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## PowerPoint Slideshow about 'CSPC 352: Computer Graphics' - JasminFlorian

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Overview

- Local and global illumination
- Phong reflectance model (local illumination)
- Flat, Gouraud, and Phong
- Shading on a polygonal mesh
- Surface subdivisions
- Shading in OpenGL

Perspective

- Lighting and shading are accomplished by modeling the world and simulating the laws of physics
- Short story [Stanislaw Lem, The Cyberiad]: The constructor Trurl creates a tiny simulation of a kingdom in a box to make a deposed, exiled despot happy. Trurl’s friend thinks that is terrible…
- There are those who say that we exist in the mind of God. What do you think of that idea?
- Pascal, Pensées: “The arithmetical machine produces effects which come closer to thought than anything which animals can do; but it can do nothing which might lead us to say that it possesses free will, as the animals have.”

Need for shading

- Was it hard to make the 3D flower (first program) look 3D?
- Shading that is appropriate for the lighting is the primary cue to 3D appearance
- [What are some other cues?]

Illumination models

- General approach:
- model the world
- simulate physics
- Global illumination models (ray tracing, radiosity) determine shading by bouncing light around an entire environment (too slow for interactive graphics)
- Local illumination models consider only the surface, light direction, and viewing direction

Local illumination

- To make lighting fast enough, we will initially restrict our attention to:
- Light source, one surface, and viewer (ignore inter-object reflections, shadows)
- Ambient, diffuse, and specular reflection (ignore transparency, refraction, reflection, …)

Light sources

- In general, a light source is a rather complicated thing. It can emit different amounts of light for each
- Location (x, y, z)
- Direction (, f)
- Wavelength (l)
- Illumination function:I(x, y, z, , f, l)
- Examples: ambient, point, area, spot,distant, …

Colored lights

- Intensity of emitted light can also be a function of wavelength
- We usually model as I = [Ir, Ig, Ib] components
- Some experiments have been done with a whole spectrum of color values, giving more realistic results in some cases

Ambient light

- Intensity doesn’t vary with x, y, z, , f
- I = [Iar, Iag, Iab]

Point lights

- Point lights have a location (so farther objects receive less light) but are not directional
- I(p0) = [Ir(p0), Ib(p0), Ig(p0)]
- How would you compute the illumination at point p?
- Illumination proportional to inverse square of distance
- I(p, p0) = (1/d2) [Ir(p0), Ib(p0), Ig(p0)]

Limitations of point lights

- Usually result in artificiallyhigh-contrast images
- Can generate umbra (full shadow) but notpenumbra (partial shadow)
- Area lights generate softershadows, but are usuallyused only in raytracing or radiosity

Distant (directional) lights

- Light point lights, but
- Without attenuation based on the distance
- Without difference in direction (parallel rays)
- Location of light source becomes [x, y, z, 0]; noattenuation
- More efficient to computethan point sources

Spotlights

- Directional, i.e. light is emitted in a narrow range of angles, q
- More realistic spotlights wouldmodel a gradual fall-off of light
- E.g. cosef= (s • l)e if s is direction ofsource, l direction to source, both unit vectors

Illumination and shading

- How do these light sources affect brightness of a surface point?
- Most commonly used model for interactive graphics: Phong Illumination Model
- Involves terms:
- Ambient
- Diffuse
- Specular
- It is a (simplified) model of the physics of reflection

Vectors used by Phong model

- The directions used by the phong model
- n: surface outward normal
- v: direction to viewer
- l: direction to light source
- r: reflection direction
- Since these are directions, theyare unit vectors.

Ambient term of Phong model

- An object has an ambient reflectivity coefficient, ka
- A light source gives off a certain amount of ambient light, La
- Total ambient illumination: Ia = ka La
- (For colored light, we repeat this computation for R, G, and B ambient light values and reflectivity coefficients)

Diffuse term

- A perfectly diffuse reflector is so rough that it scatters light equally in all directions
- But note that when thelight comes in at an angle,the same energy is spreadout over larger area
- Such surfaces are calledLambertian surfaces (obeying Lambert’s Law)

Diffuse shading

- At noon, illum. is 1
- As the angle q (u infigure) decreases, illumination goes to zero
- Illumination is proportional to cos(q) (Lambert’s law)
- cos(q) = l • n
- Id = kdl • n Ld

Specular Term

- Specular term adds highlights in the reflection direction
- Note that the smoother and shinier the object, the tigher and brighter thehighlight
- Highlight power falls as viewer v moves away from reflection dir, r. (cos f = v•r)
- Modeled as cosaf, a is shininess coefficient (1..200)
- Is = ks Ls (r•v)a

Phong illumination model

- Phong illumination model:

I = Ambient + Diffuse + Specular

= Ia + Id + Is

= ka La + kd Ldl • n + ks Ls (r•v)a

- May add light attenuation term

1/(a+bd+cd2) ( ka La + kdl • n Ld) + ks Ls (r•v)a

- Parameters needed:
- Light: La, Ld, Ls for each light
- Surface: ka, kd, ks, a
- Repeat for each color component, light source
- How hard to calculate?

Polygon shading

- How do you use the Phong Illumination Model to render an object with shading?
- Consider a polygonal sphere approximation
- How do you find the normals to the faces?
- Shade a face with a constant color?

glShadeModel(GL_FLAT);

- Called flat shading or Constant shading
- How much computation would this require
- Per pixel?
- Per vertex?

Flat shading drawbacks

- The human visual system enhances edges
- We see stripes (known as MachBands) along edges
- Much like aconvolution!
- How to avoid?

Gouraud shading

- Gouraud shading:
- Define vertex normals as averageof surrounding faces
- Compute lighting equation at each vertex
- Interpolate colors across polygon

glShadeModel(GL_SMOOTH);

- Computation required
- Per pixel?
- Per vertex?
- Very fast! Especially with reasonably large polygons and hardware color interpolation

Gouraud drawbacks

- Drawbacks of Gouraudshading?
- Polygon edges are still visible
- Brightness is modelled asa linear function, but that’snot really accurate
- Real highlights are smalland bright and drop off sharply
- If polygons are too large, highlights get distorted and dimmed (notice the funny shape)
- How to avoid these artifacts?

Phong shading

- To eliminate artifacts, interpolate normals
- Results: better shading, much nicer highlights
- Computation required per pixel?
- This is still too expensive to do in hardware, in general

Shading summary

- Don’t confuse Phong Illumination Model and Phong Shading
- Gouraud shading: compute illumination model at each vertex. Interpolate colors. (Often done in hardware)
- Phong shading: interpolate vertex normals. Compute illumination model at each vertex

Specifying lights in OpenGL

- OpenGL supports those four light types
- Point, directional lights

GLfloat light0_pos[] = {1.0, 2.0, 3.0, 1.0};

GLfloat light0_pos[] = {1.0, 2.0, 3.0, 0.0};

- Diffuse, Ambient, Specular coefficients

GLfloat diffuse0[] = {1, 0, 0, 1};

GLfloat ambient0[] = {1, 0, 0, 1};

GLfloat spedular0[] = {1, 1, 1, 1};

glEnable(GL_LIGHTING)

Enabling lights

- Can enable at least 8 lights:

glEnable(GL_LIGHT0);

glLightfv(GL_LIGHT0, GL_POSITION, light0_pos);

glLightfv(GL_LIGHT0, GL_AMBIENT, ambient0);

glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuse0);

glLightfv(GL_LIGHT0, GL_SPECULA, specular0);

- Spotlights: set more light parameters as above
- GL_SPOT_DIRECTION
- GL_SPOT_EXPONENT
- GL_SPOT_CUTOFF

Specifying Materials

- Material properties are part of the drawing state, specified by glMaterialfv

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};

glMaterialfv(GL_FRONT, GL_AMBIENT, ambient);

glMaterialfv(GL_FRONT, GL_DIFFUSE, diffuse);

glMaterialfv(GL_FRONT, GL_SPECULAR, specular);

glMaterialf(GL_FRONT, GL_SHININESS, 100.0);

GLfloat emission[]={0.0, 0.3, 0.3, 1.0};

glMaterialfv(GL_FRONT, GL_EMISSION, emission);

- Use GL_FRONT_AND_BACK for two-sided faces

OpenGL Gouraud Shading

- OpenGL needs to know vertex normals as well as locations

glNormal3fv(n);

glVertex3fv(p);

- How to compute vertex normals?
- Cross product for face normals
- Average normals of surrounding faces
- How to find neighboring faces?

Virtual Trackball shading

- Flat shading
- Compute a normal for each face
- Gouraud shading
- Compute a normal for each vertex as average of adjacent faces
- [Defect: you may not want to smooth-shade across every edge: how should it really be done?]
- What would you have to do to handle material properties, surface colors, etc?

Surface subdivision

- In real modelers…
- one can usually define smooth curved surfaces
- Eg spheres, quadrics, NURBS, smoothed polygons
- Modeler renders with smoothness setting
- Recursively split polygons into smaller pieces, with new vertices on the smooth surface
- Splitting a triangle can be done by
- Bisecting angles
- Computing the centrum
- Bisecting sides
- Of course, smoother surfaces take longer to draw

Chapter summary

- Phong illumination model has ambient, diffuse, and specular terms
- It can be used for Flat, Gouraud, and Phong shading
- OpenGL supports eight ambient, point, distant, and spot lights
- You must specify light and material properties with many OpenGL function calls
- Curved surfaces are modeled with polygon subdivisions

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