User's Manual

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0
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As shown, the inputs to the function are:
• PCI Register Bus: The chip internal PCI data bus and the appropriate register decodes for loading
the Gamma breakpoint values
• Red In 2e07:00: The Red color component of the Overlay Stream
• Green In 2e07:00: The Red color component of the Overlay Stream
• Blue In 2e07:00: The Red color component of the Overlay Stream
The output of the function is:
• Red Out: The Gamma corrected Red color component.
• Green Out: The Gamma corrected Green color component.
• Blue Out: The Gamma corrected Blue color component.
Gamma Correction Theory Of Operation
The Gamma Correction function outlined above is implemented via piece wise linear interpolator that
allows any arbitrary curve to be output as a function of any linear input. The function provides smooth
(continuous) interpolations efficiently in hardware while taking little compromise in the preciseness of
the output curve relative to the ideal. The breakpoints of the curve are defined via software and loaded
via the PCI internal data bus.
In general, the implementation outlined above allows for 7 logarithmically spaced breakpoints, of which
the 5 internal breakpoints are software definable, the 2 endpoints are predefined at zero and 255
(decimal). The value at a given breakpoint, and the breakpoint +1 (using the MSB which is 1 of an 8 bit
color are looked up in the table simultaneously). These two points define the exact desired values at
those breakpoints. The values between the breakpoints are linearly interpolated using the remaining bits
of the color. The output of the function therefore evaluates the equation:
Result = mX + B
Where:
m = Slope
B = Initial Value defined at table entry N
The slope, m, is derived by taking the difference between the breakpoint defined by [N] and the next
successive breakpoint [N+1]. The value at [N+1] is simply derived via the input wiring to the mux. Since
all slopes are defined to be positive for this function, the use of an unsigned multiplier is sufficient for
this function.
Latch Output Valu
e
Latch Address
Area Of Interest