Rendering theory & practice

Introduction

We’ve looked at modelling, surfacing and animating.

The final stage is rendering. This can be the most time consuming part

of the process depending on the complexity of you scene.

Many people underestimate the time needed But you won’t of course!

Some basic things to remember

Render to frame files rather than movie files

Use file formats that use no compression or loss-less compression

Use anti-aliasing (within reason) Don’t raytrace if you can avoid it!

Simple shading

We’ve already considered this in the simplest sense: Flat, Gouraud and Phong shading

None of these consider inter-face reflections or shadows.

We need these for visual realism. For these we need global

illumination algorithms

Global illumination

This simulates the interaction of light with the entire environment rather than individual surfaces.

Light is tracked from emitters to sensors. Shadows are automatically generated, as

are interactions between surfaces. There are two common approaches: ray

tracing and radiosity Before we look at these in detail, we

should look at some general features of global illumination

Global illumination (2)

Ignoring the fact that the calculations (as we shall see later) are complex, the solution to global illumination is simple:

Start at a light source Trace every light path through the

environment it either:* hits the eye point* has its energy reduced below a threshold* travels out of the environment

A first attempt: the rendering equation

where I(x,x’) is the transport intensityg(x, x') is the visibility function(x, x') transfer emittance(x, x', x'') is the scattering term

s

xdxxIxxxxxxxgxxI ,,,,,,

Describes what happens at point x on a surface due to light travelling from it

Another attempt: surface-surface interactions

We can also model the way one surface interacts with another

This is easier to consider non-mathematically

Four different interactions:diffuse to diffusespecular to diffusediffuse to specularspecular to specular

Mechanisms of light transport

Diffuse to diffuse Specular to diffuse

Specular to specularDiffuse to specular

Mechanisms of light transport (2)

Specular-specular transfer can be calculated using ray-tracing

Diffuse-diffuse transfer can be calculated using radiosity

Specular-diffuse and diffuse-specular need a combination

We can categorise the type of transfer so that we know how to handle a given situation

Categories of light transfer

Light-Diffuse-Diffuse-Eye (LDDE) Light-Specular-Diffuse-Eye (LSDE) Light-Diffuse-Specular-Eye (LDSE) Light-Specular-Specular-Eye (LSSE) …

Examples of light transfer

Ray tracing

Initial ray

Transmitted ray

Reflected ray

Reflected ray

Transmitted ray

Eye

Initial ray

Transmitted ray Reflected ray

Including a local model

A classic ray-traced scene1

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Radiosity

This implements diffuse-diffuse transfer. Instead of following individual rays,

interaction between patches in a scene are considered.

This is different from other global illumination algorithms in two important ways:* the solution is view independent* the scene must be divided into patches

Radiosity (2)

Consider a light source as an array of emitting patches

Light is shot from these into the scene and we consider the diffuse-diffuse interaction between the light patch and the first hit patch

The energy arriving at the hit patch is then re-emitted according to the surface properties, hitting other patches…

This process iterates until there are no further significant changes in energy distribution

Radiosity example

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