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Physics and Game Development; or, What Goes Up Better Come Down
by Marc Mencher on 03/29/13 04:13:00 pm   Expert Blogs

The following blog post, unless otherwise noted, was written by a member of Gamasutra’s community.
The thoughts and opinions expressed are those of the writer and not Gamasutra or its parent company.


Unless you're making a text-only game, you'll need to apply some form of Newtonian physics to pretty much every action that occurs on the screen. Computer and video games apply the laws of physics so that objects "behave" as they do in the normal world. This means that programmers need to know the physical science equations to apply them accurately and appropriately to the game code. (Even objects in an "alternate" or cartoon universe have to follow the rules of physics to be believable.)

The online book description for Physics for Game Development states, "Colliding billiard balls. Missile trajectories. Cornering dynamics in speeding cars... By applying the laws of physics, you can realistically model nearly everything in games that bounces around, flies, rolls, slides, or isn't sitting still, to create compelling, believable content for computer games, simulations, and animation."

Game elements that require application of physics include things we take for granted in the real world, like elasticity, light, sound, reactions and interactions, and especially gravity.

  • Gravity – This basic principle is at the core of motion in our world; all falling objects respond to the force of gravity according to the laws of physics. Falling objects accelerate according to certain equations but all objects fall at the same rate, unless they are affected by air resistance. Likewise, if a game character jumps in the air, that character must come back down to earth according to the rules of gravity. (Even the girl in Leisure Suit Larry fell back down at some point . . . )
  • Motion – A game in which things don't move is going to be pretty boring. Even old school side scrollers have at least some type of motion on the screen, even if it's
    rolling terrain and limited pixel animations. The more you understand physics and programming, the more efficiently and you'll be able to add realism to your game—which means the more you can do with limited funds.
  • Elasticity - Things bounce, stretch and snap back to their original form depending on how they react to the force effecting change are—in other words, depending on how "elastic" they are. This principle can be applied to create realism in an MMO, humor in a cartoon-based world and action in a super-hero type of game.
  • Light - There are laws about how a beam of light will reflect off an object or how it will bend or refract when going through something transparent. Not only will application of these rules affect the overall realism in your game, they will also affect the game's visual appearance. Myst explored the use of these principles to great effect. Modern
    MMOs like Guild Wars 2 and action games like Call of Duty set a very high standard for the dramatic use of physics in this area.
  • Sound - Sound characteristics like echoes or the Doppler effect (the effect of relative motion on sound waves) are applications of physical laws on sound. While we know
    that there's no sound in space, we accept the convention of explosions, collisions and engines in our entertainment media. Lack of sound would not only hamper the
    storytelling (and the drama) but it would mean that a lot of talented sound engineers don't have anything to do!

· Collision detection – Used to determine how two solid objects interact in the envrionment. This could be as simple as whether your avatar walks through a fence or over it, whether you go around an NPC or through her, and whether you can shoot through rocks or not (applied as "line of sight.")

· Particle systems - A common aspect of computer games are explosions. Early computer games used the simple expediency of repeating the same explosion in each circumstance. However, in the real world, explosions vary depending on the terrain, altitude of the explosion and the type of solid bodies that come in contact with the explosion. A particle system model allows a variety of other physical phenomena to be simulated, including smoke, moving water, precipitation, and so forth. An environment's realism is limited by processor power, the knowledge of the programmers and of course, time and money available to make it fancy.

· Ragdoll physics - A procedural animation and simulation technique that displays the movement of a character when it drops to the ground, usually when it's killed. The character's body is a series of rigid bones connected with hinges at the joints; simulations model what happens to the body as it collapses to the ground. More sophisticated physics models of creature movement and collision interactions require greater level of computing power and a more accurate simulation of physical principles and elements. Programmers need to understand how physical principles in the environment affect anatomy—and they also need to be able to communicate really well with artists!

· Cartoon physics – We're all familiar with hapless characters who fun off a cliff and continue to move horizontally until they realize that oh no! there's not more ground and down they go. Sonic the Hedgehog moves at superspeed, characters survive being crushed by a heavy safe and just about any object can endure the classic "squash and stretch" process. These physical anomalies are intended to provide humor and create improbable situations for superheroes. While we don't necessarily want to see them in Call of Duty, we would feel cheated if they didn't show up in a cartoon or superhero-based game.

The bottom line is that while you might be able achieve these important (and expected) game elements without a knowledge of physics, the quality and believability of your product would probably not support a high level of art, action and story without them. Without the appropriate application of physics in a computer or video game, you might as well be attacking creeping coins or going through a maze of twisty little passages. As Alice said, "What is the use of a book without pictures . . .?"


[1] Pertaining to the laws developed by Sir Isaac Newton (1642-1727).


[3] Carroll, Lewis. Alice's Adventures in Wonderland.

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