Light, shading and materials

1. Topic 3 Lighting and Shading
Dr. Collins Oduor
Kenyatta University
Lighting and
Shading

2. 2
Topics
 Introduction
 Geometric Model in Lighting
 Colored Surfaces and Lights
 Shading and Graphics Pipeline
 Flat Shading and Smooth Shading

3. 3
Introduction
 Lighting
 Process of computing the luminous intensity reflected
from a specific 3-D point
 Shading
 Process of assigning colors to pixels
 Shading model dictates how light is scattered or
reflected from a surface
 We will begin with achromatic light then colored lights

4. 4
Introduction
 Two types of light sources
 Point light source
 Ambient light
 Light interacts surfaces in different ways
 Absorbed by surface
 Reflected by surface
 Transmitted into the interior
 What absorbs all of the incident achromatic light?

5. 5
Introduction
 Types of reflection of incident light
 Diffuse scattering
 Some of the incident light penetrates the surface
slightly and is re-radiated uniformly in all directions
 Scattering light interacts strongly with surface  color
is affected by nature of surface material
 Specular reflections
 Incident light does not penetrate the surface
 Reflected directly from the surface
 More mirror like and highly directional
 Highlight, shiny, plastic like

6. 6

7. 7
Shading and Graphics Pipeline
 Vertices are sent down the pipeline along with their
associated normals
 All shading calculations are done on vertices
VM clip
projection
matrix
viewport
matrix
shading is
applied here
v1, m1
v2, m2
v0, m0

8. 8
Shading and Graphics Pipeline
 Lights are objects and the positions of light sources are
also transformed by the modelview matrix
 After all quantities are expressed in camera
coordinates, colors are attached to vertices using the
formula
 If an object is clipped, normals of newly generated
vertices are calculated by interpolation

9. 9
Flat Shading and Smooth Shading
 Polygonal face in 3D space
 Individual face
 Underlying surface approximated
 Shading methods
 Flat shading
 Smooth shading
 Gouraud shading
 Phong shading

10. 10
Comparison of Shading Methods

11. 11
Flat Shading
 Entire face is drawn with the same color
 Lateral inhibition
 When there is a discontinuity across an object, the
eye manufactures a Mach band at the discontinuity
and a vivid edge is seen
 Specular highlights are rendered poorly
 Either no highlight at all
 Or highlight on the entire face

12. 12
Smooth Shading
 Smooth shading computes colors at more points on
each face to de-emphasize edges between adjacent
faces
 Use linear interpolation
 Gouraud shading
 Interpolate vertex colors
 Phong shading
 Interpolate vertex normals
 Interpolate normal for each pixel

13. 13
Gouraud Shading
 Used by OpenGL
 Example
 colora: by interpolating
color3 and color4
 colorb: by interpolating
color1 and color2
 Colors of pixels on the horizontal line segment is
obtained by interpolating colora and colorb
 Does not picture highlights well
color1
color2
color3
color4
colora
colorb

14. 14
Phong Shading
 Compute normal at each pixel by interpolating the
normals at the vertices
 Apply the shading model to to every point to find
the color
 Example
 ma: by interpolating m3 and m4
 mb: by interpolating m1 and m2
 Normals of pixels on the
horizontal line segment is
obtained by interpolating ma and mb
 Colors of the pixels are then computed
m1
m2
m3
m4
ma
mb

15. 15
Phong Shading
 Very smooth appearance
 Highlights are approximated better
 Principle drawback
 Heavy computation  slow speed
 Not supported by OpenGL
 Can be approximated using texture mapping
 Phong shading  Phong model
Don’t be
confused!!

16. 16
Introduction
 Lighting
 Process of computing the luminous intensity reflected
from a specific 3-D point
 Shading
 Process of assigning colors to pixels
 Shading model dictates how light is scattered or
reflected from a surface
 We will begin with achromatic light then colored lights

17. 17
Introduction
 Two types of light sources
 Point light source
 Ambient light
 Light interacts surfaces in different ways
 Absorbed by surface
 Reflected by surface
 Transmitted into the interior
 What absorbs all of the incident achromatic light?

18. 18
Introduction
 Types of reflection of incident light
 Diffuse scattering
 Some of the incident light penetrates the surface
slightly and is re-radiated uniformly in all directions
 Scattering light interacts strongly with surface  color
is affected by nature of surface material
 Specular reflections
 Incident light does not penetrate the surface
 Reflected directly from the surface
 More mirror like and highly directional
 Highlight, shiny, plastic like

19. 19
Introduction
 Total light reflected from the surface in a certain
direction is the sum of
 Diffuse components
 Specular components
 We calculate the size of each component that reaches
the eye for each point of interest on surfaces

20. 20
Comparison of Shading Methods

21. 21
Flat Shading
 Entire face is drawn with the same color
 Lateral inhibition
 When there is a discontinuity across an object, the
eye manufactures a Mach band at the discontinuity
and a vivid edge is seen
 Specular highlights are rendered poorly
 Either no highlight at all
 Or highlight on the entire face

22. 22
Smooth Shading
 Smooth shading computes colors at more points on
each face to de-emphasize edges between adjacent
faces
 Use linear interpolation
 Gouraud shading
 Interpolate vertex colors
 Phong shading
 Interpolate vertex normals
 Interpolate normal for each pixel

23. 23
Gouraud Shading
 Used by OpenGL
 Example
 colora: by interpolating
color3 and color4
 colorb: by interpolating
color1 and color2
 Colors of pixels on the horizontal line segment is
obtained by interpolating colora and colorb
 Does not picture highlights well
color1
color2
color3
color4
colora
colorb

24. 24
Phong Shading
 Compute normal at each pixel by interpolating the
normals at the vertices
 Apply the shading model to to every point to find
the color
 Example
 ma: by interpolating m3 and m4
 mb: by interpolating m1 and m2
 Normals of pixels on the
horizontal line segment is
obtained by interpolating ma and mb
 Colors of the pixels are then computed
m1
m2
m3
m4
ma
mb

25. 25
Phong Shading
 Very smooth appearance
 Highlights are approximated better
 Principle drawback
 Heavy computation  slow speed
 Not supported by OpenGL
 Can be approximated using texture mapping
 Phong shading  Phong model
Don’t be
confused!!

26. •Lighting
•Lighting models
•Ambient
•Diffuse
•Specular
•Surface Rendering Methods
•Ray-Tracing

27. “Lighting”
 Two components:
 Lighting Model or Shading
Model - how we calculate
the intensity at a point on
the surface
 Surface Rendering Method
- How we calculate the
intensity at each pixel

28. Jargon
 Illumination - the transport of light from a
source to a point via direct and indirect paths
 Lighting - computing the luminous intensity
for a specified 3D point, given a viewpoint
 Shading - assigning colors to pixels
 Illumination Models:
 Empirical - approximations to observed
light properties
 Physically based - applying physics
properties of light and its interactions with
matter

29. The lighting problem…
 What are we trying to
solve?
 Global illumination –
the transport of light
within a scene.
 What factors play a part
in how an object is “lit”?
 Let’s examine different
items here…

30. Two components
 Light Source Properties
 Color (Wavelength(s) of light)
 Shape
 Direction
 Object Properties
 Material
 Geometry
 Absorption

31. Light Source Properties
 Color
 We usually assume the light
has one wavelength
 Shape
 point light source -
approximate the light source
as a 3D point in space. Light
rays emanate in all directions.
 good for small light sources
(compared to the scene)
 far away light sources

32. Distributed Lights
 Light Source Shape continued
 distributed light source - approximating the light
source as a 3D object. Light rays usually emanate in
specific directions
 good for larger light sources
 area light sources

33. Light Source Direction
 In computer graphics, we usually treat lights as rays
emanating from a source. The direction of these rays
can either be:
 Omni-directional (point light source)
 Directional (spotlights)

34. Ambient Term - Background Light
 The ambient term is a HACK!
 It represents the approximate
contribution of the light to
the general scene, regardless
of location of light and object
 Indirect reflections that are
too complex to completely
and accurately compute
 Iambient = color

35. Diffuse Term
 Contribution that a light has on
the surface, regardless of
viewing direction.
 Diffuse surfaces, on a
microscopic level, are very rough.
This means that a ray of light
coming in has an equal chance of
being reflected in any direction.
 What are some ideal diffuse
surfaces?

36. Different for shiny vs. dull objects

37. Snell’s Law is for IDEAL surfaces
• Think about the amount of light reflected at
different angles.
N
L
R
V
 


38. Combining the terms
 Ambient - the combination of light reflections
from various surfaces to produce a uniform
illumination. Background light.
 Diffuse - uniform light scattering of light rays on
a surface. Proportional to the “amount of light”
that hits the surface. Depends on the surface
normal and light vector.
 Sepecular - light that gets reflected. Depends
on the light ray, the viewing angle, and the
surface normal.

39. Ambient + Diffuse + Specular

40. Lighting Equation
Ilambient = light source l’s ambient component
Ildiffuse = light source l’s diffuse component
Ilspecular = light source l’s specular component
kambient = surface material ambient reflectivity
kdiffuse = surface material diffuse reflectivity
kspecular = surface material specular reflectivity
shininess = specular reflection parameter (1 -> dull, 100+ -> very shiny)
   
   













1
0
lights
l
shininess
specular
l
diffuse
l
ambient
l
final
shininess
specular
specular
diffuse
diffuse
ambient
ambient
final
H
N
k
I
L
N
k
I
k
I
I
H
N
k
I
L
N
k
I
k
I
I
specular
diffuse
ambient
N
L
R
V


41. Attenuation
 One factor we have yet to take into account is that a
light source contributes a higher incident intensity to
closer surfaces.
 The energy from a point light source falls off
proportional to 1/d2
.
 What happens if we don’t do this?

42. What would attenuation do for:
 Actually, using only 1/d2
,
makes it difficult to correctly
light things. Think if d=1 and
d=2. Why?
 Remember, we are
approximating things.
Lighting model is too simple
AND most lights are not point
sources.
 We use:
  2
2
1
0
1
d
a
d
a
a
d
f




43. Shading
 When do we do the lighting equation?
 What is the cost to compute the lighting for a 3D
point?
     
 








1
0
lights
l
shininess
specular
l
diffuse
l
l
ambient
l
final H
N
k
I
L
N
k
I
d
f
k
I
I specular
diffuse
ambient

44. Shading
 Shading is how we “color” a triangle.
 Constant Shading
 Gouraud Shading
 Phong Shading

45. Constant Shading
 Constant Intensity or Flat Shading
 One color for the entire triangle
 Fast
 Good for some objects
 What happens if triangles are small?
 Sudden intensity changes at borders

46. Gouraud Shading
 Intensity Interpolation Shading
 Calculate lighting at the vertices. Then
interpolate the colors as you scan convert

47. Gouraud Shading
 Relatively fast, only do three calculations
 No sudden intensity changes
 What can it not do?
 What are some approaches to fix this?
 Question, what is the normal at a vertex?

48. Phong Shading
 Interpolate the normal, since that is
the information that represents the
“curvature”
 Linearly interpolate the vertex
normals. For each pixel, as you scan
convert, calculate the lighting per
pixel.
 True “per pixel” lighting
 Not done by most
hardware/libraries/etc

49. Shading Techniques
 Constant Shading
 Calculate one lighting calculation (pick a vertex) per triangle
 Color the entire triangle the same color
 Gouraud Shading
 Calculate three lighting calculations (the vertices) per triangle
 Linearly interpolate the colors as you scan convert
 Phong Shading
 While you scan convert, linearly interpolate the normals.
 With the interpolated normal at each pixel, calculate the lighting
at each pixel