1. What is Colour?
Colour is the result of the interaction between physical light and the human visual system.
Colour is a psychological perception, not a physical property of an object.
🧠Remember:
Colour exists in the brain, not the object.
2. Newton's Discovery
In 1666, Isaac Newton showed that white sunlight can be separated into a spectrum of colours using a prism.
White light contains many wavelengths of visible light.
3. Colour and Wavelength
Visible light occupies approximately 400 nm to 700 nm of the electromagnetic spectrum.
| Colour |
Approximate Wavelength |
| Violet |
~400 nm |
| Green |
500–570 nm |
| Red |
~700 nm |
4. Human Colour Perception
The retina contains light-sensitive photoreceptors called rods and cones.
| Photoreceptor |
Function |
| Rods |
Brightness and night vision |
| Cones |
Colour vision |
5. Types of Cones
| Cone Type |
Most Sensitive To |
| S Cone |
Blue |
| M Cone |
Green |
| L Cone |
Red |
The brain combines signals from these three cone types to perceive colour.
Exam Fact:
Human colour vision is trichromatic.
6. Additive Colour Mixing
Colours are created by adding coloured light together.
Red + Green = Yellow
Red + Blue = Magenta
Green + Blue = Cyan
All three primary lights combined produce white.
🧠Additive:
Black → White
7. Subtractive Colour Mixing
Colours are produced by removing wavelengths from white light.
Cyan + Magenta + Yellow → Black
Used in printing and pigments.
🧠Subtractive:
White → Black
8. Problems with Colour Images
- Each pixel contains three values.
- Colours change under different lighting conditions.
- Different cameras record colours differently.
Important:
Lighting greatly affects recorded RGB values.
9. Colour Spaces
A colour space is a coordinate system used to represent colours numerically.
Each colour corresponds to a point within the colour space.
10. Common Colour Spaces
| Colour Space |
Main Use |
| RGB |
Displays |
| CMY / CMYK |
Printing |
| HSV |
Human-friendly colour representation |
| YCbCr |
Digital video |
| CIE LAB |
Perceptual colour measurement |
11. RGB Colour Space
RGB represents colour using Red, Green and Blue components.
It is an additive colour model commonly used in displays.
🧠RGB = Screens
12. RGB Colour Cube
RGB colours form a cube in 3D coordinate space.
- Black = (0,0,0)
- White = (1,1,1)
- Each corner represents a pure colour.
13. CMY and CMYK
CMY uses Cyan, Magenta and Yellow as primary colours.
Conversion:
C = 1 − R
M = 1 − G
Y = 1 − B
CMYK adds black ink (K) for true black printing.
14. Colour Gamut
Gamut refers to the range of colours represented by a colour model.
RGB generally has a larger gamut than CMY.
🧠Not all screen colours can be printed.
15. HSV Colour Space
HSV represents colours using Hue, Saturation and Value.
| Component |
Meaning |
| Hue |
Dominant colour |
| Saturation |
Colour purity |
| Value |
Brightness |
16. Why HSV is Popular
HSV is more intuitive and closer to human colour perception than RGB.
Useful for colour segmentation and object detection.
🧠HSV = Human-Friendly RGB
17. YCbCr Colour Space
YCbCr separates brightness information from colour information.
| Component |
Description |
| Y |
Luminance |
| Cb |
Blue difference |
| Cr |
Red difference |
Commonly used in digital video systems.
18. Colour Difference
Colour difference measures how different two colours are.
Euclidean distance is the simplest colour distance measure.
Colour Difference:
Distance between colour coordinates.
19. Perceptually Uniform Colour Spaces
Equal distances should correspond to equal perceived colour differences.
RGB, HSV and YCbCr are not perceptually uniform.
Better Choices:
CIE LAB and CIE LUV
20. CIE LAB and CIE LUV
Designed so colour distances better match human perception.
| Component |
Meaning |
| L* |
Lightness |
| a* |
Green ↔ Red axis |
| b* |
Blue ↔ Yellow axis |
21. Colour-Based Image Retrieval (CBIR)
Images can be retrieved based on colour similarity.
Colour histograms are commonly used as image descriptors.
🧠Similar Histogram = Similar Colour Distribution
22. Colour Histograms
A colour histogram describes the distribution of colours within an image.
Independent of image position, rotation and scale.
23. Colour-Based Skin Detection
Human skin tones occupy specific regions in colour spaces such as RGB, HSV and YCbCr.
Frequently used in face detection systems.
24. Colour Harmonization
Adjust colours within an image to create visually pleasing combinations.
Used in photography, graphic design and image editing.
25. Colourization
Automatically adds colour to grayscale images.
Modern colourization systems commonly use deep learning.
26. Final Exam Summary
Most Important Points
- Colour: Interaction between light and visual perception.
- Visible Spectrum: Approximately 400–700 nm.
- Rods: Brightness vision.
- Cones: Colour vision.
- Three Cone Types: S, M and L.
- RGB: Additive colour model.
- CMY/CMYK: Subtractive colour model.
- HSV: Hue, Saturation, Value.
- YCbCr: Luminance + Colour Difference.
- Colour Gamut: Range of representable colours.
- RGB is not perceptually uniform.
- CIE LAB and CIE LUV: Perceptually meaningful colour spaces.
- CBIR: Uses colour histograms for image retrieval.
- Applications: Skin detection, harmonization and colourization.