Intel 815 manual Mathematical Gamma Correction For Overlay, Three times

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

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15.3.1.2.Mathematical Gamma Correction For Overlay

Gamma correction is a function that corrects for non-linearity between display phosphor brightness as a function of electron beam current. Depending on the actual phosphor used, and/or whether the event that data streams have been pre-gamma corrected for different phosphor characteristics than the display device being used, the digital display stream must be corrected to achieve equally spaced increase in brightness on the display for equally spaced increases in color intensity values.

Gamma correction shall be chiefly implemented via a piecewise linear approximation of a curve. There are 6 individual breakpoint values, logarithmically spaced in the Color Intensity domain. Only one set of 8 values is provided and shared for each of the Red, Green, and Blue intensity components. These values shall be loadable via software control and are programmable. For each of Red, Green, and Blue, the appropriate Gamma breakpoint pairs are looked up and smoothly interpolated between to arrive at the final Red, Green, and Blue values that are output to the DAC.

Figure 41. Gamma Correction Unit Block Diagram

Xxrgb23:1

6

stage 1 stage 2 stage 3

			0x00
			latch8to15	MUX(4:1)
					latchlow
										Box denotes
		latch16to31
						latchlow
		latch32to63								logic replicated
						MUX(2:1)	latchN			three times
		latch64to127				latchhigh
		latch128to91		MUX(4:1)				Inverter
					latchhigh
		latch192to25
	1:0		5
		latch192to25
			5			2		invlatchN	Cin='1'
			1:0
Priority Encoder /				2						latchlow
Down Counter						RedGAMMA BLOCK
1) Enables signals required to	Muxsel		2						Adder	Slope
						DIAGRAM			Adder to calculate
select appropriate data for slope
									the slope.
calculation.										ipoint
		1:0	1:0	2
										6:0
			latch128to19			2				14:7
			1
			latch192to25	MUX(4:1)
			5		latchhigh					Slope
			0x00
										ipoint
			0x00			latchhigh				8X7Multiplier
			1:0			MUX(2:1)	latchN+1			remainder
			latch8to15			latchlow
			latch16to31	MUX(4:1)
					latchhigh
			latch32to63						7:0
			latch64to128

stage 4

Cin='0'

Adder	Grgb23:
Adder to get
	16
gamma
corrected value.

Shifter: shifts data to get 8 bits (Lower bits are padded by zeros). remainder

remainder

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