Black Prophecy's Game Physics: Introduction

Hi, my name is Cyrus Preuss and I am the Technical Director at Reakktor Media. Before taking that position, I was responsible for the integration of game physics into the game engine. Basically I am still responsible for this area as physics programmer although the main part of the work is already done.

In my developer diary I would like to outline the „mechanical side” of our game and give you a rough overview about how the single areas work hand in hand in order to create a whole picture. Some of the used terms will sound very technical but I will try to explain them as best as possible.

But one thing at a time. Where does everything begin?

Game Physics

A well known game producer once said “Hey, what’s so difficult at integrating physics into a game? It’s completely easy. For example: I clap my hands – collision – and it works…”. As far as I know he raised some uncomprehending and puzzled glances from the present programmers. If it would be that easy then some programmers would be redundant. Actually physics do work very simple at the first glance, but to deduce how easy or difficult it is to integrate physics into a game from that is of course simply wrong.

Physical objects

To understand what game physics are about it is necessary to explain some concepts first.

Rigid Bodies
A rigid body describes the physical model of a not deformable body. The essential characteristics are specified by its mass centre, its mass, as well as its mass distribution. Ideally the mass distribution is deduced from the so called collision geometry. Simplifications of the mass distribution happen as approximation by simple primitives, for example spheres or boxes. A single rigid body is neither limited in its movement direction nor in its rotation axis. On one hand the movement of a rigid body is determined by collisions with other Rigid Bodies. On the other hand it is possible for the programmer to attach forces and torques with different characteristics. Rigid bodies in relation to Black Prophecy are, for example, player ships or projectiles.

Constraints
Rigid bodies can be correlated with each other by constraints. These constraints are limiting the involved rigid bodies in their freedom of movement. The simplest example is a fixed constraint, which firmly interconnects two rigid bodies. Thus both bodies cannot rotate separately or change their relative position to each other anymore. As constraints do not play a role for Black Prophecy at the current time we can set them aside in the further course. Ragdolls and their articulated joints are a popular example for the use of different kinds of constraints.

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