal component contained in the imaged subject. Therefore, the edge crispening effect is affected greatly by the color of the subject.
For example, where the subject is a totally blue pattern, the G signal level is low and so the level of the edge crispening signal DTL1 is low. As a result, sufficient edge crispening effect cannot be obtained.