1. CSPC 352: Computer Graphics
Chapter 6:
Lighting
and
Shading

2. Overview Local and global illumination
Interactive Computer Graphics
Overview
Local and global illumination
Phong reflectance model (local illumination)
Flat, Gouraud, and Phong
Shading on a polygonal mesh
Surface subdivisions
Shading in OpenGL
WebGL lighting demo

3. Interactive Computer Graphics

4. Need for shading How do you make something look 3D?
Interactive Computer Graphics
Need for shading
How do you make something look 3D?
Shading that is appropriate for the lighting is the primary cue to 3D appearance
[What are some other cues?]
other cues: stereo, parallax, optical flow, object size,

5. Illumination models General approach:
Interactive Computer Graphics
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, usually [1] [2])
Local illumination models consider only the surface, light direction, and viewing direction

6. Interactive Computer Graphics

7. Interactive Computer Graphics
Local illumination
To make lighting fast, 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, …)

8. Interactive Computer Graphics

9. Interactive Computer Graphics
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, …

10. Interactive Computer Graphics
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

11. Ambient light Intensity doesn’t vary with x, y, z, , f
Interactive Computer Graphics
Ambient light
Intensity doesn’t vary with x, y, z, , f
I = [Iar, Iag, Iab]

12. Interactive Computer Graphics
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)]

13. Limitations of point lights
Interactive Computer Graphics
Limitations of point lights
Usually result in artificially high-contrast images
Can generate umbra (full shadow) but not penumbra (partial shadow)
Area lights generate softer shadows, but are usually used only in non-interactive graphics (e.g. raytracing or radiosity)

14. Distant (directional) lights
Interactive Computer Graphics
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]; no attenuation
More efficient to compute than point sources

15. Interactive Computer Graphics
Spotlights
Directional, i.e. light is emitted in a narrow range of angles, q
More realistic spotlights would model a gradual fall-off of light
E.g. cose f = (s • l)e if s is direction of source, l direction to source, both unit vectors

16. Illumination and shading
Interactive Computer Graphics
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

17. Vectors used by Phong model
Interactive Computer Graphics
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, they are unit vectors.

18. Ambient term of Phong model
Interactive Computer Graphics
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)

19. Interactive Computer Graphics

20. Interactive Computer Graphics
Diffuse term
A perfectly diffuse reflector is so rough that it scatters light equally in all directions
But note that when the light comes in at an angle, the same energy is spread out over larger area
Such surfaces are called Lambertian surfaces (obeying Lambert’s Law)

21. Diffuse shading At noon, illum. is 1
Interactive Computer Graphics
Diffuse shading
At noon, illum. is 1
As the angle q (u in figure) decreases, illumination goes to zero
Illumination is proportional to cos(q) (Lambert’s law)
cos(q) = l • n
Id = kd l • n Ld

22. Interactive Computer Graphics

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

24. Interactive Computer Graphics

25. Phong illumination model
Interactive Computer Graphics
Phong illumination model
Phong illumination model:
I = Ambient + Diffuse + Specular
= Ia + Id + Is
= ka La + kd Ld l • n + ks Ls (r • v)a
May add light attenuation term
1/(a+bd+cd2) ( ka La + kd l • 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?

26. Interactive Computer Graphics
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?

27. Interactive Computer Graphics

28. Flat shading drawbacks
Interactive Computer Graphics
Flat shading drawbacks
The human visual system enhances edges
We see stripes (known as Mach Bands) along edges
Much like a convolution!
How to avoid?

29. Gouraud shading Gouraud shading: Computation required Example
Interactive Computer Graphics
Gouraud shading
Gouraud shading:
Define vertex normals as average of 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
Example

30. Bilinear interpolation
Interactive Computer Graphics
Bilinear interpolation
Rasterization is done a scan-line at a time
Bilinear interpolation: given a color at each vertex, how do you render a scan-line with smooth shading?
interpolate vertex colors along each edge of the polygon to get colors for start and end of scanline
interpolate colors along the scanline

31. Interactive Computer Graphics

32. Gouraud drawbacks Drawbacks of Gouraud shading?
Interactive Computer Graphics
Gouraud drawbacks
Drawbacks of Gouraud shading?
Polygon edges are still visible
Brightness is modelled as a linear function, but that’s not really accurate
Real highlights are small and bright and drop off sharply
If polygons are too large, highlights get distorted and dimmed (notice the funny shape)
How to avoid these artifacts?

33. Phong shading To eliminate artifacts, interpolate normals
Interactive Computer Graphics
Phong shading
To eliminate artifacts, interpolate normals
Results: better shading, much nicer highlights
Computation required per pixel?
Computationally much more expensive, but modern graphics cards can do it
Example

34. Interactive Computer Graphics

35. Interactive Computer Graphics
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

36. Specifying lights in OpenGL
Interactive Computer Graphics
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)

37. Interactive Computer Graphics
Top left: pale blue ambient light and white diffuse light source.
Top right: pale blue diffuse light source and almost no ambient light.
Bottom left: directional light source
Bottom right: point light. Notice that the highlights are no longer centered on the objects and that there is (exaggerated?) distance attenuation.

38. Enabling lights Can enable at least 8 lights:
Interactive Computer Graphics
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

39. Interactive Computer Graphics
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

40. Interactive Computer Graphics
Top row: no ambient reflection.
Middle row: Gray ambient reflection.
Bottom row: blue ambient reflection.
First column: blue material property; no specular reflection
Second column: white specular reflection with small shininess exponent.
Third column: high shininess exponent.
Fourth column: blue diffuse color and emissive property instead of specular.

41. OpenGL Gouraud Shading
Interactive Computer Graphics
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?

42. Virtual Trackball shading
Interactive Computer Graphics
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?]
Phong shading?
What would you have to do to handle material properties, surface colors, etc?

43. Surface subdivision In real modelers…
Interactive Computer Graphics
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

44. Interactive Computer Graphics
What’s missing?
Now that the rendering pipeline is all software, we can add additional effects
What’s missing?

45. Fresnel Effect Surface reflectance depends on viewing angle
Interactive Computer Graphics
Fresnel Effect
Surface reflectance depends on viewing angle
Fresnel shaders: allow reflection, specularity, other attributes to depend on viewing angle

46. Interactive Computer Graphics
How would you do this?
Refraction: 1 2 3

47. Interactive Computer Graphics

48. Interactive Computer Graphics
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