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CSPC 352: Computer Graphics Chapter 6: Lighting and Shading 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

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Cspc 352 computer graphics l.jpg

CSPC 352: Computer Graphics

Chapter 6:

Lighting

and

Shading


Overview l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
Ambient light

  • Intensity doesn’t vary with x, y, z, , f

  • I = [Iar, Iag, Iab]


Point lights l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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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