Lec 10a - Colour & RGB Images

Question 1

What is light?

Answer 1

Light is electromagnetic radiation in the wavelength range of 400nm (blue) to 700nm (red).
Almost all light is a mixture of wavelengths, e.g. the rainbow spectrum of white light from the sun.

Question 2

What is colour vision based on?

Answer 2

Colour vision is based on the tri-stimulus theory of colour perception where three
kinds of cones are sensitive to red, green and blue light.

Question 3

What are examples of tri-stimulus colour systems?

Answer 3

RGB model and hue, saturation, brightness (HSB) model

Question 4

What is the relative response of human cone photo-receptors?

Answer 4

human cone photo-receptors get heavily excited for green and red, but not as much for blue

Question 5

What does colour perception depend on?

Answer 5

Colour perception depends on colour context (e.g. what colour and intensity resides next to what other colour etc.)
• Blue alone tends to be perceived quite weakly. The greatest colour discrimination is in the green to yellow range.

Question 6

What forms of colour blindness are there?

Answer 6

Forms of colour blindness include red-green, blue-yellow and achromatic, has
obvious implications for the display of computer graphics.

Question 7

Give an example where the reflectivity of a material depends on the wave length of the light source

Answer 7

For example, an intrinsically “red” material would have high reflectivity for longer (red) wavelengths, and low reflectivity for other, shorter wavelengths. Under “white” light (an uniform distribution of power across λ), it would reflect mostly red. Under “blue” light (a distribution with higher power in the blue or short wavelengths, and lower or zero power in the red (long) wavelengths, it would appear dark, since the product p(λ)r(λ) would be low all the way across.

Question 8

What are images?

Answer 8

Images are two-dimensional (2D) patterns of light, that possess intensity and colour
properties, and can be considered as a function (bounded by visibility) that maps a
plane into some space of measurements

Question 9

What is digitisation?

Answer 9

Digitisation involves sampling over a regular spatial grid, typically square or
rectangular (rarely hexagonal or triangular, but occasionally in robotics).

Question 10

What is sampling?

Answer 10

Sampling involves quantisation over intensity values, for each of the pixels in an
image

Question 11

What are the different forms of pixels in images?

Answer 11

• in linear proportion to intensity or in non-linear (adaptive) proportion (as in night
vision in the human eye), and either
• continuous in value (grey-scale or colour) or binary in value (bitmap representation of 0’s and 1’s).

Question 12

What are some of the trade offs with digital images?

Answer 12

• resolution (fineness of sampling or quantisation) versus fidelity, and
consequently
• fidelity(accuracy) versus storage and processing costs.

Question 13

What does an image format contain?

Answer 13

header plus pixel data

Question 14

What is the header?

Answer 14

Header may be fixed record structure, or variable (e.g. attribute/value list), or some
combination

Question 15

When should you use internal vs external formats?

Answer 15

Depends whether the purpose of the format is for the interchange of images (e.g. streaming of images or video over the Internet) or for optimised, random-access
during memory and processor operations.

Question 16

What is the mandatory information that the image header must contain?

Answer 16

• Magic number (that indicates what format image is, generally used in conjunction with file suffix)
• Pixel type such as bit, byte or integer sometimes unsigned) float, double or complex (imaginary numbers are used by the Fourier transform for smoothing images)
• Image size (in rows and columns)

Question 17

What is the optional information that the image header may contain?

Answer 17

• Statistics such as minimum and maximum values of intensities or colour values
(very useful for display, means don’t have to parse entire image first)
• Additional geometrical information such as aspect-ratio or origin offset
• Ownership, program that created image and name of organisation or company

Question 18

How are images generally saved?

Answer 18

in order of raster scanning, like mapping a
two-dimensional (2D) array into linear storage (used by computer memory).
• Normally left-to-right then top-to-bottom, however sometimes in other orders
(always check before making assumptions, when given images to work with)
• Pixels may be either be stored using (natural machine units) bytes, integers,
floats or (packed) bit fields that need to be converted (unpacked) before
displaying.
• Often contain padding, to make them an even size for optimised file input and
output in many graphics application programming interfaces (API’s).

Question 19

What are multi-band or multi-dimensional images?

Answer 19

Multi-band images include multiple values at each pixel, the most common being the
colour red, green, blue (RGB) and hue, saturation and brightness (HSB) formats.

Question 20

When do you use offset image coordinates?

Answer 20

Offset coordinate systems are often used for specific or internal image format
applications where it is important to set up a relative coordinate system for an
image’s data.

Question 21

What are properties of offset image coordinates?

Answer 21

• involves embedding an image into an absolute coordinate system,
• can measure the offset by various conventions ±
• can be implemented in any of the other coordinate systems