raster graphics and color aaron bloomfield cs 445: introduction to graphics fall 2006
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Raster Graphics and Color
Aaron Bloomfield
CS 445: Introduction to Graphics
Fall 2006
2
Overview
Display hardware 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 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 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)
5. Anode connection
6. Mask for separating beams for RGB part of displayed image
7. Phosphor layer with RGB zones
8. Close-up of the phos-phor-coated inner side of the screen
1. Electron guns
2. Electron beams
3. Focusing coils
4. Deflection coils
Image via Wikipedia: http://en.wikipedia.org/wiki/Cathode_ray_tube
6
Liquid Crystal Display (LCD)
Figure 2.16 from H&B
7
Display Hardware
Video display 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 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
DisplayProcessorDisplay
ProcessorSystem
MemorySystem
MemoryCPUCPU
FrameBufferFrameBuffer
MonitorVideoController
VideoController
System Bus
I/O Devices
Figure 2.29 from H&B
10
Frame Buffer
Frame Buffer Figure 1.2 from FvDFH
11
Frame Buffer Refresh
Figure 1.3 from FvDFH
Refresh rate is usually 60-120 Hz for CRTs
12
DAC
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
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
Color indices
DAC
15
Color CRT
Figure 2.8 from H&B
16
Overview
Display hardware 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
Figure 15.16&15.17 from H&B
H S V Color 0 1.0 1.0 Red120 1.0 1.0 Green240 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 ?
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 fromthe 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 15.3-4 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
BRG
C BGRC
26
Linear Color Matching
Grassman’s Laws:
1. Scaling the color and the primaries by the same factor preserves the match:
2C = 2R + 2G + 2B
2. 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
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
Human Color Vision
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
From http://pages.infinit.net/graxx/Theorie4.html
Human perceptual gamut The cone keeps going
towards the right Brightness (not whiteness!)
keeps increasing
36
Chromaticity Diagram
X
Y
Z
2.77 1.75 1.13
1.00 4.59 0.06
0.00 0.57 5.59
R
G
B
ZYX
Yy
ZYX
Xx
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 http://ian-albert.com/graphics/rgb.php
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
B
G
R
Y
M
C
1
1
1
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
KYY
KMM
KCC
YMCK
,,min
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
B
G
R
Q
I
Y
311.0528.0212.0
321.0275.0596.0
114.0587.0299.0
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)
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