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Advanced Computer Graphics: Colour

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  1. Advanced Computer Graphics:Colour James Gain Department of Computer ScienceUniversity of Cape Town jgain@cs.uct.ac.za Advanced Computer GraphicsCollaborative Visual Computing Laboratory

  2. Objectives • To describe the human visual system and our perception of colour • To introduce the physics of colour • To cover a range of colour models and their relative merits Advanced Computer Graphics

  3. What is Colour? • physical description: • a spectra of wavelengths. • psychological perception: • a stimulus sent from the optic system to the brain. • computer description: • different sets of bases and coordinates, depending on the type of display and application. • Problem: • How do we ensure that the different representations create the same visual effect? Advanced Computer Graphics

  4. Sight • Arguably our principle modality. • An area of considerable research and development. • Components: • Wetware: The human visual system. • Hardware: Visual display systems (e.g. computer monitor). • Software: Rendering of 3D scenes (e.g. PSC). • Perceptual Terms: • Hue: Distinguishes among colours such as red, green, purple and yellow. • Saturation: Refers to how far a colour is from a grey of equal intensity. (How intense is the hue) • Brightness: the perceived intensity of a self-luminating object. (How much light is emitted). Alternatively, lightness refers to intensity from a reflecting object. Advanced Computer Graphics

  5. Human Visual System • The lens of the eye forms an inverted image of the world on the back surface (retina) of the eye. • Cones: • 150000 per square millimeter in the fovea. • High resolution, colour. • Rods: • Lower resolution • monochromatic. • Peripheral vision: so we keep the high-res region in context and avoid being hit by passing branches. • Information is passed to the visual cortex (which performs extremely complex processing). Advanced Computer Graphics

  6. Visual Cortex: Tristimulus Reality? • Types of Cones: • Low: 560 nm red ? • Medium: 530 nm green ? • High: 420 nm blue ? • Signal to brain: • L - M ®red-green • H - (L+M) ®blue-yellow • L + M ®red+green» overall luminance • Red/Green colour blindness means no signal L – M signal. Advanced Computer Graphics

  7. Hardware: The Ultimate Display • Ivan Sutherland’s “Ultimate Display” [1965] postulated a display which produced images indistinguishable from reality. • Requirements of ultimate Head Mounted Display (HMD): • Viewing distance: 10cm • Human visual acuity: 1’ ( ) of visual arc on average 5” ( ) in the best-case • Field of View (FOV): Horizontal = 200 deg Vertical = 100 deg • Update Rate: 60Hz • Must also match other aspects of Human Visual System. • Required Flat Screen Resolution: • 1’ visual acuity requires 12000x6000 pixels • 5” visual acuity requires 144000x72000 pixels Advanced Computer Graphics

  8. Saturation Tints Tones Brightness Greys Shades Artists’ Perception of Colour • Artists mix white and black with pure colours • Tint = Pure colour + white • Shade = Pure colour + black • Tone = Pure colour + black + white White Pure Colour Black Advanced Computer Graphics

  9. Physics of Colour: Colorimetry • Light is electromagnetic energy in the 400-760nm wavelength • Perceived as a spectrum of colours: violet, indigo, blue, green, yellow, orange, red. • Light energy, present simultaneously at each wavelength, is represented as a spectral energy distribution, P(l). • Physics vs. Perception: • Hue = dominant wavelength • Saturation = excitation purity • Brightness (self-luminous objects), lightness (reflecting objects) = luminance (amount of light) • Pure colour: a single highly dominant wavelength Advanced Computer Graphics

  10. Spectral Distribution • One colour = one spectral energy distribution Energy P(l) Wavelength l 400 nm Violet 760 nm Red Advanced Computer Graphics

  11. Hue, Saturation and Brightness Energy P(l) Dominant wavelength e2 Brightness = area under the curve Saturation e1 l 400 nm Violet 760 nm Red Hue Advanced Computer Graphics

  12. Retina Response • Fraction of light absorbed by the three types of cones (R, G, B): 0.2 G R Light Absorbed B l 0.0 760 nm Red 400 nm Violet Advanced Computer Graphics

  13. Retina Luminous Efficiency • The eye’s response to light of constant luminence with varying dominant wavelength. • Peak sensitivity (ability to resolve detail) is to yellow-green light. 100 Relative Sensitivity l 0 760 nm Red 400 nm Violet Yellow-Green 550 nm Advanced Computer Graphics

  14. Colour Definition • Define a colour by its spectral distribution: • Large memory requirements to create an adequate sampling of the distribution. • Dominant wavelength (hue) is not always immediately obvious. • Metamers: Different spectra with the same colour response. No uniqueness. • Need for simpler bases: • describing all colours • with a unique set of coordinates Advanced Computer Graphics

  15. Metamers Same orange-like colour Energy P(l) l 400 nm Violet 760 nm Red Advanced Computer Graphics

  16. Primary Colours • Since our cones are sensitive to red, green and blue why not describe all colours as a linear combination of these primaries? • Below: value of three primaries required to match all wavelengths. Values V(l) l 400 nm Violet 760 nm Red Advanced Computer Graphics

  17. CIÉ Model • Commission Internationale de l’Éclairage defined a colour model in 1931. • Three standard primary functions: • X, Y, Z • Defines all visible colours • Only positive coefficients • Y = retina luminance perception function • X and Y = chromaticity Advanced Computer Graphics

  18. CIÉ Basis Functions Values V(l) Z Y X l 400 nm Violet 760 nm Red Advanced Computer Graphics

  19. CIÉ Chromaticity Diagram • Project the CIÉ plane of constant luminosity (X+Y+Z=1) onto the X-Y plane. Advanced Computer Graphics

  20. CIÉ Chromaticity Diagram • Colours on the horseshoe perimeter are fully saturated. • Complementary colours are those that can be mixed to produce white light. • Diagram factors out luminence so brown (orange-red at very low level luminance relative to its surrounding area) does not appear. • Dominant wavelength can be calculated by drawing a line from white through the colour and intersecting the perimeter. • Nonspectral colours (purple and magenta) intersecting the base of the horseshoe do not have a dominant wavelength. Advanced Computer Graphics

  21. CIÉ Chromaticity Perimeter y Green 520 540 560 Yellow 500 580 White 600 Cyan Red 480 700 Purple x 400 Advanced Computer Graphics

  22. CIÉ Chromaticity Attributes y Green B Yellow A D Cyan Red C E F G Purple x Advanced Computer Graphics

  23. Mixing Colours y Green • All colours on a straight line segment can be created by mixing the endpoints. • Similarly, colours within a triangle are a combination of the endpoints. Yellow I Cyan Red J K Purple x Advanced Computer Graphics

  24. CIÉ Colour Gamuts Monitor RGB Gamut Film Gamut x Advanced Computer Graphics

  25. Beyond CIÉ 1931 • Equal steps across the Chromaticity Diagram do not equate to equal perceptual distances. • CIÉ 1976: perceptually uniform • LUV: • L for lightness • U and V for chromaticity Advanced Computer Graphics

  26. Computer description • Display device based colour-spaces: • RGB (monitor) • CMYK (printer) • YIQ (television) • Perception based colour-spaces: • HSV (user interaction) • Need to: • Correct for different display characteristics • Convert between colour models Advanced Computer Graphics

  27. RGB Colour Model • Red, Green and Blue primaries. • Most well known colour space. • Used (internally) in every monitor. • Additive (colours added to a black background) • Black = (0,0,0), White = (1,1,1) Advanced Computer Graphics

  28. CMY Colour Model • Cyan, Magenta and Yellow Primaries • Used (internally) in colour printers • Substractive (colours subtracted from a white background) • Black = (1,1,1), White = (0,0,0) • Complementary to RGB: Advanced Computer Graphics

  29. The RGB/CMY cube Blue Cyan Magenta White Black Green Red Yellow Advanced Computer Graphics

  30. CMYK Colour Model • CMYK (Cyan, Magenta, Yellow, blacK) • Mostly for printer use • Saves on coloured inks by using black ink as far as possible • As a result dark colours dry more quickly • Converting from CMY to CMYK • Black is used instead of equal amounts of C, M, Y. • K = min(C,M,Y) • C = C – K • M = M – K • Y = Y – K Advanced Computer Graphics

  31. YIQ, YUV Colour Model • U.S. commercial television broadcasting • Recoding of RGB for: • Transmission efficiency • Backward compatibility with Black and White TV (drop I and Q) • YIQ = NTSC • Y is luminance • I and Q are chromaticity • YUV=PAL • Exploits visual system: • More sensitive to changes in luminance (Y) than chromaticity (I and Q) -> more bandwidth for Y • Objects covering a small field of view produce limited colour sensation -> either I or Q can have lower bandwidth. • Ratio Y:I:Q = 4:1.5:0.6 Advanced Computer Graphics

  32. HSV Colour Model • Hue, Saturation and Value Primaries • Cylindrical co-ordinate system: • Hue (H = 0 – 360 deg) rotation • Saturation (S = 0.0 – 1.0) radius • Value (V = 0.0 – 1.0) height • User oriented and based on the intuitive appeal of artist’s colours (hue, tint, shade). • Represented by a hexcone. Advanced Computer Graphics

  33. The HSV cone Green Yellow Cyan Red White Blue Magenta Black Advanced Computer Graphics

  34. HSV conversion • Flatten RGB cube along the diagonal to create HSV hexagon • There is an algorithm for converting in both directions • RGB to HSV: max = max(r,g,b); min = min(r,g,b)[ V = max S = (max-min)/max H = more complicated Green Yellow Red Cyan Blue Magenta Advanced Computer Graphics

  35. HSV properties • Interpolating Colour: • Linear interpolation of the same two colours in RGB, CMY, YIQ and CIE all produce the same result. Appropriate for Gouraud shading • Not true for HSV. But interpolating between colours while keeping Hue, Saturation or Value constant is obviously easier. • Effective for visualization: • change saturation, at constant hue • change hue, at constant saturation (maps) Advanced Computer Graphics

  36. Colour Fidelity • Problem: ensure that colours look the same when changing display devices • Sample the RGB phosphors with colorimeters • Sometimes device description are available from manufacturers • Convert to XYZ space: • From XYZ to device dependent RGB uses the inverse. Advanced Computer Graphics

  37. Intensity Perception • Human perception of intensity is logarithmic: • Sensitive to ratios of intensity levels not absolute steps • I = 0.1 to 0.11 has same as I = 0.5 to 0.55 • Cause of Mach-Banding • Need more steps at lower intensity levels • CRT intensity related to voltage: • Constants depend on the particular CRT • Gamma Correction: lookup table which correct for both hardware and perceptual issues with intensity. • Can lead to strange effects if forgotten. Transferring a picture between monitors with different gamma values causes problems. Advanced Computer Graphics

  38. Displaying colours • Colour ability of the display • measured in bits: • 1 bit=2 levels, 8 bits=256 levels • bits per colour primitive: • 24 bits=256 levels for each of R,G,B • Depends on the medium: • TV Screen: 30 dpi, 8bits colour • Computer Screen: 70-100 dpi, 24 bits colour • Laser Printer: 300-2400 dpi, 3 bits colour (8 colours) • Photo: 800 dpi, 36 bits colour Advanced Computer Graphics

  39. Gaining colour resolution • Sometimes, this colour resolution is not enough. How do we display more colours? • Basic idea: • Sacrifice some of the spatial resolution for intensity resolution • Halftoning and dithering • Halftoning: • For top quality printers • Discs of size varying inversely with Intensity • Pattern angle (called screen angle) • Halftone resolution (different from spatial resolution) • 60-80 dpi for newspapers, 120-200 dpi for books Advanced Computer Graphics

  40. Dithering • Used if halftoning is not possible (because of the wrong type of printer or use of a monitor) • Dither patterns • Group of n*n pixels = n*n+1 intensity levels • 2x2 example: • Matrix notation: 0 1 2 3 4 Advanced Computer Graphics

  41. Dithering Properties • For level i, display pixels with v < i • Requirement of matrix: • Don’t produce visual artefacts (e.g. straight lines) • Growth sequence (minimizes differences in nearby areas) • dispersed dots for CRT, clustered dots for printers Advanced Computer Graphics

  42. Dithering: the next step • One image pixel = 4 or 9 printer pixels • What if I want one image pixel = one printer pixel? • Use modulo: • i = x modulo n • j = y modulo n • Light the pixel if value (i, j) in dithering matrix is smaller than I(x, y) • Error diffusion: • There is an error at each pixel • Dithering pattern is sometimes visible • Floyd-Steinberg error diffusion spreads the error to neighbouring pixels Advanced Computer Graphics

  43. Using Colours in CG • Meaningless colours reduce performance by up to one third. • Design colour schemes first in monochrome. This does not prejudice the colour blind. • Aesthetics: Use even perceptual steps between colours, complementary colours, even intensities. • Using colours for codes can have unintended meanings. • Perception: • Colour of area is affected by surroundings. • Colours seem to advance (red) or recede (blue). • Blocks of the same size in different colours appear to have different sizes (red > green) Advanced Computer Graphics

  44. Conclusion • Several ways to represent colours. Each one suited to a specific task: • Conversion between colour models is often required: • for colour fidelity (convert to CIE and back to device-space) • for displaying the image in a different medium • But remember colour is heavily influenced by our perceptions! Advanced Computer Graphics