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Creating and managing systems

See system.h reference

If you want to add physics processing to your game, you can use gama built in physics.

# Creating a system

You create a system using gm_system_create.

        gmSystem world = gm_system_create();

      

This creates a new structure of type gmSystem which you can configure, for properties, you can see gm_system reference

And for more information about systems, you can view the system.h reference.

# Creating bodies

A body is an entity in the system, it is like a physics body with it's own mass, shape and forces acting on it. Gama currently supports only two shapes of bodies, circles and rectangles, and provide differenc functions for creating objects of those different shapes:

• gm_rectangle_body
• gm_circle_body

body.h reference

Bodies have properties you can modify to change their behavious

• mass: holds the mass of the body which is used in collisions, a body with a mass of zero is considered having infinite mass, and for example won't move during collisions but can still have a velocity.
• is_active: set this to zero and gama will completely ignore the body.
• is_static: marks the body as non-moving
• position, velocity, acceleration: they are all structs of type gmPos, having .x and .y.
• ...

You can view other properties in the gmbody reference.

# Adding the bodies to the system

The last step is to add the bodies into the system, to do this, you are going to pass a reference to you body object to the system. you can use the gm_system_push and other gm_system_pushN functions where N is the number of shapes you want to add less than six.

# Updating the system

You can update the system with gm_system_update function.

# Example

        #include <gama.h>

int main() {
  gmSystem world = gm_system_create();
  world.acceleration.y = -0.9; // add gravity
  world.damping = 0.1;         // add damping(... air resistance?)

  gmBody ground_body = gm_rectangle_body(0, 0, -1, 2, 0.1);
  ground_body.is_static = 1;     // ground never moves with system
  ground_body.restitution = 0.6; // somehow ellastic, maybe there is grass on it
  gm_system_push(&world, &ground_body);

  gmBody ball_body = gm_circle_body(1, 0, 0, 1); // unit circle of mass 1
  gmBody player_body = gm_rectangle_body(34, 0, 0, 0.3, 0.3);
  gm_system_push2(&world, &ball_body, &player_body);

  gmBody other_bodies[57] = ...;
  gm_system_push_array(&world, 57, other_bodies);

  do {
    gm_system_update(&world); // update the system

    gm_draw_rect_body(&ground_body, GM_GREEN);
    // .. draw other things
  } while (gm_yield());
  gm_system_destroy(&world);
  return 0;
}

      

# Getting collisions

When two bodies collide, gama creates a new gmcollision object which holds information about the collision, you can get and test for a collision with gm_collision_detect

        gmCollision myCollision;

if(gm_collision_detect(&myCollision,&mySystem, &bodyA, &bodyB) 
    // process the collision
 
 
// OR if you don't need the collision

if(gm_collision_detect(NULL, &mySystem, &bodyA, &bodyB))
    // process the collision

      

# References

• physics.h reference
• collision.h reference
• system.h reference
• body.h reference