# Overview

## Basics

There are 4 basic classes you will use in Pymunk.

Rigid Bodies (`pymunk.Body`)
  A rigid body holds the physical properties of an object. (mass, position, rotation, velocity, etc.) It does not have a shape by itself. If you’ve done physics with particles before, rigid bodies differ mostly in that they are able to rotate. Rigid bodies generally tend to have a 1:1 correlation to sprites in a game. You should structure your game so that you use the position and rotation of the rigid body for drawing your sprite.
Collision Shapes (`pymunk.Circle`, `pymunk.Segment` and `pymunk.Poly`)
  By attaching shapes to bodies, you can define the a body’s shape. You can attach many shapes to a single body to define a complex shape, or none if it doesn’t require a shape.
Constraints/Joints (`pymunk.constraint.PinJoint`, `pymunk.constraint.SimpleMotor` and many others)
  You can attach joints between two bodies to constrain their behavior.
Spaces (`pymunk.Space`)
  Spaces are the basic simulation unit in Pymunk. You add bodies, shapes and joints to a space, and then update the space as a whole. They control how all the rigid bodies, shapes, and constraints interact together.

The actual simulation is done by the Space. After adding the objects that should be simulated to the Space time is moved forward in small steps with the `pymunk.Space.step()` function.

## Model your physics objects

### Object shape

What you see on the screen doesn’t necessarily have to be exactly the same shape as the actual physics object. Usually the shape used for collision detection (and other physics simulation) is much simplified version of what is drawn on the screen. Even high end AAA games separate the collision shape from what is drawn on screen.

There are a number of reasons why its good to separate the collision shape and what is drawn.

- Using simpler collision shapes are faster. So if you have a very complicated object, for example a pine tree, maybe it can make sense to simplify its collision shape to a triangle for performance.
- Using a simpler collision shape make the simulation better. Lets say you have a floor made of stone with a small crack in the middle. If you drag a box over this floor it will get stuck on the crack. But if you simplify the floor to just a plane you avoid having to worry about stuff getting stuck in the crack.
- Making the collision shape smaller (or bigger) than the actual object makes gameplay better. Lets say you have a player controlled ship in a shoot-em-up type game. Many times it will feel more fun to play if you make the collision shape a little bit smaller compared to what it should be based on how it looks.

You can see an example of this in the using_sprites.py example included in Pymunk. There the physics shape is a triangle, but what is drawn is 3 boxes in a pyramid with a snake on top. Another example is in the platformer.py example, where the player is drawn as a girl in red and gray. However the physics shape is just a couple of circle shapes on top of each other.

### Mass, weight and units

Sometimes users of Pymunk can be confused as to what unit everything is defined in. For example, is the mass of a Body in grams or kilograms? Pymunk is unit-less and does not care which unit you use. If you pass in seconds to a function expecting time, then your time unit is seconds. If you pass in pixels to functions that expect a distance, then your unit of distance is pixels.

Then derived units are just a combination of the above. So in the case with seconds and pixels the unit of velocity would be pixels / second.

(This is in contrast to some other physics engines which can have fixed units that you should use)

### Looks before realism

How heavy is a bird in angry birds? It doest matter, its a cartoon!

Together with the units another key insight when setting up your simulation is to remember that it is a simulation, and in many cases the look and feel is much more important than actual realism. So for example, if you want to model a flipper game, the real power of the flipper and launchers doesn’t matter at all, what is important is that the game feels “right” and is fun to use for your users.

Sometimes it make sense to start out with realistic units, to give you a feel for how big mass should be in comparison to gravity for example.

There are exceptions to this of course, when you actually want realism over the looks. In the end it is up to you as a user of Pymunk to decide.

## Game loop / moving time forward

The most important part in your game loop is to keep the dt argument to the `pymunk.Space.step()` function constant. A constant time step makes the simulation much more stable and reliable.

There are several ways to do this, some more complicated than others. Which one is best for a particular program depends on the requirements.

Some good articles:

- http://gameprogrammingpatterns.com/game-loop.html
- http://gafferongames.com/game-physics/fix-your-timestep/
- http://www.koonsolo.com/news/dewitters-gameloop/

## Unstable simulation?

Sometimes the simulation might not behave as expected. In extreme cases it can “blow up” and parts move anywhere without logic.

There a a number of things to try if this happens:

- Make all the bodies of similar mass. It is easier for the physics engine to handle bodies with similar weight.
- Dont let two objects with infinite mass touch each other.
- Make the center of gravity in the middle of shapes instead of at the edge.
- Very thin shapes can behave strange, try to make them a little wider.
- Have a fixed time step (see the other sections of this guide).
- Call the Space.step function several times with smaller dt instead of only one time but with a bigger dt. (See the docs of Space.step)
- If you use a Motor joint, make sure to set its max force. Otherwise its power will be near infinite.

(Most of these suggestions are the same for most physics engines, not just Pymunk.)

## Copy and Load/Save Pymunk objects

Most Pymunk objects can be copied and/or saved with pickle from the standard library. Since the implementation is generic it will also work to use other serializer libraries such as jsonpickle (in contrast to pickle the jsonpickle serializes to/from json) as long as they make use of the pickle infrastructure.

See the copy_and_pickle.py example for an example on how to save, load and copy Pymunk objects.

## Additional info

As a complement to the Pymunk docs it can be good to read the Chipmunk docs. Its made for Chipmunk, but Pymunk is build on top of Chipmunk and share most of the concepts, with the main difference being that Pymunk is used from Python while Chipmunk is a C-library.