In the editing system, it has been a conventional practice to make various processing for image signals by the key signal which is generated with reference to the luminance signal as a standard.
Specifically, the editing system of this type extracts the luminance signal form an image signal which is an object of processing and makes the gain of the luminance signal variable after clamping it to generate the key signal. Further, with reference to the key signal, an image signal of background and the image signal to be processed are mixed each other, thereby causing the image as the object of processing (see FIG. 14B) to be fit into the image of background (see FIG. 14A) for producing a composed image (see FIG. 14C), for example.
In the meantime, the editing system may generate the key signal taking an image signal produced by the computer graphics as the object of processing. In such an image signal as produced by the computer graphics, there may sometimes be included a picture of blue color, etc. having little luminance signal components.
If the key signal is generated with the luminance signal as a standard in connection with the picture of blue color, etc. having little luminance components, the key signal will be generated in thinner form. Consequently, when processing the picture produced by the computer graphics by the key signal, the edition processed picture will be displayed played in the thinner shape. This will make the edited picture to cause a sense of incongruity.
Thus, when letters of Y and C in FIG. 14B are displayed in white and blue, respectively, there is an inconvenience that, as is shown in FIG. 14C, the letter C will be composed to be thinner than the letter Y in the composed picture.