CompSci 372 – Tutorial
Part 10
Colour
20/10/2008 CompSci 372 - Tutorial 10 1 Electromagnetic Radiation
20/10/2008 CompSci 372 - Tutorial 10 2 Spectrum
● Spectral Density Function (SDF) Energy Energy
400nm Wavelength 700nm 400nm Wavelength 700nm Energy Energy
400nm Wavelength 700nm 400nm Wavelength 700nm
20/10/2008 CompSci 372 - Tutorial 10 3 Spectrum Energy
400nm Wavelength 700nm
● Hue: Dominant wavelength ● Luminance: Sum of energy, Brightness ● Saturation: Ratio Hue part/Luminance
20/10/2008 CompSci 372 - Tutorial 10 4 Spectrum Energy
400nm Wavelength 700nm
● Hue High S High S Med L Low L ● Luminance ● Saturation
20/10/2008 CompSci 372 - Tutorial 10 5 Spectrum Energy
400nm Wavelength 700nm
● Hue Low S Low S High L Low L ● Luminance ● Saturation
20/10/2008 CompSci 372 - Tutorial 10 6 Interaction with Materials
● Absorption ● Transmission ● Reflection ● (Combination)
● Preservation of Energy
20/10/2008 CompSci 372 - Tutorial 10 7 Spectrum
● Spectral Response Function (SRF) Energy Energy
400nm Wavelength 700nm 400nm Wavelength 700nm Energy Energy
400nm Wavelength 700nm 400nm Wavelength 700nm
20/10/2008 CompSci 372 - Tutorial 10 8 Result
● SDF · SRF = Result SDF Energy
400nm Wavelength 700nm Energy
400nm Wavelength 700nm Energy
400nm Wavelength 700nm
20/10/2008 CompSci 372 - Tutorial 10 9 Result
● Light sources defined by their SDF ● Materials defined by their SRF – Absorbed Light SRF ● Surfaces, Eye, CCD chip, ... – Reflected Light SRF ● Mirror, polished surface,... – Transmitted Light SRF ● Glass, water, ...
20/10/2008 CompSci 372 - Tutorial 10 10 Human Eye
● Receptors for – Long/Medium/Short wavelengths (Cone cell, high intensity) – “At night, Gray value/Intensity all cats are gray” (Rod cell, low intensity)
Amplified by neurons
20/10/2008 CompSci 372 - Tutorial 10 11 Human Eye
● We can perceive different SDF's as the same colour! Energy
400nm Wavelength 700nm
20/10/2008 CompSci 372 - Tutorial 10 12 Colour Spaces
● RGB, sRGB, CMYK, YIQ, ypbPr, xvYCC, HSV, HSL ● CIE 1931 XYZ – First attempt to produce a colour space based on human perception ● Most common: tristimulus colour space – Each dimension a primary colour – Values [0...1]
20/10/2008 CompSci 372 - Tutorial 10 13 RGB Colour Space
● Components: – Red Additive – Green – Blue ● Easy to understand ● Difficult to handle – What is “Brown”? – Interpolation problems
20/10/2008 CompSci 372 - Tutorial 10 14 CMYK Colour Space
● Components: – Cyan Subtractive – Magenta – Yellow – (Black/Key) ● Used in Print ● Also not trivial to handle
20/10/2008 CompSci 372 - Tutorial 10 15 HSL Colour Space
● Components: – Hue – Saturation – Lightness ● Interpolation is easier and intuitive
20/10/2008 CompSci 372 - Tutorial 10 16 HSL Colour Space
● Components: – Hue – Saturation – Lightness
20/10/2008 CompSci 372 - Tutorial 10 17 HSV Colour Space
● Components: – Hue – Saturation – Value
20/10/2008 CompSci 372 - Tutorial 10 18 CIE XYZ Colour Space (1931)
● Mother of all colour spaces ● Chromacity Diagram: – Normalised coordinates: X x= X Y Z Y y= X Y Z
20/10/2008 CompSci 372 - Tutorial 10 19 CIE XYZ Colour Space (1931)
● Easy to interpolate ● Gamut of human perception (average person) ● Gamuts of devices are subsets
20/10/2008 CompSci 372 - Tutorial 10 20