CS 586/480 Computer Graphics IIdavid/Classes/CS586/Lectures/CGII_Pres3.pdfCS 586/480 Computer...
Transcript of CS 586/480 Computer Graphics IIdavid/Classes/CS586/Lectures/CGII_Pres3.pdfCS 586/480 Computer...
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CS 586/480Computer Graphics II
Dr. David BreenKorman 207Thursday 6PM Æ 8:50PM
Presentation 310/9/03
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Logisticsn Take attendancen Read research paper and prepare
questionn R.A. Drebin, L. Carpenter, and P. Hanrahan,
"Volume Rendering", ACM Computer Graphics(SIGGRAPH '88 Proceedings), 1988, pp. 65-74
n Michael Thomas presents next weekn Assignment due dates
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Slide Creditsn Jonathan Cohen - Johns Hopkinsn Yizhou Yu - University of Illinoisn Kevin Suffern -University of Technology,
Sydney, Australian G. Drew Kessler, Larry Hodges - Georgia
Institute of Technology
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Color Models
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
Frequency SpectrumFrequency SpectrumFrequency Spectrum
Spectrum describes frequency distribution of Spectrum describes frequency distribution of a light sourcea light source
Wavelength (nm)Wavelength (nm)
Am
plitu
deA
mpl
itude
400400 450450 500500 550550 600600 650650 700700
redredyelyel..greengreenblueblue
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
DefinitionsDefinitionsDefinitions
Hue: quality that distinguishes one color Hue: quality that distinguishes one color family from another (i.e. red, yellow, green, family from another (i.e. red, yellow, green, blue, etc.)blue, etc.)
ChromaChroma: degree of color’s departure from : degree of color’s departure from greyscalegreyscale
Value/Lightness: quality distinguishing light Value/Lightness: quality distinguishing light from dark colorsfrom dark colors
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
More definitionsMore definitionsMore definitions
Achromatic light: literally light without Achromatic light: literally light without chromachroma, or , or greyscale greyscale lightlight
•• fairly uniform frequency distributionfairly uniform frequency distribution
Monochromatic light: light which has all Monochromatic light: light which has all intensity near a single frequencyintensity near a single frequency
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
Color Mixture - SubtractiveColor Mixture Color Mixture -- SubtractiveSubtractive
Applies when mixing pigments and dyesApplies when mixing pigments and dyes
••Each substance absorbs certain frequenciesEach substance absorbs certain frequencies
••Combining substances absorbs the union of Combining substances absorbs the union of these frequenciesthese frequencies
••Resulting reflected light is intersection of colors Resulting reflected light is intersection of colors reflected by eachreflected by each
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
Subtractive Mixture ExampleSubtractive Mixture ExampleSubtractive Mixture Example
from Gerald from Gerald MurchMurch, “Color Displays and Color Science”, in , “Color Displays and Color Science”, in Color and the Color and the ComputerComputer, H. John , H. John DurrettDurrett, ed., page 10., ed., page 10.
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
Color Mixture - AdditiveColor Mixture Color Mixture -- AdditiveAdditive
Applies to mixing of luminescent colors, such Applies to mixing of luminescent colors, such as color CRT and LCD displays, etc.as color CRT and LCD displays, etc.
••Color refers to actual frequency spectrum of Color refers to actual frequency spectrum of lightlight
••Combining lights adds their frequency spectraCombining lights adds their frequency spectra
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
Additive Color ExampleAdditive Color ExampleAdditive Color Example
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
3 Types of retinal cones3 Types of retinal cones3 Types of retinal cones
From Foley, From Foley, vanDamvanDam, , FeinerFeiner, , and Hughes, and Hughes, Computer Computer Graphics: Principles and Graphics: Principles and Practice, 2nd edition, page Practice, 2nd edition, page 577577
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
Efficient Color Computations in Computer GraphicsEfficient Color Computations in Efficient Color Computations in Computer GraphicsComputer Graphics
Represent frequency spectrum as discrete set Represent frequency spectrum as discrete set of samplesof samples
••Typically 3 samples: red, green, and blueTypically 3 samples: red, green, and blue
••Monitors also use samples corresponding to Monitors also use samples corresponding to different phosphorsdifferent phosphors
••Eye also has 3 samples (types of cones)Eye also has 3 samples (types of cones)
Does Does notnot imply that three samples for initial imply that three samples for initial and intermediate produce accurate and intermediate produce accurate computationscomputations
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
Color Space GamutColor Space GamutColor Space Gamut
Color gamut: subspace of visible colorsColor gamut: subspace of visible colorsNo system of mixing colors from fixed No system of mixing colors from fixed
number of primaries can represent all number of primaries can represent all visible colorsvisible colors
from Gerald Murch, “Color Displays and Color Science”, in Color and the Computer, H. John Durrett, ed., page 13.
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
Color Spaces - RGB cubeColor Spaces Color Spaces -- RGB cubeRGB cube
Shortcomings:Shortcomings:••perceptually nonperceptually non--linearlinear••nonnon--intuitive for human specificationintuitive for human specification
From Alan Watt, 3D Computer Graphics, 2nd edition, p. 416
from Foley, vanDam, Feiner, and Hughes, Computer Graphics: Principles and Practice, plate II.4
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
Color Spaces - HSV hexaconeColor Spaces Color Spaces -- HSV HSV hexaconehexacone
Still not perceptually Still not perceptually linearlinear
Axes correspond to Axes correspond to more intuitive more intuitive perceptual qualitiesperceptual qualities••Selection similar to Selection similar to
artist color mixingartist color mixing••choose hue of base choose hue of base
pigment, add white, pigment, add white, add blackadd black
Derived from Derived from projections of RGB projections of RGB cubecube
From Alan Watt, 3D Computer Graphics, 2nd edition, p. 419
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
CIE Color SpaceCIE Color SpaceCIE Color Space
Employs 3 artificial primaries: X, Y, ZEmploys 3 artificial primaries: X, Y, Z
••Mathematical abstractions, not physically Mathematical abstractions, not physically realizablerealizable
••Allow Allow supersaturationsupersaturation
Larger than visible spectrumLarger than visible spectrum
Standard for representing colors and Standard for representing colors and converting between spacesconverting between spaces
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
CIE Space and Device GamutsCIE Space and Device CIE Space and Device GamutsGamuts
from Foley, vanDam, Feiner, and Hughes, Computer Graphics: Principles and Practice, plates II.1 and II.2
Chromaticity DiagramChromaticity Diagram
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Johns Hopkins Department of Computer ScienceCourse 600.456: Rendering Techniques, Professor: Jonathan Cohen
Gamma CorrectionGamma CorrectionGamma Correction
Exponential function converts from deviceExponential function converts from device--independent RGB space to deviceindependent RGB space to device--dependent RGBdependent RGB
••Gamma is exponentGamma is exponent
••Every monitor is differentEvery monitor is different
••Monitor color intensities are nonMonitor color intensities are non--linear with linear with respect to phosphor excitation levelsrespect to phosphor excitation levels
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Light Models
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Spot Light
Dl
Pl
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f
q
n Specified byn Position, Directionn Cone angle, sharpness
n L = Pl - Psn if f > qn IL= 0
n elsen IL= cosn((p/2)(f/q))
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Shading Models
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 5
Consider the process of looking at (or photographing) a scene through awindow.
At every point on the window the light from the scene has a certainintensity I(x, y).
Intensity is a measure of the brightness of the light.
Generally there will be a mixture of light of different wavelengths ateach point on the window: I = I(x, y, λ) where λ is the wavelength.
We don't discuss the wavelength dependency in these notes.
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 6
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 8
Types of illumination
Direct illumination
Light striking the objects comes directly from the light sources
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 9
Indirect illumination
Light striking the objects has been reflected off at least one other objectsince leaving the light source.
The light could also have been refracted through transparent objects.
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 10
Reflection Models
There exists a number of reflection models that model the real world tovarying degrees of realism.
In general, the more realistic a model is, the more physics andcomputation is involved.
Most reflection models incorporate direct illumination.
The indirect illumination is very expensive to calculate.
Only the most advanced rendering techniques incorporate indirectillumination.
Examples are ray tracing and radiosity.
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 11
Some more terminology
Direct illumination is referred to as local illumination.
Indirect illumination is referred to as global illumination.
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 15
Interaction of light and objects.
We must also model the way that light interacts with the surfaces ofobjects.
When light hits the surface of an object, it can be:
• Reflected
• Absorbed
• Transmitted (for transparent objects)
We see opaque objects by the light they reflect.
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 16
The way in which light interacts with real objects is very complicated.
The interaction depends on:
• the material properties of the object
• the wavelength of the incident light
• the angle of incidence of the light and the surface
The polarisation of the light is also involved, but rarely modelled incomputer graphics.
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 22
Diffuse reflection
The incident light is scattered equally in all directions
incident light
surface
This is characteristic of dull, matt surfaces such as paper, bricks, carpet,etc.
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 23
Specular reflection
The reflected light is concentrated around the direction of mirrorreflection, and is spread out.
incident light
surface
mirror reflectiondirection
This can be used to model shiny surfaces.
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Calculating the Reflected Ray
n r = 2 * (n · L) * n - Ln Derivation left for the studentsn Clamp all dot products to zero. They
shouldn’t be negative, but they can ben MAX (0, n · L)
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 40
θ90
o−90
o
1.0 cos θ
cos θ2
cos θ10
cos θ25
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 46
N
RL
V
θ θ φdiffuse
diffuse + specular
n = 10
n = 25
Diffuse + specular reflection
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Computer Graphics Rendering Techniques: Reflection Models © KG Suffern 1991-20001 50
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Originally from Larry F. Hodges, G. Drew Kessler 7
Light-source AttenuationThus far we have ignored the inverse square law: energy decays with the
inverse square of the distance dL to the light source. Including this termwe get
I = Iaka + Ipkd (L•N)/dL2
However, due to our previous assumptions of a point light source anduniform ambient light, using the dL
2 term gives too rapid of a decreasein illumination intensity to look realistic. The 1/dL
2 term is usuallyreplaced by fatt where
fatt = MIN (1/(c1dL), 1)
I = Iaka + Ipkd (N•L)*fatt
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Including Surface Color
n I = IL*kd * C * cos(q) + IL*ks * C * cosn(f)!!!!+ Ia*ka * C
n C - RGB surface colorn Could define Cd, Cs and Ca, but you
don’t have to
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Larry F. Hodges, G. Drew Kessler 11
Too Intense
With multiple light sources, it is easy to generated values of I > 1
One solution is to set the color value to be MIN(I, 1)
• An object can change color, saturating towards whiteEx. (0.1, 0.4, 0.8) + (0.5, 0.5, 0.5) = (0.6, 0.9, 1.0)
Another solution is to renormalize the intensities to vary from 0 to 1if one I > 1.
• Requires calculating all I’s before rendering anything.
• No over-saturation, but image may be too bright, and contrasts alittle off.
Image-processing on image to be rendered (with original I’s) willproduce better results, but is costly.
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Calculating Normalsv -1 -1 -1v 1 -1 -1v -1 1 -1v 1 1 -1v -1 -1 1v 1 -1 1v -1 1 1v 1 1 1f 1 3 4f 1 4 2f 5 6 8f 5 8 7f 1 2 6f 1 6 5f 3 7 8f 3 8 4f 1 5 7f 1 7 3f 2 4 8f 2 8 6
vertices
triangles
n Create vector structure(for normals) same size asvertex structure
n For each facen Calculate unit normaln Add to normal structure
using vertex indexn Normalize all the normalsn N(a,b,g)= aNa+ bNb+ gNc
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Supersample & Average Imagen hres and vres are even
for (i=0; i++; i < hres/2)for (j=0; j++; j < vres/2) {
offset = 2 * [i,j]; newImage[i,j] = (oldImage[offset] +
oldImage[offset + [0,1]] + oldImage[offset + [1,0]] + oldImage[offset + [1,1]])/4;}
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Wrap Upn Discuss next week’s programming
assignmentn Add point lights with colorn Add color, shading properties and
normals to modelsn Phong shadingn Supersample image
n Discuss status/problems/issues withthis week’s programming assignment