CS781 - Colour for Computer Graphics - Winter 2009
Lecture 13
Creating Colour: Self-Luminous Devices
`Self-luminous' means producing light without an external illuminant.
Colour CRT
Colour CRT
What is it?
- Three monochrome CRTs
- explicitly true for projectors
- old projectors
- new projectors
- three in one glass envelope for ordinary CRTs
- three is a commitment made very early in the process
- basis of hardware design
- basis of television standards
- Shadowmask
Usability tests
- monochrome CRT -> colour CRT -> MPEG
- how little we have progressed in seventy years
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 abstraction
- Three `phosphors' emitting light of constant chromaticity.
- Three `guns', each of which controls the intensity of light emitted by
one of the phosphors
- Pixels are independent of one another
- Light output from phosphors depends only on the input to a single
gun.
- Display surface has uniform characteristics
- Light output is stable over time
The colour gamut
The colour cube
The colour cube in colour space
The colour cube in chromaticity coordinates
Emitted colour
Emitted light
- \Phi(R,G,B, \lambda) = \sum_{i=R,G,B} \phi_i(\lambda) f(i)
- R,G,B are the `primaries' of the device
Tristimulus values
- For (j=X,Y,X) j(R,G,B) = \sum_{i=R,G,B} f(i) \sum_\lambda
\phi_i(\lambda){\bar x}_j(\lambda)
- j(R,G,B) = \sum_{i=R,G,B} f(i) a_{ij}
- where a_{ij} = \sum_\lambda \phi_i(\lambda){\bar x}_j(\lambda)
- To get the tristimulus values we need to know
- a_{ij}: the chromaticity coordinates of the phosphors
- f(i): the non-linear responses of the primary intensities to the
input
Measuring the Colour of CRT Output
Geometry
Problems
- temporal stability
- spatial variation
Building a Mapping
Model-based
Look-up Table Based
Calibrating a Colour CRT
The Television Signal
The White Axis
Gamma Correction
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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