Illumination and Shading

Question 1

Distinguish between illumination and shading

Answer 1

Illumination refers to the use of lights in virtual world. Shading on the other hand, refers to effects that light have on 3D objects in the scene.

Question 2

Outline 4 types of Light Sources

Answer 2

1. Point light source - Parts of the polygons which are closer to the light source appear brighter than those that are further away.
2. Sunlight source - Light rays strike the polygons in the scene from single parallel direction with equal intensity everywhere.
3. Spotlight source - The light is focused and forms a cone-shaped envelop as it projects out from the light source.
4. Area light source - Light source comes from a rectangular area & projects the light from one side of the rectangle. e.g. florescent light fixture in a ceiling panel.

Question 3

Define Shading model?

Answer 3

A shading model checks lighting conditions and figures out what surface should look like based on lighting conditions & surface parameters.

Question 4

Differentiate the terms hard shadows and soft shadows

Answer 4

Hard shadows are those caused by very distant light sources, like the sun. Soft shadows are caused by close light sources like light bulbs

Question 5

Describe the following with an example: local light source and infinite light source

Answer 5

Rays from a local light source emanate in different directions. Rays from the finite light source travel in the same direction.

Question 6

In the shading model, describe the light of concern used in computer graphics.

Answer 6

In computer graphics, we are concerned with the reflected light - light which bounces off object and enters eye.

Question 7

Compare and contrast the following surface properties: ambient lighting, diffuse lighting and specular lighting

Answer 7

Ambient light accounts for indirect light and accounts of light effects that are difficult to compute i.e. secondary reflections
Unlike ambient reflection, diffuse reflection is dependent on location of light source relative to the object.
Specular lighting models reflections on shiny surfaces and is view dependent i.e. it changes as camera’s position changes

Question 8

Describe three shading models

Answer 8

1. Flat shading - perform one lighting calculation per polygon
2. Gouraud shading - performs one lighting calculation per vertex
3. Phong shading - performs one lighting calculation per pixel

Question 9

List any two problems associated with Gouraud shading model

Answer 9

1. Gouraud shading interpolates linearly and so can make the highlight much bigger
2. Gouraud shading can miss highlights that occur in the middle of a polygon

Question 10

One of the problems associated with interpolation shading is that Vertex normals can be incorrect when calculated as average of face normal. Suggest two solutions

Answer 10

1. Add more polygons

2. Test for angles and use different vertex normals for adjacent polygons