CS 445 / 645 Introduction to Computer Graphics

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CS 445 / 645 Introduction to Computer Graphics. Lecture 13 Color. Assignment 3. Due March 23 Fourth years plan for thesis collision We’ll provide lots of details to keep the assignment doable Morphing algorithm. Last Class. We discussed vision physiology and perception of gradients

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### CS 445 / 645Introduction to Computer Graphics

Lecture 13

Color

Assignment 3
• Due March 23
• Fourth years plan for thesis collision
• We’ll provide lots of details to keep the assignment doable
• Morphing algorithm
Last Class
• We discussed vision physiology andperception of gradients
• Today:
• Color perception
• Color representations
Specifying Color
• Color perception usually involves three quantities:
• Hue: Distinguishes between colors like red, green, blue, etc
• Saturation: How far the color is from a gray of equal intensity
• Lightness: The perceived intensity of a reflecting object
• Sometimes lightness is called brightness if the object is emitting light instead of reflecting it.
• In order to use color precisely in computer graphics, we need to be able to specify and measure colors.
Combining Colors

Shining colored lightson a white ball

Subtractive (CMYK)

Mixing paint colors andilluminating with white light

How Do Artists Do It?
• Artists often specify color as tints, shades, and tones of saturated (pure) pigments
• Tint: Gotten by adding white to a pure pigment, decreasing saturation
• Tone: Gotten by adding white and black to a pure pigment

White

Tints

Pure Color

Tones

Grays

Black

HSV Color Space
• Computer scientists frequently use an intuitive color space that corresponds to tint, shade, and tone:
• Hue - The color we see (red, green, purple)
• Saturation - How far is the color from gray (pink is less saturated than red, sky blue is less saturated than royal blue)
• Brightness (Luminance) - How bright is the color (how bright are the lights illuminating the object?)
HSV Color Model
• Hue (H) is the angle around the vertical axis
• Saturation (S) is a valuefrom 0 to 1 indicatinghow far from the verticalaxis the color lies
• Value (V) is the height of the hexcone”
HSV Color Model

H S V Color

0 1.0 1.0 Red

120 1.0 1.0 Green

240 1.0 1.0 Blue

* 0.0 1.0 White

* 0.0 0.5 Gray

* * 0.0 Black

60 1.0 1.0 ?

270 0.5 1.0 ?

270 0.0 0.7 ?

Figure 15.16&15.17 from H&B

Intuitive Color Spaces
• A top-down view of hexcone
HSV Color Space
• A more intuitive color space
• H = Hue
• S = Saturation
• V = Value (or brightness)

Saturation

Value

Hue

Precise Color Specifications
• Pigment-mixing is subjective --- depends on human observer, surrounding colors, lighting of the environment, etc
• We need an objective color specification
• Light is electromagnetic energy in the 400 to 700 nm wavelength range
• Dominant wavelength is the wavelength of the color we “see”
• Excitation purity is the proportion of pure colored light to white light
• Luminanceis the amount (or intensity) of the light
Electromagnetic Spectrum
• Visible light frequencies range between ...
• Red = 4.3 x 1014 hertz (700nm)
• Violet = 7.5 x 1014 hertz (400nm)

Figures 15.1 from H&B

Visible Light
• Hue = dominant frequency (highest peak)
• Saturation = excitation purity (ratio of highest to rest)
• Lightness = luminance (area under curve)

White Light

Orange Light

Figures 15.3-4 from H&B

How well do we see color?
• What color do we see the best?
• Yellow-green at 550 nm
• What color do we see the worst?
• Blue at 440 nm
• Flashback: Colortables (colormaps) for color storage
• Which RGB value gets the most bits?
• Can perceive color differences of 10 nm at extremes (violet and red) and 2 nm between blue and yellow
• Metamers – different energy radiations look like the same color
• Color perception also affected by surrounding light and adaptation
Just noticeable difference (JND)
• 128 fully saturated hues can be distinguished
• Cannot perceive hue differences with less saturated light.
• Sensitivity to changes in saturation for a fixed hue and brightness ranges from 16 to 23 depending on hue.
• Talked about representing intensities last lecture
Human Color Vision
• Humans have 3 light sensitive pigments in their cones, called L, M, and S
• Each has a different spectral response curve:
• “Tristimulus” color theory
Color Spaces
• Three types of cones suggests color is a 3D quantity. How to define 3D color space?
• Idea:
• Shine given wavelength () on a screen
• User must control three lasers producing three wavelengths (say R=700nm, G=546nm, and B=436nm)
• Adjust intensity of RGB until colors are identical
• Note phosphors of TV are not perfect RGBemitters as the results to right demonstrate
A Problem Exists
• Exact target match (l) with phosphors not possible
• Some red had to be added to target color to permit exact match using “knobs” on RGB intensity output of CRT
• Equivalently (theoretically), some red could have been removed from CRT output
• Figure shows that red phosphor must remove some cyan for perfect match
• CRT phosphors cannot remove cyan, so 500 nm cannot be generated
CIE Color Space
• No standard set of three wavelengths can be combined to generate all other wavelengths.
• The CIE(Commission Internationale d’Eclairage) defined three hypothetical lights X, Y, and Z with these spectra:
• Idea: any wavelength  can be matched perceptually by positivecombinations of X, Y, and Z

x ~ R

y ~ G

z ~ B

CIE Color Space
• The gamut of all colors perceivable is thus a three-dimensional shape in X, Y, Z
• Color = xX + yY + zZ
CIE Chromaticity Diagram (1931)
• For simplicity, we often project to the 2D plane x + y + z = 1
• x = x / (x+y+z)
• y = y / (x+y+z)
• z = 1 – x - y
Device Color Gamuts
• Since X, Y, and Z are hypothetical light sources, no real device can produce the entire gamut of perceivable color
• Example: CRT monitor
Device Color Gamuts
• We can use the CIE chromaticity diagram to compare the gamuts of various devices:
• Note, for example, that a color printercannot reproduceall shades availableon a color monitor
A Problem With XYZ Colors
• If we have two colors C1 and C2, and we add DC to both of them, the differences between the original and new colors will not be perceived to be equal
• This is due to the variation of the just noticeable differences in saturated hues
• XYZ space is not perceptually uniform
• LUV space was created to address this problem
RGB Color Space (Color Cube)
• Define colors with (r, g, b) amounts of red, green, and blue
RGB Color Gamuts
• The RGB color cube sits within CIE color space something like this:
Converting Color Spaces
• Simple matrix operation:
• The transformation C2 = M-12 M1 C1yields RGB on monitor 2 that is equivalent to a given RGB on monitor 1
YIQ Color Space
• YIQis the color model used for color TV in America. Y is brightness,I (orange-cyan) & Q (green-magenta) are color
• Note: Yis the same as CIE’s Y
• Result: Use the Y alone and backwards compatibility with B/W TV!
• These days when you convert RGB image to B/W image, the green and blue components are thrown away and red is used to control shades of grey (usually)
Converting Color Spaces
• Converting between color models can also be expressed as such a matrix transform:
• Note the relative unimportance of blue in computing the Y
Perceptually Uniform Color Space
• Color space in which Euclidean distance between two colors in space is proportional to the perceived distance
• CIE, RGB, not perceptually uniform
• Example with RGB
• LUV was created to be perceptually uniform
The CMY Color Model
• Cyan, magenta, and yellow are the complements of red, green, and blue
• We can use them as filters to subtract from white
• The space is the same as RGB except the origin is white instead of black
• This is useful for hardcopy devices like laser printers
• If you put cyan ink on the page, no red light is reflected
• Add black as option (CMYK) to match equal parts CMY
Halftoning
• A technique used in newspaper printing
• Only two intensities are possible, blob of ink and no blob of ink
• But, the size of the blob can be varied
• Also, the dither patterns of small dots can be used
Halftoning – Moire Patterns
• Repeated use of same dot pattern for particular shade results in repeated pattern
• Perceived as a moire pattern