VIDEO LEVELS AND CHROMA KEY
May9, 2002 SuperView3000 User Manual 51
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chromakey. This is because the frame buffer color must lie within the correct
rangefor all three color components simultaneously. The fraction of all colors
reservedfor the chroma key is the product of the individual fractions. For the
examplegiven, this is equal to 0.25 x 0.25 x 0.25 = 0.016 = 1.6%.
Wecalculate 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
issuethe command:
>ckc -270 175 525 1250 525 1250
Lookfor a red level less than 175 mV by specifying a red level between -270
(theminimum level the hardware understands) and 175. Similarly, look for
greenand blue levels greater than 525 mV by specifying green and blue levels
between525 and 1250 (the maximum level the hardware understands).
Theabove procedure should work reasonably for computer frame buffers with
voltageranges of 714 mV and 1 V as well. A 700 mV will have a slightly
smallermargin at the low voltage (black) end, and a 1 V buffer will have a
substantially larger margin at the high voltage (white) end.