# lecture7 - CSE472 Computer Graphics Light and Color Light....

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1 CSE472 Computer Graphics Light and Color Light…

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2 CSE472 Computer Graphics Achromatic Light Achromatic light   light represented by shades of a single color.   Light is described with a single parameter Intensity or luminance  (physical properties) Brightness (perceived)
3 CSE472 Computer Graphics Monitors (CRT’s) Cathode Ray Tubes - Phosphor Attractor Grid + Control Voltage Deflection Magnets Electron Beam Control Grid

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4 CSE472 Computer Graphics Monitors (LCD’s) Polarizer Polarizer Liquid Crystal Backlight Viewer
5 CSE472 Computer Graphics Gamma A CRT electron gun does not have a linear  relationship between voltage and intensity.  The   relationship is: k -  constant of total intensity N - number of electrons falling upon the phosphor.   I - intensity.   The value  γ  is a parameter of the CRT and varies in  the range 2.2-2.5 for most CRT’s.  This is called  gamma   LCD’s have a similar property I = kN γ

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6 CSE472 Computer Graphics Gamma Correction If gamma is not corrected for, a non-linearity  will appear in the output.   Gamma Correction K and  γ  are functions of the monitor design.   Real systems do this (c in range [0,1]): V = (I / K) 1/ γ γ / 1 * 255 c I =
7 CSE472 Computer Graphics Gamma Correction Curves 0 0.2 0.4 0.6 0.8 1 1.2 0 0.2 0.4 0.6 0.8 1 1.2 Gamma 1.3 Gamma 2.2

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8 CSE472 Computer Graphics Gamma Correction Values Monitors typically range from 2.2-2.5 Correction values are often more like 1.3 Why? Where should gamma correction be done? When do we need it?
9 CSE472 Computer Graphics Intensity Levels Graphics systems are typically discrete Fixed number of intensity steps How many do we need? Dynamic range Ratio between largest and smallest value If min=light of one candle and max=light of the sun, even 65536 steps will be very perceptible

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10 CSE472 Computer Graphics Common Bit Sizes Used Computer graphics 8 bits = 256 steps You can see the steps Commercial television 10 bits = 1024 steps Motion pictures 10-12 bits = 1024-4096 steps Generally imperceptible
11 CSE472

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## This note was uploaded on 07/25/2008 for the course CSE 472 taught by Professor Owen during the Spring '08 term at Michigan State University.

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lecture7 - CSE472 Computer Graphics Light and Color Light....

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