I’ve decided to try something a bit different and mix in physics posts while I wrap up the math primer. Many of you might be more interested in the physics, or may already be familiar with the math topics, so this gives me a chance to have something for everyone.
Today, we’ll look at some of the core concepts around physics simulations in video games. We’ll go over some terminology and approaches so that we’re all on the same page when discussing more involved topics in later posts. First off, we’ll discuss what it means to have physics in a video game, and how does that compare with physical simulation in other software fields. Then, we’ll discuss the basic steps in simulating physics in a game, and how it integrates with the rest of the game code.
Physics in Video Games
Video games have become incredibly complicated pieces of software over the past 20 years. I recall a time when PC games could fit on a single density 720KB 5.25” floppy disk. Several years later when CDs were initially becoming available, you could find a single CD with over 1000 games on it! These games ran at measly 320x240 resolutions with 8 or 16 colors.
Today, games easily weigh in at several gigabytes, with many being tens of gigabytes. They have nearly photo-realistic graphics with incredibly high resolutions. Along with that extra visual realism comes an expectation of immersion from the user. The point of having incredibly realistic graphics is to make the player feel like they’re actually in the game. However, for the illusion to hold, the virtual world around them must also behave in a believable manner. The best graphics in the world wouldn’t make a player feel like they’re actually immersed if the characters move unnaturally, pass through walls or floors, or things don’t fall and topple as if they had realistic mass and material properties. These are just some of the challenges of making the world feel believable.
Physical simulation software is nothing new, and has been used for decades for scientific research and study. Even the use of physics in games isn’t entirely new, though it’s gotten a lot more attention in recent years. Even some of the earliest games, such as Pong, had very primitive physics simulations in the sense that the trajectory of the ball bouncing off the paddles was determined in a way that would seem intuitive and consistent with our understanding of physics. What is new, however, is the recent adoption of general purpose physics simulations in video games. The physics in early games was very much written for the specific scenarios of that game. For example, it could handle projecting the trajectory of a ball off of a paddle, but nothing else. This meant that for each new effect the game wanted to have, a new piece of physics code would have to be written to handle it. Additionally, a new game written would have to have new physics code written for it as well. As game worlds became more complicated, the need for more physical scenarios increased, and the cost of writing specialized functions for each scenario became too prohibitive. During this time, the concept of having a general purpose piece of code that, given some overall parameters, could simulate arbitrary physical conditions and scenarios came to be. These general purpose physics engines could allow game developers to create games with many more interesting scenarios, and do it much more quickly.
Before moving on to discussing the anatomy of these physics engines, there’s one more important point to consider. Physics simulations are just that, simulations. Physics, in general, is very complex and not possible to replicate in code. However, if we take certain core aspects or principles of physics, and simulate them with some acceptable level of accuracy, then performing this in code becomes much more reasonable. The most common class of physics simulated in games is called Rigid Body Dynamics. Rigid body simulations use Newtonian principles of movement and mass to simulate rigid objects (spheres, boxes, a tower, a boulder, a wall, etc…). In games, the physical accuracy of the simulation only needs to be high enough to be believable, and is often times reduced to improve performance. A smooth running, albeit slightly less accurate, game is much more immersive than a perfectly accurate game that runs poorly and constantly drops frames.
Basic Components of Physics Engines
There are many different physics engines out on the market today. Each one works somewhat differently than the next, and each uses different approaches to solving the general physics problem for games. However, in order to solve the problem for games, there are certain things that every engine must do. For the purposes of this post, we’ll be focusing on rigid body dynamics as it’s the most common type of physics in games. I’ll try and include posts later to discuss other types of simulations such as cloth, fluid, or gas. All of the components of the physics engine can be summarized in a single general operation called the simulation step.
Video games display many rendered frames per second. Comparing this to animation, many small in-between images are shown quickly to give the impression of movement. The de facto standard for most PC and console games is to target 60fps, which means that each frame represents about 0.01667s of time. For the physics simulation to match, only 0.01667s worth of physics should happen each frame. That means an object moving forward at a velocity of 1m/s should only move 0.01667 meters that frame, and then again the next frame, and then again, and finally after 60 frames have gone by, the object would have covered 1 meter of distance.
These intervals of moving objects along their trajectories is called the physics simulation step. Almost every physics simulation equation involves time, and the time used for solving these is this time slice determined by the game. Most engines will take in the time as a value each frame, as it could change from frame to frame. Some games use variable frame rates, and so a static number should never be assumed.
So, what exactly happens during the simulation step?
A typical physics engine normally tracks all the objects that it’s simulating in a data structure somewhere. For simplicity let’s call it a list, though this isn’t always the case. For each object, the physics engine needs to know some important information, such as it’s mass, current velocity, current position and orientation, and outside forces acting on the object. Each step, the integration portion of the physics code will solve for the new current positions and velocities for every object. This involves using equations of motion from physics to approximate the positions and velocities for an object in some future time (current time plus the time slice for the frame), using the current positions and velocities as starting points. In addition, outside forces such as gravity, springs, friction, or anything else relevant to the game are also considered to ensure the newly computed positions and velocities make sense.
If there were only a single object moving in a vacuum, then we’d be done! However, most games involve more than one object, and these objects move in interesting environments. Therefore, there arises a situation where two objects are moving towards each other or end up running into each other. What happens? If we don’t do anything, these objects will just pass right through each other. The renderer certainly has no knowledge that these are separate, solid objects that shouldn’t overlap. It will happily draw them intersecting each other. In most games, however, you don’t want the objects to pass through each other. An asteroid colliding with a ship should crash into it, possibly destroying the ship. A character walking on top of a floor should stay above it, gravity shouldn’t pull the character down through the ground. In order to handle these scenarios, the game needs to know that two objects are overlapping. The process of identifying these scenarios is called collision detection, and is normally one of the other major tasks the physics code must do. Generally, the job of the collision detection code is to determine all such pairs of overlapping objects, possibly including some additional data such as how far they overlap and in what orientation, and providing this data to the game for further processing.
Once the physics engine has identified that a pair (or many pairs) of objects are overlapping, then what do we do? In many cases, this is something specific to the rules of the game. For instance, in a space shooter game, when an asteroid overlaps the player’s ship, the game may decide to stop drawing the player’s ship and instead draw an explosion animation. Following that, the game would probably start the level over and reduce the number of lives of the player. All of these reactions to the collision are driven entirely by the game itself, and not the physics engine. This is because they are very specific to the game in question. However, there are certainly cases where the game doesn’t care to be involved. For instance, in a first person shooter, if the player knocks over a chair in a room, the game doesn’t need to do anything specific for this case. There are no game rules invoked, and therefore the game just wants the motion of this chair and player to continue to be simulated in a realistic fashion. This likely means the chair falls over in response to being bumped by the player, and when it strikes the floor, it probably rolls or tumbles slightly. This class of reaction is another common, and arguably the most complex, job of the physics engine and is generally referred to as collision response or collision resolution.
We’ve covered the basics of all physics engines. Every simulation suitable for games will have these general components, though it’s common to include many more features such as joints, cloth simulations, physics-based animation, and other interesting things. I’ll be going into a lot more details for each component as we continue to discuss physics in games. I’m planning on presenting a sample architecture for a physics engine, and then drilling into each section piece by piece to discuss the design, why it was designed that way, what algorithms are used, what other alternate algorithms are there, and what optimizations where made and why. I’ll be providing sample code snippets and probably entire posts dedicated to certain algorithms and optimizations. Please note that the architecture used will be something of my own invention, and is not that of any other physics engine out there. While there will certainly be similarities due to the fact that we’re solving the same problem, all of the code and design presented will have been created specifically for the purpose of education on this blog.