shading - university of missouri
TRANSCRIPT
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1Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Shading
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2Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Why we need shading
● Build a sphere– Many polygons
– glColor
● Want
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3Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Shading
● Why does the image of a real sphere look like
● Light-material interactions cause each point to have a different color or shade
● Need to consider – Light sources– Material properties– Location of viewer– Surface orientation
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Scattering
● Light strikes A – Some scattered– Some absorbed
● Some of scattered light strikes B– Some scattered– Some absorbed
● Some of this scatteredlight strikes A
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5Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Global Effects
translucent surface
shadow
multiple reflection
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Rendering Equation
● The infinite scattering and absorption of light can be described by the rendering equation – Cannot be solved in general– Ray tracing is a special case for perfectly
reflecting surfaces● Rendering equation is global and includes
– Shadows– Multiple scattering from object to object
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Local vs Global Rendering
● Correct shading requires a global calculation involving all objects and light sources– Incompatible with pipeline model which shades
each polygon independently (local rendering)● However, in computer graphics, especially
real time graphics, we are happy if things “look right”– Exist many techniques for approximating
global effects
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Light-Material Interaction
● Light that strikes an object is partially absorbed and partially scattered (reflected)
● The amount reflected determines the color and brightness of the object– A surface appears red under white light because
the red component of the light is reflected and the rest is absorbed
● The reflected light is scattered in a manner that depends on the smoothness and orientation of the surface
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Light Sources
General light sources are difficult to work with because we must integrate light coming from all points on the source
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Simple Light Sources
● Point source– Model with position and color– Distant source = infinite distance away
(parallel)● Spotlight
– Restrict light from ideal point source● Ambient light
– Same amount of light everywhere in scene– Can model contribution of many sources and
reflecting surfaces
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Surface Types
● The smoother a surface, the more reflected light is concentrated in the direction a perfect mirror would reflected the light
● A very rough surface scatters light in all directions
smooth surface rough surface
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Phong Model
● A simple model that can be computed rapidly● Has three components
– Diffuse– Specular– Ambient
● Uses four vectors – To source– To viewer– Normal– Perfect reflector
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Phong Model
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Ideal Reflector
● Normal is determined by local orientation● Angle of incidence = angle of reflection● The three vectors must be coplanar
r = 2 (l · n ) n - l
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Lambertian Surface
● Perfectly diffuse reflector● Light scattered equally in all directions● Amount of light reflected is proportional to
the vertical component of incoming light– reflected light ~cos θi
- cos θi = l · n if vectors normalized
– There are also three coefficients, kr, kb, kg that show how much of each color component is reflected
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Specular Surfaces
● Most surfaces are neither ideal diffusers nor perfectly specular (ideal reflectors)
● Smooth surfaces show specular highlights due to incoming light being reflected in directions concentrated close to the direction of a perfect reflection
specularhighlight
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Modeling Specular Relections
● Phong proposed using a term that dropped off as the angle between the viewer and the ideal reflection increased
φ
Ir ~ ks I cosαφ
shininess coef
absorption coef
incoming intensityreflectedintensity
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The Shininess Coefficient
● Values of α between 100 and 200 correspond to metals
● Values between 5 and 10 give surface that look like plastic
cosα φ
φ 90-90
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Ambient Light● Light reflected so many times from so
many surfaces it appears to come equally from all directions
● Amount and color depend on:– color of the light(s)
– material properties of the object
● Add ka Ia to diffuse and specular terms
reflection coef intensity of ambient light
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Distance
● Light on surface is inversely proportional to the square of the distance between
– Light point– Surface point
● Diffuse and specular:– 1/d^2
● Constant and linear terms – soften the effect of the point source
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28
Distance
I = 20 I = 10
I = 5Squared distance
Linear distance
D = 2
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Light Sources
● Phong Model, add results from each light source
● Each light source has– Diffuse
– Specular
– Ambient ● Maximum flexibility ● No physical justification
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Adding up the Components
Each light source and each color component, the Phong model can be written (without the distance terms) as
I =kd Id l · n + ks Is (v · r )α + ka Ia
Each color component:add contributions fromall sources
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Light Sources
● Separate red, green and blue components● Hence, 9 coefficients for each point source
- Idr, Idg, Idb, Isr, Isg, Isb, Iar, Iag, Iab
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Material Properties
● Material properties match light source properties– Nine absorption coefficients
• kdr, kdg, kdb, ksr, ksg, ksb, kar, kag, kab
– Shininess coefficient α
http://www.real3dtutorials.com/tut00008.php
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Modified Phong Model
● The specular term in the Phong model is problematic because it requires the calculation of a new reflection vector and view vector for each vertex
● Blinn suggested an approximation using the halfway vector that is more efficient
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The Halfway Vector
• h is normalized vector halfway between l and v
h = ( l + v )/ | l + v |
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Using the halfway vector
● Replace (v · r )α by (n · h )β
● β is chosen to match shininess● Known as the modified Phong or Blinn
lighting model
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Example
Only differences in these teapots are the parametersin the modifiedPhong model
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Computation of Vectors
● l and v are specified by the application● Can compute r from l and n● Problem is determining n● For simple surfaces n can be determined but
how we determine n differs depending on underlying representation of surface
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Plane Normals
● Equation of plane: ax+by+cz+d = 0● Plane is determined by three points p0, p2,
p3 or normal n and p0
● Normal can be obtained by
n = (p2-p0) × (p1-p0)
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Normal to Sphere
● Implicit function f(x,y.z)=0● Normal given by gradient● n = [∂f/∂x, ∂f/∂y, ∂f/∂z]T=p
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Parametric Form
● For sphere
● Tangent plane determined by vectors
● Normal given by cross product
x=x(u,v)=cos u sin vy=y(u,v)=cos u cos vz= z(u,v)=sin u
∂p/∂u = [∂x/∂u, ∂y/∂u, ∂z/∂u]'∂p/∂v = [∂x/∂v, ∂y/∂v, ∂z/∂v]'
n = ∂p/∂u × ∂p/∂v
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Flat Shading
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Interpolative Shading
• Enable with glShadeModel(GL_SMOOTH);
• Calculate color at each vertex
• Interpolate color in interior
• Compute during scan conversion (rasterization)
• Much better image
• More expensive to calculate
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Gouraud Shading
* Need to know adjacent faces
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Averaging Normals
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Barycentric Coordinates
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Steps in OpenGL shading
1. Enable shading and select model
2. Specify normals
3. Specify material properties
4. Specify lights
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Normals
• In OpenGL the normal vector is part of the state
• Set by glNormal*()glNormal3f(x, y, z);glNormal3fv(p);
• Usually we want to set the normal to have unit length so cosine calculations are correct
- Length can be affected by transformations
- Note that scaling does not preserved lengthglEnable(GL_NORMALIZE) allows for
autonormalization at a performance penalty
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Normal for Triangle
p0
p1
p2
n
plane n ·(p - p0 ) = 0
n = (p2 - p0 ) ×(p1 - p0 )
normalize n ← n/ |n|
p
Note that right-hand rule determines outward face
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Enabling Shading
• Shading calculations are enabled byglEnable(GL_LIGHTING)
- Once lighting is enabled, glColor() ignored
- glEnable(GL_COLOR_MATERIAL);
• Must enable each light source individuallyglEnable(GL_LIGHTi) i=0,1…..
• Can choose light model parametersglLightModeli(parameter, GL_TRUE)
•GL_LIGHT_MODEL_LOCAL_VIEWER do not use simplifying distant viewer assumption in calculation
•GL_LIGHT_MODEL_TWO_SIDED shades both sides of polygons independently
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Defining a Point Light Source
• For each light source, we can set an RGBA for the diffuse, specular, and ambient components, and for the positionGL float diffuse0[]={1.0, 0.0, 0.0, 1.0};GL float ambient0[]={1.0, 0.0, 0.0, 1.0};GL float specular0[]={1.0, 0.0, 0.0, 1.0};Glfloat light0_pos[]={1.0, 2.0, 3,0, 1.0};
glEnable(GL_LIGHTING);glEnable(GL_LIGHT0);glLightv(GL_LIGHT0, GL_POSITION, light0_pos);glLightv(GL_LIGHT0, GL_AMBIENT, ambient0);glLightv(GL_LIGHT0, GL_DIFFUSE, diffuse0);glLightv(GL_LIGHT0, GL_SPECULAR, specular0);
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Distance and Direction
• The source colors are specified in RGBA
• The position is given in homogeneous coordinates
- If w =1.0, we are specifying a finite location
- If w =0.0, we are specifying a parallel source with the given direction vector
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Global Ambient Light
•Ambient light depends on color of light sources
- A red light in a white room will cause a red ambient term that disappears when the light is turned off
•OpenGL also allows a global ambient term that is often helpful for testingglLightModelfv(GL_LIGHT_MODEL_AMBIENT, global_ambient)
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Moving Light Sources
•Light sources are geometric objects whose positions or directions are affected by the model-view matrix
•Depending on where we place the position (direction) setting function, we can
- Move the light source(s) with the object(s)- Fix the object(s) and move the light source(s)
- Fix the light source(s) and move the object(s)
- Move the light source(s) and object(s) independently
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Material Properties
• Material properties are also part of the OpenGL state and match the terms in the modified Phong model
• Set by glMaterialv()GLfloat ambient[] = {0.2, 0.2, 0.2, 1.0};GLfloat diffuse[] = {1.0, 0.8, 0.0, 1.0};GLfloat specular[] = {1.0, 1.0, 1.0, 1.0};GLfloat shine = 100.0glMaterialf(GL_FRONT, GL_AMBIENT, ambient);glMaterialf(GL_FRONT, GL_DIFFUSE, diffuse);glMaterialf(GL_FRONT, GL_SPECULAR, specular);glMaterialf(GL_FRONT, GL_SHININESS, shine);
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Front and Back Faces
• The default is shade only front faces which works correctly for convex objects
• If we set two sided lighting, OpenGL will shade both sides of a surface
• Each side can have its own properties which are set by using GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK in glMaterialf
back faces not visible back faces visible
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Transparency
•Material properties are specified as RGBA values
•The A value can be used to make the surface translucent
•The default is that all surfaces are opaque regardless of A
•Later we will enable blending and use this feature
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Efficiency
• Because material properties are part of the state, if we change materials for many surfaces, we can affect performance
• We can make the code cleaner by defining a material structure and setting all materials during initialization
• We can then select a material by a pointer
typedef struct materialStruct { GLfloat ambient[4]; GLfloat diffuse[4]; GLfloat specular[4]; GLfloat shineness;} MaterialStruct;
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Polygonal Shading
•Shading calculations are done for each vertex
- Vertex colors become vertex shades
•By default, vertex shades are interpolated across the polygonglShadeModel(GL_SMOOTH);
• If we use glShadeModel(GL_FLAT); the color at the first vertex will determine the shade of the whole polygon
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Polygon Normals
• Polygons have a single normal- Shades at the vertices as computed by the Phong model can be almost same
- Identical for a distant viewer (default) or if there is no specular component
• Consider model of sphere
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Smooth Shading
•We can set a new normal at each vertex
•Easy for sphere model - If centered at origin n = p
•Now smooth shading works
•Note silhouette edge
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Mesh Shading
•The previous example is not general because we knew the normal at each vertex analytically
•For polygonal models, Gouraud proposed we use the average of the normals around a mesh vertex
n = (n1+n2+n3+n4)/ |n1+n2+n3+n4|
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Gouraud and Phong Shading
• Gouraud Shading- Find average normal at each vertex (vertex normals)- Apply modified Phong model at each vertex- Interpolate vertex shades across each polygon
• Phong shading- Find vertex normals
- Interpolate vertex normals across edges
- Interpolate edge normals across polygon- Apply modified Phong model at each fragment
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67Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009
Comparison
• If the polygon mesh approximates surfaces with a high curvatures, Phong shading may look smooth while Gouraud shading may show edges
• Phong shading requires much more work than Gouraud shading
- Until recently not available in real time systems
• Both need data structures to represent meshes so we can obtain vertex normals
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