Topic 5 - Imaging Mapping - II

DIGITAL IMAGE PROCESSING

Course 3624

Department of Physics and Astronomy

Professor Bob Warwick

5.1 The Perception of Colour

• Although the human eye has only a limited ability to discern

shades of grey (~ 2 5), it does much better with colour

gradations (> 2 8)

• Inclusion of colour invariably adds to the visual content of an

image (eg black and white versus colour TV pictures)

• The human perception of colour is a complex subject. It

depends on both physiological and psychological factors. Also

the eye/brain system is highly adaptive

The Nature of White Light

Combining colour pigments

Colour Mixing Experiments

Magenta, cyan, yellow, and black are the colours often used in printing

Magenta, cyan, yellow, and black are the colours often used in printing

5

The Human Eye

The Retina

Fovea

Macula Optic nerve

Spectral Sensitivity Curves of Rods and Cones

The range of sensitivity is roughly 400 - 650 nm.

There are three types of cone (blue, green, red).

The rods (dashed) primarily give sensitivity in low illumination

Tri-Stimulus Colour Theory

Any colour can be reproduced by mixing an appropriate set of three “primary colours” – Thomas Young 1802

A More Complex Model of Human Colour Perception

Colour Matching Experiments

Choose three “primary” light sources with spectra P1(λ), P2(λ) & P3(λ)

Colour matching involves adjusting the input light levels β1 β2 β3 to match the tristimulus values of the test colour

C(λ) =

Colour Matching with Monochromatic Primaries

435.8 nm 546.1 nm 700nm

The CIE 1931 (All Positive) Colour Matching Functions

The corresponding tristimulus values for an arbitrary spectral colour I are:

The CIE XYZ system was designed so that the Y parameter was a measure of brightness ie luminance. Then

x=X/(X+Y+Z) & y=Y/(X+Y+Z) represent "chromaticity" parameters, (since z = 1 - x – y gives no further colour information)

Thus three parameters define an input in the CIE xyY colour space.

CIE 1931 Chromaticity Diagram

The outer curve boundary represents the track of monochromatic colours in the x,y plane. (Although in practice the colours shown are restricted by the performance of the display devices.)

The gamut of colours that can be generated using monochromatic primaries at 435.8, 546.1 and 700 nm, lie within the triangle. Similarly the xy coordinates of the three phosphors in a display will define the "realizable" colours of the device

Alternative Colour Specifications

Colour shade (red, green,blue etc..)

Depends on peak λ

Measures the purity of the colour

Degree of dilution by white light

Luminance Hue Saturation (LHS) Colour Space

Complementary Colours

Author: Richard Alan Peters II

Colours opposite each other on the colour disk are called “complementary”.

CYAN - REDCYAN - RED GREEN - MAGENTAGREEN - MAGENTA BLUE - YELLOWBLUE - YELLOW

R

B

G

5.2 Pseudo-Colour TechniquesColour displays utilize three different phosphors (RGB) to generate a set of "realizable" colours. These colours occupy the "colour cube" of the device, which can be visualized as a 3-d space.

The Colour Cube has R,G & B axes normalised so that at full intensity R=G=B=1.

Then R=G=B=0 is black R=G=B=1 is full intensity white

R=G=B= 0 1 represents a grayscale (along the leading diagonal)

Pseudo-Colour ProcessingPseudo-colour processing involves the use of colour to enhanced the display of a single band (monochrome) image. The input grayscale is subdivided into intervals, against which a set of colours are assigned.

If the number of intervals

is small Density Slicing

Colour Assignment

Grey Level ( f )

Yellow

Red

Blue

Black

The Colour TableMore sophisticated applications of pseudo-colour involve the use of a colour table. The Colour Table represents a specific (and more systematic) mapping from grey level to RGB colour.

For example:

0 63 127 191 255 f

R,G,B

1

0

Example: Colour Table with 64 levels

A single band image in pseudo-colour.

XMM Image of SNR G21.5-0.9

5.3 True Colour and False Colour Techniques

In many applications (e.g., remote sensing, astronomy etc..) more than one image is recorded of the same scene using different spectral filters. Up to three such images can be displayed simultaneously with a "three channel" display system.

In this case the possibilities are:

TRUE COLOUR PROCESSING - where the images are recorded through R,G & B filters and are (respectively). displayed on the R, G and B channels of the display system. In many applications (e.g. photography, TV etc..) special care is needed to achieve an acceptable level of colour fidelity.

FALSE COLOUR PROCESSING - where images recorded in three different wavebands are displayed arbitrarily on the R, G and B channels of the display system.

Implementation of Colour Enhancementvia the device Colour (Look-up) Table

D/A

Image Store

Colour Look-up

Table

D/A

D/A

Video Out

R

G

B

Single Channel System – Pseudo-Colour

Image Store 1

Colour Look-up

TableD/A Video

OutR

G

B

Image Store 2

Colour Look-up

TableD/A Video

Out

Image Store 3

Colour Look-up

TableD/A Video

Out

Three Channel System – True or False Colour

DISPLAY

I/P CHANNELS

True Colour

10 micron image

1.2 micron image

2.2 micron image

DISPLAY

I/P CHANNELS

False Colour

DISPLAY

I/P CHANNELS

Pseudo Colour

Pseudo Colour Examples

True, False or Pseudo Colour ?

Studying Temporal Changes in Remote SensingExample: Panama Canal