Lecture 7 - Color (slides) - Interactive Computer Graphics...

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Interactive Computer Graphics: Lecture 7 Colour
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Graphics Lecture 7: Slide 2 Ways of looking at colour 1. Physics 2. Biology (how do human visual receptors work?) 3. Psychology (how do humans subjectively assess colour?)
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Graphics Lecture 7: Slide 3 The physics of colour A pure colour is a wave with: Wavelength ( λ ) Amplitude (intensity or energy) (I)
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Graphics Lecture 7: Slide 5 Colours are energy distributions Color Frequency Wavelength violet 668–789 THz 380–450 nm blue 631–668 THz 450–475 nm cyan 606–630 THz 476–495 nm green 526–606 THz 495–570 nm yellow 508–526 THz 570–590 nm orange 484–508 THz 590–620 nm red 400–484 THz 620–750 nm
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Graphics Lecture 7: Slide 6 Colours are energy distributions Lasers are light sources that contain a single wavelength (or a very narrow band of wavelengths) In practice light is made up of a mixture of many wavelengths with an energy distribution.
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Graphics Lecture 7: Slide 7 Light distribution for red Energy 300 nm (violet) Wavelength Light distribution perceived as red 700 nm (red)
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Graphics Lecture 7: Slide 8 Sunlight Energy 300 nm (violet) 500 nm (green) 700 nm (red) Wavelength Light energy distribution in sunlight
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Graphics Lecture 7: Slide 9 Human Colour Vision Human colour vision is based on three ‘cone’ cell types which respond to light energy in different bands of wavelength. The bands overlap in a curious manner.
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Graphics Lecture 7: Slide 10 Human receptor response 400 500 600 700 Blue Green Red Wavelength Relative Sensitivity
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Graphics Lecture 7: Slide 11 Tri-Stimulus Colour theory The receptor performance implies that colours do not have a unique energy distribution. And more importantly: Colours which are a distribution over all wavelengths can be matched by mixing three. R G B
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Graphics Lecture 7: Slide 12 Colour Matching Given any colour light source, regardless of the distribution of wavelengths that it contains, we can try to match it with a mixture of three light sources X = r R + g G + b B where R, G and B are pure light sources and r, g and b their intensities For simplicity we can drop the R G B.
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Graphics Lecture 7: Slide 13 Colour Matching
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Graphics Lecture 7: Slide 14 Subtractive matching Not all colours can be matched with a given set of light sources (we shall see why later) However, we can add light to the colour we are trying to match: X + r = g + b With this technique all colours can be matched.
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