Material and Lighting (2)

Plan

• Recap last time, using tutors/lightmaterial and answer questions.
• Look at twColor() and twGrayLight in TW.py
• Discuss next homework (creative scene)
  • Look at pytw/demos/modeling/Scene-Using-Library-Temple.py.
  • Run some the examples in pytw/.
  • How to divide up your work into modules
  • License request
• (10) lab exercise using materialTutor2, which uses TW's restricted API for material and lighting. You can still use the more powerful stuff!
• Defining surface normals. Why and how. Look at twSolidBarn() in TW.py
• (30) lab exercise on material and lighting
• (20) If there's time, we'll discuss issues of material and lighting, using demos
  • diffuse versus specular demo
  • flat versus smooth shading
  • directional versus positional lights
  • zero ambient light
  • spotlights, and lighting big quads
• (if time) Second exercise on the more powerful OpenGL material and lighting API.

Exercise

Today, I want you to create a gray light illuminating a slightly bluish sphere and a slightly greenish triangle. We'll proceed in steps.

• I suggest starting with the material-and-lighting/TeddyBearLit.py code. Copy it to light-lab.py
• Remove the bear code and expand the bounding box some, so you can work with reasonable size numbers.
• First, get gray light falling on a blueish sphere. You'll know you've succeeded when the sphere looks the right color.
• Adjust the position and direction of the light so that you get a nice specular highlight on the sphere. Use smooth shading so that it looks nicer.
• Now, make the triangle. We save the triangle for last because we need to be sure to remember the surface normal (the sphere automatically has them) and because we have to worry about which side of the triangle we're viewing. The triangle should be angled so that neither the eye nor the light are hitting it perpendicularly. First make the triangle white, then adjust the color.
• The result of phase 1 might be:

  

• Try making the light pinkish.
• If there's time:
  • Experiment with global ambient.
  • Experiment with the shininess of the sphere so you can see the specularity

Demos

Next time, we'll look at the following issues and demos:

• pytw/demos/material-and-lighting/DiffuseVsSpecular.py A demo of the difference between diffuse and specular light, falling on the same material. The left ball only gets specular light, the right ball only gets diffuse light. Neither has any ambient light.

  

• pytw/demos/material-and-lighting/JewelAndBall.py A demo of the difference between flat and smooth shading, using the same material. The left ball uses flat shading, looking like a jewel, and the right one uses smooth shading, looking like a ball. Slices and stacks are the same!

  

• pytw/demos/material-and-lighting/PointVsDirLights.py A demo of the difference between directional light (on the left) and point sources (on the right)

  
  Front View
  
  Top View
  

• pytw/demos/material-and-lighting/Moon.py A demo of the moon, showing zero ambient light. It's cool to rotate this.

  
  

• pytw/demos/material-and-lighting/SpotlightDemo.py An interactive demo of lighting a large quad using a spotlight.

  
  

  It's important to interact with this:

  • + increases the number of squares
  • - decreases the number of squares
  • h hides the lines
  • d draws the lines
  • f flat shading
  • s smooth shading
  • w widens the cutoff angle
  • n narrows the cutoff angle

Exercise Solution

Here is a solution to the exercise. The goal is not to duplicate mine exactly, but to gain control of the tools of material and lighting.

Written by Scott D. Anderson
scott.anderson@acm.org

This work is licensed under a Creative Commons License.