Week 6 Colour
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Transcript of Week 6 Colour
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Week 6Colour
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Overview
By the end of this lecture you will be familiar with:– Human visual system– Foundations of light and colour– HSV and user-oriented colour models
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The Human Eye
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HVS Function
• Part of the nervous system• Light enters through the pupil (2mm-9mm)• The lens focuses the light onto the retina• Retina cells are of two types: cones and rods• Huge dynamic range of around 80dB
– 30,000 candel/m2 (white A4 in bright sunlight)– 0.03 cd/m2 (same page in moonlight)
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The Human Visual System• Retina
– light-sensitive membrane consisting of three types of colour sensors (cones)
– each type is most receptive to wavelengths in either the red, green or blue ranges
– perceived colour is a result of the relative excitation of each group of cones
• Leads to a 3-D representation of colour based on red, green & blue primaries
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Contrast Sensitivity of the Eye
• The perception of fine detail is dependent on luminance level
• The eye is good at identifying sharp boundaries• Detail can be missed if the changes are gradual• The eye prefers richness in colour than
resolution– consider PlayStation or Nintendo
• The eye can only resolve about 40 grey levels
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Implications for Multimedia
• Preserve edge information• Consider brightness for fine detail• Consider number of colour shades in use• Consider when to use inverse video• Carefully choose contrasting colours• Useful properties for image & video coding...
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Colour Models
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Light• A narrow frequency band within the spectrum
of electromagnetic energy– the visible spectrum from 400-700nm – others are ultraviolet, infrared, microwaves etc.– each wavelength within the visible spectrum
produces light of a different colour– 400nm=violets, blues, greens, yellows,
700nm=oranges, reds
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Spectral Density• P() - The power per unit wavelength of a
coloured light
P()
(nm)400 700
viol
et
Indi
go
Blu
e
Gre
en
Yel
low
Ora
nge
Red
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Why we see colours?
• Materials have different absorption and scattering amounts for the different wavelengths of light
• Examples:– A yellow object absorbs a lot of blue light, but scatters in the
longer (red and green) wavelengths
– Black clothing gets very hot in sunlight, because it doesn’t scatter much light (obviously not, as it’s black!) so it absorbs a lot (as heat)
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Additive Colour Matching
• Mixing different amounts and wavelengths of light together produces colours
• Maxwell’s trichromatic colour theory
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Colour Description• Coloured light is described in terms of :
– Hue: the “perceived colour” (red/yellow etc) determined by the dominant wavelength. No dominant wavelength = achromatic
– Saturation: the purity of the colour– Brightness/Luminance: the perceived intensity
of the light
• The Chrominance of a colour is the combination of hue and saturation
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Colour Definition
• Dominant wavelength method– useful for description of colours, but not for
precisely obtaining and representing colour values
• A more precise method is based on the reception of coloured light by the human eye
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RGB Colour CubeGreen
Red
Blue
Green (0,1,0)
Cyan (0,1,1)
Yellow (1,1,0)
White (1,1,1)
Black (0,0,0)
Blue (0,0,1) Magenta (1,0,1)
Red (1,0,0)
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Using RGB• Hardware-oriented model
– Equally defined & independent RGB values are well suited to graphics architecture
– Greyscales along line where R=G=B– Additive colour specification (additive primaries)
• colours defined in terms of an addition to black
– Linear colour combination operation• suitable for colour merging operations, but less useful for
colour mixing
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HSV (Hue, Saturation, Value)• Developed by AR Smith (1978)
– re-coding of RGB colour cube to generate a user-view of colour
– set RGB cube on black vertex and look down from the white vertex
– primary and secondary colours are arranged radially around the centre axis
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The HSV Hexicone
Black
Red
Green
Blue
Yellow
Cyan
Magenta
White
Saturation
Value
Hue
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Using HSV• First, select the colour
(pure hue) which most closely matches the desired colour (hue)
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Using HSV cont.
• Lighten the colour by adding white (saturation)
• Saturation indicates the degree to which the hue differs from a neutral gray. The values run from 0%, which is no color saturation, to 100%, which is the fullest saturation of a given hue at a given percentage of illumination.
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Using HSV cont.
• Darken or lighten the colour appropriately by changing the level of illumination. Values run as percentages; 0% appears as black (no light), whilst 100% is full illumination, which washes out the colour.
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Overview of the HSV cone
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Other Hardware Oriented Models
• CMYK– Cyan, Magenta, Yellow, Black– Subtractive primaries
• colour is specified as a subtraction from white• used in printing industry
• YUV, YIQ– Broadcast standards (YUV=UK, YIQ=USA)– Y=Luminance, UV/IQ are chrominance– RGB re-coded for narrow transmission
bandwidth
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User-Oriented Models
• RGB-based models are derived from a good fit with hardware requirements– but they do not provide an intuitive means of user
colour specification– e.g. how to specify brown, gold, etc.?
• User-oriented models attempt to view colour using the perceptive terms identified earlier