CS781 - Colour for Computer Graphics - Winter 2009
Lecture 14
Creating Colour: Self-Luminous Devices
`Self-luminous' means producing light without an external illuminant.
Colour CRT
Calibrating a Colour CRT
Calibration
- putting a device into a standard configuration
- essential for broadcast applications
Characterization
- having a reliable mapping from device coordinates to standard colour
coordinates (tristimulus coordinates)
- ideally one-to-one
- which allows give me colour X
- adequate for device-to-device applications
- could be the basis for broadcast applications
Modelling a Colour CRT
The colour gamut
The colour cube
The colour cube in colour space
The colour cube in chromaticity coordinates
Measuring the Colour of CRT Output
Geometry
Problems
- temporal stability
- spatial variation
Building a Mapping for XYZ input
Model-based
Forward
- X = A Ii(vi) + Black
- Ii(v) = Gi f(vi)
- f(vi) = vi^\gamma
Invert
- Ii(vi) = invA (X - Black)
- fi = Ii(v) / Gi
- vi = invf( fi ) = fi^{-\gamma}
Look-up Table Based
Calibrating a Colour CRT
The Television Signal
Viewing Conditions
Reference white
- D6500 at 70 lumens per square metre
Viewing distance
- 5 screen heights from image
- about 10 degrees of visual angle
Surround
- 3 times the linear dimensions of the image
- 10 degree surround
- total field of view 30 degrees
- D65 at 10 lumens per square metre
The White Axis
- black
- white
- grey: test of how well the gamma functions match
Colour of grey (all guns equal: vR = vG = vB) depends on surround
Gamma Correction
What do you want gamma to be?
- For artists
- mode of luminance histogram is at 18%
- gamma = 2.6 based on ...
- For untrained eyes
- mode of luminance histogram is at 25%
- gamma = 2.0 based on ...
Colour LCD
Technological basis
- backlight
- polarizers
- liquid crystal between the plates of a capacitor
- colour filters
What if we had four filters?
- Why? We are surely overdoing the blue
Colour OLED
Solid state photodetectors and LEDs are very closely related
Creating Colour: Illuminated Devices
Half-tone Printing
Photography
Continuous-tone Printing
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