Tutorial
Colour Representation
Solutions
- Describe three possible ways of creating a suitable colour-map
table to display a given image.
- Broadcast colour television uses a YIQ colour system. The Y channel
is called the luminance and is the same as the CIE Y primary. The Y
component of a YIQ colour signal is used for display on a black-and-white
television. The RGB-to-YIQ mapping is defined by:
Which RGB colours will look the same on a black-and-white TV?
- Image reconstructions such as dithering rely on the fact
that the eye integrates spatially. That is, for a small enough
area, the eye will only perceive the average intensity. Suppose
we create a dithered image using a 4x4 dither pattern where
each pixel can have one of 8 intensities.
How many different intensities can be generated for a single 4x4 dithering
tile?
- When graphics algorithms model the transfer of light, the light is
usually represented using an RGB triple. Thus, instead of a complete spectral
distribution, the RGB model uses an appropriate mix of three discrete
wavelenths such that they are perceptually equivalent to the complete
spectrum. Using the RGB model in surface reflection calculations can lead
to errors, however. Suppose we know the spectral reflectance distribution
of a known material. This type of distribution simply tells us to what
extent different wavelengths are absorbed or reflected by a material.
Construct an example reflectance distribution function that would cause
a simplified RGB model of light to yield the wrong answer.
- Could one build a four-phosphour monitor? What would be the tradeoffs in
the design?
- Could a perfect (complete) colour gamut ever be achieved, if
phosphours producing pure spectral colours could be manufactured?
- Could cyan, magenta, and yellow phosphours be used to construct a monitor?
Why or why not?
- What are some possible methods for performing colour conversion of an image
when it is known that some of the colours in the image will fall
outside of the gamut of the desired output device?