1. Visual Appearance Chapter 4
Tomas Akenine-Möller
Department of Computer Engineering
Chalmers University of Technology

2. Overview of today’s lecture
Refresher on simple lighting models
Plus some new stuff
Fog
Gamma correction
Transparency and alpha
Tomas Akenine-Mőller © 2002

3. Compute lighting at vertices, then interpolate over triangle
Geometry
blue
red
green
Rasterizer
How compute lighting?
We could set colors per vertex manually
For a little more realism, compute lighting from
Light sources
Material properties
Geometrical relationships
Tomas Akenine-Mőller © 2002

4. Refresher on lighting Diffuse component : idiff
i=iamb+idiff+ispec
Diffuse is Lambert’s law:
Photons are scattered equally in all directions
Tomas Akenine-Mőller © 2002

5. Lighting Specular component : ispec
Diffuse is dull (left)
Specular: simulates a highlight
Tomas Akenine-Mőller © 2002

6. Specular component: Phong
Phong specular highlight model n l
Reflect l around n: r -l
Read about Blinns highlight formula: (n.h)m
Tomas Akenine-Mőller © 2002

7. Ambient component: iamb
Ad-hoc – tries to account for light coming from other surfaces
Just add a constant color:
Tomas Akenine-Mőller © 2002

8. Lighting i=iamb+idiff+ispec
DEMO
This is just a hack!
Has little to do with how reality works!
Tomas Akenine-Mőller © 2002

9. Additions to the lighting equation2
Depends on distance: 1/(a+bt+ct )
Can have more lights: just sum their respective contributions
Different light types:
Tomas Akenine-Mőller © 2002

10. What’s lighting and what’s shading?
Lighting: the interaction between light and matter
Shading: do lighting (at vertices) and determine pixel’s colors from these
Three types of shading:
Flat, Goraud, and Phong
Tomas Akenine-Mőller © 2002

11. Tomas Akenine-Mőller © 2002
Fog
Simple atmospheric effect
A little better realism
Help in determining distances
Color of fog: color of surface:
How to compute f ?
3 ways: linear, exponential, exponential-squared
Linear:
Tomas Akenine-Mőller © 2002

12. Tomas Akenine-Mőller © 2002
Fog example
Often just a matter of
Choosing fog color
Choosing fog model
Turning it on
Tomas Akenine-Mőller © 2002

13. Tomas Akenine-Mőller © 2002
Gamma correction
If input to gun is 0.5, then you don’t get 0.5 as output in intensity
Instead, gamma correct that signal: gives linear relationship
Tomas Akenine-Mőller © 2002

14. Tomas Akenine-Mőller © 2002
Gamma correction
I=intensity on screen
V=input voltage (electron gun)
a,e, and g are constants for each system
Common gamma values:
Assuming e=0, gamma correction is:
Tomas Akenine-Mőller © 2002

15. Why is it important to care about gamma correction?
Portability across platforms
Image quality
Texturing
Interpolation
One solution is to put gamma correction in hardware…
Tomas Akenine-Mőller © 2002

16. Transparency and alpha
Very simple in real-time contexts
The tool: alpha blending (mix two colors)
Alpha (a) is another component in the frame buffer, or on triangle
Represents the opacity
1.0 is totally opaque
0.0 is totally transparent
The over operator:
Rendered object
Tomas Akenine-Mőller © 2002

17. Tomas Akenine-Mőller © 2002
Transparency
Need to sort the transparent objects
Render back to front
Lots of different other blending modes
Can store RGBa in textures as well
Two ways:
Unmultiplied
Premultiplied
Tomas Akenine-Mőller © 2002