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VIDEO LEVELS AND CHROMA KEY
May 9, 2002 SuperView 3000 User Manual 51
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chroma key. This is because the frame buffer color must lie within the correct
range for all three color components simultaneously. The fraction of all colors
reserved for the chroma key is the product of the individual fractions. For the
example given, this is equal to 0.25 x 0.25 x 0.25 = 0.016 = 1.6%.
We calculate that the low threshold is 175 (25% of 700 mV) and the high
threshold is 525 (75% of 700 mV). Using the ChromaKeyColor command, we
issue the command:
>ckc -270 175 525 1250 525 1250
Look for a red level less than 175 mV by specifying a red level between -270
(the minimum level the hardware understands) and 175. Similarly, look for
green and blue levels greater than 525 mV by specifying green and blue levels
between 525 and 1250 (the maximum level the hardware understands).
The above procedure should work reasonably for computer frame buffers with
voltage ranges of 714 mV and 1 V as well. A 700 mV will have a slightly
smaller margin at the low voltage (black) end, and a 1 V buffer will have a
substantially larger margin at the high voltage (white) end.