cs781 - Colour for Computer Graphics - Winter 2012

Course Notes

Lecture 7 - Creating Colour with a Computer

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Self-Luminous Devices

`Self-luminous' means producing light without an external illuminant.

Colour CRT

Monochrome CRT

What is it?

evacuated tube
electron beam
- hot electrode is surrounded by a plasma of electrons
- focussed
- gated
- directed
- accelerated
phosphor
- back of faceplate
- constant chromaticity:
  - actually relative spectral power distribution
- variable intensity
  - proportional to power delivered by electron beam

How is an image created?

The electron beam is moved in one of two ways
1. To draw a series of curves of varying brightness that form a picture
  - most often a picture of text
  - Often called a calligraphic display
2. To scan the entire display while varying in brightness
  - The scan pattern is called raster (from Latin, rastrum meaning rake), hence raster display.
Additive mixing by temporal succession
- Remember flicker photometry
The brightness depends on the energy density of the electron beam
- Brightness limitations are not phosphor-related: it's pretty tough
- Brightness problems come from heating of the phosphor which can boil off the screen.
- Therefore calligraphic displays can be much brighter than raster ones

What light is emitted?

programmer puts a value into a function call ( wrpixel( int x, y, int fbv ) )
- The value in the frame buffer is modified to produce power deposited in the phosphor: e(fbv)
- The transformation is usually the composition of two transformations
  1. The analogue function created by the video amplifier and cathode ray tube
  2. A digital colour lookup table
Thus, the SPD of the emitted light is e(fbv) * relative SPD
- e(fbv):
  1. most naturally a non-linear function of the form (fbv)^(1/3). Why?
    - The exponential correction is called gamma correction.
    - Responds to the
  2. sometimes linearized to assist anti-aliasing. How does it help?
- BUT, if we measure intensity and plot log intensity against voltage
  - There is a non-zero intercept. Why?
  - The intercept, the exponent, and the dynamic range of this function depend strongly on the settings of the brightness and contrast controls.
artifacts
- reflected ambient illumination
- light scattered within faceplate
- rise and fall time of amplifiers

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
- shape of a pixel

What light is emitted?

Simply the additive mixture of the three lights

Calibrating a Colour CRT

The Video Signal

RGB versus YIQ
- answer to a trivia question:
  - Y the CIE luminance value
  - I a chrominance signal encoded in-phase, roughly red/green
  - Q a chrominance signal encoded in quadrature, roughly yellow/blue

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
D6500 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 = 2.5 based on putting the mode of the luminance histogram half way up the scale.
For untrained eyes
- mode of luminance histogram is at 25%
- gamma = 2.0

The Colour Output of a CRT

The colour gamut

The colour cube

The colour cube in colour space

The colour cube in chromaticity coordinates

Potential Problems

temporal stability
spatial variation
variation in line voltage

Colour LCD

These are widely used because they occupy little volume, use little energy, and weigh little compared to CRTs. Their colour performance is comparable to CRTs, but the structure of the images they produce can be intrusive

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

high efficiency

Plasma Display Panel (PDP)

Fuorescent Lights

They consist of

A gas tube coated on the inside with a phosphor.
It is filled mostly with inert gases, to which a small amount of mercury vapour is added.
Electrodes at each end of the tube

They make light by

ionizing the internal gases so that current flows
- This is called electrostatic breakdown.
as current flows collisions with electrons excite mercury atoms to high energy states
decay of these states emits untraviolet light at 180 nm and 240 nm.
the phosphor absorbs the ultraviolet photons going into high energy states
decay of these states emits visible photons.

PDPs

A PDP is an array of electrodes and phosphors which, in the off state maintain a gas just below it breakdown level.

Fluorescent light is turned off and on by applying varying voltages to the electrodes.

Thus, a PDP is in effect an array of tiny fluorescent lamps..

Field Effect Display (FED)

Just as the PDP is an array of tiny fluorescent lamps, an FED is an array of tiny CRTs.

A vacuum is made in a thin flat volume.
On one face is phosphor-coated glass and a transparent anode.
On the other face is an array of tiny cathodes.
Between the two is an array of gates (like the screens in an ordinary CRT), one per cathode.
The screens modulate the current deposited in the phosphor closest to its cathode.

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