Cathedral Demo v 1.01

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Introduction

This interactive demo combines several lighting and shadowing techniques in order to simulate realistic lighting for a game environment. Stencil shadows are used to achieve real-time shadowing, and per-pixel lighting techniques are used to render the different surfaces in a realistic way

Explore the scene and reposition lights in order to see the effects of the dynamic lighting system. Further options can be specified using the init.xml config file (see below).

System Requirements

A DX9-class video card (ATi Radeon 9500 / Geforce FX 5200 or higher)
1.2 GHz CPU with SSE capability (P3/Athlon XP or higher)
256MB RAM
Windows 2000/XP

Recent video card drivers are also fairly important.

Important: Unless your video card is very expensive, DISABLE ANTI-ALIASING in your drivers. Stencil shadowing is fill-rate intensive.

Note: if you have a non-SSE machine, such as an old Athlon (pre-XP), download the patch from my web site.

Program Instructions

There are two camera modes in the demo. These can be toggled using the F1 key.

Movement

In the walk mode, the camera acts like a camera in a first person shooter game. The player can move around the scene, using the mouse to rotate and the cursor keys to walk..

In the orbital mode, the camera orbits the scene. The camera can be zoomed using the mouse wheel, or A and Z on the keyboard. To rotate the camera, hold the right mouse button and drag.

Light Control

Light tracking is enabled or disabled with the L key. Light tracking causes one of the lights in the scene to track the camera position. Disabing light tracking causes the light to remain where it was last placed. Using this method it is possible to position the movable light anywhere in the scene.

XML Configuration

Various options can be configured in the init.XML file, which can be found in the config files directory. Changing many of these options is inadvisable unless you know how the engine works, but it’s relatively simple to add lights to the scene.

For example, to add a blue light to the scene, navigate to the scene element, and add the following code:

</light>

Note: Currently the range attribute is unsupported, so leave this at 0.

Technical Details

The demo uses a multi-pass algorithm, similar to Doom 3’s to render multiple lights. An initial ambient pass writes the Z buffer values and fills the ambient colour. Each light adds another pass.

Lighting Method

Each lighting pass performs diffuse and specular bump mapping. Gloss maps used to vary shininess at each point. Unlike Doom 3, the shininess power coefficient (and thus the highlight size) can be varied based on the gloss map texels, which means more realistic surface refraction can be achieved (see John Carmack’s speech to DoomCon, 2004).

Shadow Volumes

The program renders shadow volumes by calculating their bounds and determining if they fall within the view frustum. The bounds are approximated using an 8-sided bounding pyramid. The camera’s position relative to this bounds pyramid is used to determine the most efficient way of rendering the shadows.

The demo uses a depth-fail shadowing algorithm where the camera lies within the shadow’s bounds pyramid. Elsewhere, a depth-pass method is used. Shadow volume caps are only rendered if necessary.

Object Shaders

A major part of this program was the development of a C-like scripting language, BunnyScript. Bunnyscript is used in order to provide per-object/mesh programmability equivalent to the per-vertex functionality that vertex shaders provide. This avoids the need to hard-code per-object functionality, saving on compile time and increasing shader flexibility.

BunnyScript is compiled to a form of bytecode, then interpreted using a virtual machine. The VM has been optimised sufficiently to run several hundred object shaders per frame with minimal overhead. BunnyScript is a flexible language, and can be applied to more general scripting tasks in addition to object shaders.

Models/scene representation

Fully animated models with transformation hierarchies are possible using the B3D file format, which was developed as part of the framework for this program. In this program, the scene is fairly static (with only one animated mesh), however, character skinning is also supported and this is used in other programs.