Week 10:Rendering 1

In the last lecture we saw how to model objects and represent them as wireframe models. Wire frame models depict the outer hull of the object but do not convey a realistic image of the model. The next step in our quest for visual realism is to paint our models so we can render realistic images. This process is called rendering. 2

In this lecture you will learn all about the components of rendering: Hidden surface removal The CG reflectance model Surface materials (including texture mapping) Lights Shading algorithms 3

What is Rendering? Rendering 3D images is a multi-step process. First, we need to identify which component surfaces (usually polygons) of the model are viewable from the current viewpoint. This process involves back-face culling as well as identifying surfaces obstructed by surfaces in front of them. Once visible surfaces have been identified, we can simply assign a color to them and paint them. 4

 In most cases, however, we do not want surfaces to be colored with just one color. Surfaces should appear shaded based on the amount of light that they receive, as is the case in real life. 5 Figure. A sphere rendered with: a) a single color, b) each polygon rendered with a single color, c) shaded smoothly

 To be able to simulate this effect, we need to define material properties of the surface: not only its color but also how it responds to light. This process is called shading.  We need to define light sources to light up the scene, which enable us to view it-a process called lighting.  Once the shading and lighting of a scene has been established, shading algorithms are then used to finally render the images. 6

Rendering is a complex process and can be very time consuming. For example, a typical image in the film Toy Story took anywhere between an hour and 72 hours to render on a workstation! Different shading algorithms use different methods and tricks to simulate the behavior of light and surfaces. 7

Obviously, which algorithm you use is based on your precise needs. An interactive game needs shaders whose output may not be very sophisticated but can be rendered quickly, whereas a blockbuster movie production can afford to use complex shaders that can take days to render images but produces spectacular results. We shall employ some simple shading algorithms in this book and shall introduce you to some more complex ones. 8