Generating a chroma keying signal

DETAILED DESCRIPTION It is an object of the invention to enable the provision of a method of and apparatus for generating chroma keying signals from R-Y and B-Y component signals.
The invention provides a method of generating a chroma keying signal comprising the steps of: (a) continuously determining the value of tan.
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-1 (R-Y)/(B-Y); (b) continuously determining the value of (R-Y).
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2 +(B-Y).
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2 ; (c) comparing the determined value of tan.
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-1 (R-Y)/(B-Y) with a pre-selected value thereof; (d) comparing the determined value of (R-Y).
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2 +(B-Y).
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2 with a pre-selected value thereof; and (e) generating a keying signal when the determined value of tan.
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-1 (R-Y)/(B-Y) is substantially equal to the pre-selected value thereof and the determined value of (R-Y).
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2 +(B-Y).
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2 is equal to or greater than the preselected value.
Step (a) of the method set forth in the preceding paragraph may comprise the steps of: (I) determining the value of log (R-Y); (II) determining the value of -log (B-Y); (III) adding log (R-Y) to -log (B-Y); (IV) determining the antilog of log (R-Y) -log (B-Y) to produce (R-Y)/(B-Y); (V) determining the quadrant in which tan.
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-1 (R-Y)/(B-Y) lies; and (VI) determining the value of tan.
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-1 (R-Y)/(B-Y) using the value (R-Y)/(B-Y) produced in step (IV) together with the quadrant determined in step (V).
The pre-selected values of tan.
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-1 (R-Y)/(B-Y) and (R-Y).
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2+ (B-Y).
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2 may be produced by performing steps (a) and (b) while viewing a displayed picture, causing a cursor to point to a desired keying colour on the displayed picture and storing the determined values of tan.
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-1 (R-Y)/(B-Y) and (R-Y).
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2 +(B-Y).
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2 corresponding to the picture position to which the cursor is pointing.
The invention further provides an arrangement for generating a chroma keying signal comprising first means for continuously determining the value of tan.
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-1 (R-Y)/(B-Y), second means for continuously determining the value of (R-Y).
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2 +(B-Y).
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2, third means for comparing the determined value of tan