1. Raster Graphics and Color
Aaron Bloomfield
CS 445: Introduction to Graphics
Fall 2006

2. Overview Display hardware Raster graphics systems Color models
How are images displayed?
Raster graphics systems
How are imaging systems organized?
Color models
How can we describe and represent colors?
All non-credited images in this slide set are from Wikipedia

3. Overview Display hardware Raster graphics systems Color models
How are images displayed?
Raster graphics systems
How are imaging systems organized?
Color models
How can we describe and represent colors?

4. Display Hardware Video display devices Hard-copy devices
Cathode Ray Tube (CRT)
Liquid Crystal Display (LCD)
Plasma panels
Thin-film electroluminescent displays
Light-emitting diodes (LED)
Hard-copy devices
Ink-jet printer
Laser printer
Film recorder
Electrostatic printer
Pen plotter

5. Cathode Ray Tube (CRT) Electron guns Anode connection Electron beams
Focusing coils
Deflection coils
Anode connection
Mask for separating beams for RGB part of displayed image
Phosphor layer with RGB zones
Close-up of the phos-phor-coated inner side of the screen
Cathode rays are electron beams emitted by a cathode (negative terminal); the annode is number 5
Emission is by heating the cathode in a vacuum tube – hence the name CRT
Image via Wikipedia:

6. Liquid Crystal Display (LCD)
Figure 2.16 from H&B

7. Display Hardware Video display devices Hard-copy devices
Cathode Ray Tube (CRT)
Liquid Crystal Display (LCD)
Plasma panels
Thin-film electroluminescent displays
Light-emitting diodes (LED)
Hard-copy devices
Ink-jet printer
Laser printer
Film recorder
Electrostatic printer
Pen plotter

8. Overview Display hardware Raster graphics systems Color models
How are images displayed?
Raster graphics systems
How are imaging systems organized?
Color models
How can we describe and represent colors?

9. Raster Graphics Systems
I/O Devices
System Bus
Display
Processor
CPU
System
Memory
Frame
Buffer
Video
Controller
Monitor
Figure 2.29 from H&B

10. Frame Buffer
Frame Buffer
Figure 1.2 from FvDFH

11. Frame Buffer Refresh Refresh rate is usually 60-120 Hz for CRTs
Figure 1.3 from FvDFH

12. Direct Color Framebuffer
Store the actual intensities of R, G, and B individually in the framebuffer
24 bits per pixel = 8 bits red, 8 bits green, 8 bits blue
To make these images: in GIMP, do a decompose (from the image->mode menu). Add a all black layer. Then choose compose (filters->colors), and compose it three times, with the two other layers being black
DAC

13. Red component vs. monochromatic
The red component only has the red components of each pixel (duh!)
Monochromatic is a gray-scale image that uses another color instead of white

14. Color Lookup Framebuffer
Store indices (usually 8 bits) in framebuffer
Display controller looks up the R,G,B values before triggering the electron guns
DAC
Color indices

15. Color CRT
Figure 2.8 from H&B

16. Overview Display hardware Raster graphics systems Color models
How are images displayed?
Raster graphics systems
How are imaging systems organized?
Color models
How can we describe and represent colors?

17. 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.

18. How Do Artists Do It?
Artists often specify color as tints, shades, and tones of saturated (pure) pigments
Tint: Adding white to a pure pigment, decreasing saturation
Shade: Adding black to a pure pigment, decreasing lightness
Tone: Adding white and black to a pure pigment
White
Pure Color
Black
Grays
Tints
Shades
Tones

19. Additive color vs. Subtractive color
Additive colors models are used in light
Start with black, and add colored light to make your desired shade
Subtractive color models are used with paint
Start with white, and add colors
A given color – red – subtracts away (from the reflected light) any wavelength that is not red
Additive color mixing:
Subtractive color mixing:

20. HSV Color Model H S V Color 0 1.0 1.0 Red 120 1.0 1.0 Green
Blue
* White
* Gray
* * 0.0 Black ? ? ?
Figure 15.16&15.17 from H&B

21. Intuitive Color Spaces
HSV is an intuitive color space
Corresponds to our perceptual notions of tint, shade, and tone
Hue (H) is the angle around the vertical axis
Saturation (S) is a value from 0 to 1 indicating how far from the vertical axis the color lies
Value (V) is the height of the “hexcone”

22. 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
Luminance is the amount (or intensity) of the light

23. 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

24. 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 from H&B

25. Color Matching
In order to match a color, we can adjust the brightness of 3 overlapping primaries until the two colors look the same.
C = color to be matched
RGB = laser sources (R=700nm, G=546nm, B=435nm)
Humans have trichromatic color vision
C = R + G + B
C + R = G + B B R G C

26. Linear Color Matching Grassman’s Laws:
Scaling the color and the primaries by the same factor preserves the match:
2C = 2R + 2G + 2B
To match a color formed by adding two colors, add the primaries for each color
C1 + C2 = (R1 + R2) + (G1 + G2) + (B1 + B2)

27. RGB Spectral Colors
Match each pure color in the visible spectrum (rainbow)
Record the color coordinates as a function of wavelength ?

28. Perception of color intensities
Which shade of gray is half-way between white and black?
It’s the second one
Humans perceive color intensity (and sound, etc.) on a logarithmic scale
The first one is (about) 3/4 lit
We perceive it as 1/2 lit
The second one is 1/2 lit
We perceive it as 1/4 lit
That exponent is called gamma ()
2.0 is a sample value for a CRT or LCD monitor

29. Human Color Vision
Humans have 3 light sensitive pigments in their cones, called L, M, and S
The cones respond to different lights:
L to red
M to green
S to blue
This leads to metamerism
“Tristimulus” color theory
Green is picked up by all the cones, so it’s the most vivid
Note the vertical axis -- amount of light absorbed – is realtive to the cone itself
S cones need to absorb much less light to hit 100%
Metamerism is when colors match under one condition (a given light source), but not another

30. Just Noticeable Differences
The human eye can distinguish hundreds of thousands of different colors
When two colors differ only in hue, the wavelength between just noticeably different colors varies with the wavelength!
More than 10 nm at the extremes of the spectrum
Less than 2 nm around blue and yellow
Most JND hues are within 4 nm.
Altogether, the eye can distinguish about 128 fully saturated hues
Human eyes are less sensitive to hue changes in less saturated light (not a surprise)

31. Luminance Compare color source to a gray source Luminance
Y = .30R + .59G + .11B
Color signal on a black and white TV

32. Chromaticity and the CIE
Negative spectral matching functions?
Some colors cannot be represented by RGB
Enter the CIE
Three new standard primaries called X, Y, and Z
Y has a spectral matching function exactly equal to the human response to luminance

33. XYZ Matching Functions
Match all visible colors with only positive weights
Y matches luminance
These functions are defined tabularly at 1-nm intervals
Linear combinations of the R,G,B matching functions

34. CIE Color Space

35. Spectral Locus Human perceptual gamut
The cone keeps going towards the right
Brightness (not whiteness!) keeps increasing
From

36. recover the X,Y,Z coordinates
Chromaticity Diagram
Converting from RGB
to XYZ is a snap:
Given x, y, and Y, we can
recover the X,Y,Z coordinates

37. Measuring Color
Colorimeters measure the X, Y, and Z values for any color
A line between the “white point” of the chromaticity diagram and the measured color intersects the horseshoe curve at exactly the dominant wavelength of the measured color
A ratio of lengths will give the excitation purity of the color
Complementary colors are two colors that mix to produce pure white
Some colors are non-spectral --- their dominant wavelength is defined as the same as their complimentary color, with a “c” on the end

38. Gamuts

39. Gamut problems Monitor gamuts are RGB Printer gamuts are CMYK
Each can display colors the other cannot

40. A Problem With XYZ Colors
If we have two colors C1 and C2, and we add C to both of them, the differences between the original and new colors will not be perceived to be equal
C1:
C2:
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
add green
add green

41. The RGB Color Model This is the model used in color CRT monitors
RGB are additive primaries
We can represent this space as a unit cube:
From

42. More on RGB
The color gamut covered by the RGB model is determined by the chromaticites of the three phosphors
To convert a color from the gamut of one monitor to the gamut of another, we first measure the chromaticities of the phosphors
Then, convert the color to XYZ space, and finally to the gamut of the second monitor
We can do this all with a single matrix multiply

43. 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

44. CMYK Most printers actually add a fourth color, black
Use black in place of equal amounts of C, M, and Y
Why?
Black ink is darker than mixing C, M, and Y
Black ink is cheaper than colored ink

45. CMY vs CMYK You can create (more or less) any color with each gamut
Colored printer ink is more expensive
Notice how much less CMY is needed in the CMYK version
One of the reasons printers use CMYK
And color mixing…

46. The YIQ Color Model YIQ is used to encode television signals
Y is the CIE Y primary, not yellow
Y is luminance, so I and Q encode the chromaticity of the color
If we just throw I and Q away, we have black and white TV
This assumes known chromaticities for your monitor
Backwards compatibility with black and white TV
More bandwidth can be assigned to Y

47. HSV color space aside Consider a HSV picture space:
Blue and red are at right angles to each other
Thus, with 2 coordinates, you can define any saturation/hue combination
Let’s call the blue axis Cb
It defines the blue/yellow combination
And the red axis Cr
It defines the red/cyan combination

48. The YCbCr Color Model Y is luma (similar to luminance)
The brightness of a pixel
Cb and Cr define the chrominance
Meaning they each define saturation and hue
Cb is the blue chroma, Cr is the red
From the last slide
Notice the murkiness of the Cr and Cb components
The human eye does not notice differences in them nearly as much

49. JPEG Image Compression
Take an image in the (r,g,b) color space
Assume it’s 8 bits per image (24 bits total)
Convert it to YCbCr
Also 8 bits per image
Downsample Cb and Cr to fewer bits
Let’s say 4 bits (24 = 16)
So it can have values 0, 15, 31, 47, … 255
Each pixel now takes up 16 bits
8 for Y, 4 for Cb and 4 for Cr
Then do some other magic (including zip-like compression)
And you have a (lossy) compressed image

50. Future of color displays
Future color displays may have more pixels
RGB plus yellow, cyan, etc.
Will allow much more vivid color
A greater gamut of color possibilities
Note that both the pictures on the right are being displayed by an RGB output device…

51. Photo printers
Photo printers use many ink colors for rich, vivid color
Also a scam to sell you more ink (the razor business model)