| 摘要 |
The linear foreground and background compositing system of the "blue-screen" type and as included in my prior U.S. Pat. No. 4,100,569, is provided with non-linear background control signals for improved composite picture quality under various adverse circumstances. More particularly, the problem of footprints, scuff marks, and dust on the "blue-screen" background for the foreground signal has been eliminated by a "clean-up" circuit 98 which shifts the background signal to "fully on" when it reaches a level in the vicinity of 80 or 90 percent "on", and making a concurrent reduction in the foreground signals, so that above the selected control level shadows or semi-transparency objects are made fully transparent and are not reproduced. The problem of "print-through" in the case of glossy black objects has been avoided by a circuit 34 which reduces background signal to zero when the level of blue foreground signals drops below a predetermined level. The problem of shadow inversion and high noise level which may arise when foreground objects are subject to back lighting has been solved by the use of a dual control signal, one operating in the blue backing area, and the second operating in the subject area, see FIG. 2, AND gate 66 and potentiometer R-7. An improved single control for increasing or decreasing the intensity of shadows provides a continuously variable shadow control for bowing the background signal level characteristic, either upward or downward throughout its complete control range, see FIGS. 4 and 5. Special circuitry 32 is also provided for accommodating black background, or white backgrounds, so that special effects involving black backgrounds, or foreground cartoon pictures having white background may be easily handled.
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