User's Manual
Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0
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The following gives a more detailed description of the algorithm. A line with two coordinate points as
(x1, y1) and (x2, y2) can be defined by the following equation:
y2 = slope (x2 - x1) + y1
This implies slope of the line is:
slope = (y2 - y1) / x2 -x1
Another point with X = x3 on the same line can be defined as (x3, y3) where y3 is
y3 = slope (x3 - x1) + y1
Value Of Latch
@ Adrs [N+1]
Value Of Latch
@ Adrs [N]
Ideal Curve
Derived Slope =
Adrs[N+1] - Adrs[N]
Result = Value Of Latch @ Adrs[N] + Slope * LSB's
Interpolated Point
Value Of LSB's
Gamma correction can be bypassed by programming the registers with data values corresponding to a
linear curve with slope = 1. The register programming for gamma bypassing is as shown below.
Adrs[8] = 8 Adrs[64] = 64
Adrs[16] = 16 Adrs[128] = 128
Adrs[32] = 32 Adrs[192] = 192
When registers are programmed with the above values, output of the gamma unit is the same as the input,
and no gamma correction occurs.
Gamma Hardware Implementation
Result = Adrs[192] + [[0x00 - Adrs[192]] * [X - 192] << 1] >> 7 192 <= X < 256
Result = Adrs[128] + [[Adrs[192] - Adrs[128]] * [X - 128] << 1] >> 7 128 <= X < 192
Result = Adrs[64 ] + [[Adrs[128] - Adrs[ 64 ]] * [[X - 64] << 1]] >> 7 64 <= X < 128
Result = Adrs[32 ] + [[Adrs[64 ] - Adrs[ 32 ]] * [[X - 32] << 2]] >> 7 32 <= X < 64
Result = Adrs[16 ] + [[Adrs[32 ] - Adrs[ 16 ]] * [[X - 16] << 3]] >> 7 16 <= X < 32
Result = Adrs[ 8 ] + [[Adrs[16 ] - Adrs[ 8 ]] * [[X - 8] << 4]] >> 7 8 <= X < 16
Result = 0x00 + [[Adrs[8 ] - 0x00 ]] * [[X - 0 ] << 4]] >> 7 0 <= X < 8
Consider Adrs[8] = 8, Adrs[16] = 16, Adrs[32] = 32, Adrs[64] = 64, Adrs[128] = 128, Adrs[192] = 192.
For X = 5, Result = 0 + [[8-0] * [5 << 4]] >> 7 == 5
For X = 11, Result = 8 + [[16-8] * [3 << 4]] >> 7 == 11
For X = 24, Result = 16 + [[32-16] * [8 << 3]] >> 7 == 24
For X = 63, Result = 31 + [[64-32] * [31 << 2]] >> 7 == 63
For X = 100, Result = 64 + [[128-64] * [36 << 1]] >> 7 == 100
For X = 156, Result = 128 + [[192-128] * [28<< 1]] >> 7 == 156
For X = 200, Result = 192 + [[256-192] * [8 << 1]] >> 7 == 200