PolygonCollider

The properties categories, collidesWith and groupIndex are used for collision filtering. That is useful if you want only some of your fixtures to collide with each other. By default, all fixtures collide with each other, so the default categories is Category1. The default collidesWith is All, and the default groupIndex is 0.

For example, say you make a character that rides a bicycle. You want the bicycle to collide with the terrain and the character to collide with the terrain, but you don't want the character to collide with the bicycle (because they must overlap). Therefore Box2D supports such collision filtering using categories and groups.

Box2D supports 16 collision categories. For each fixture you can specify which category it belongs to. You also specify what other categories this fixture can collide with. For example, you could specify in a game that all players don't collide with each other and enemies don't collide with each other, but players and enemies should collide. You might also have powerups in your game, with which the player should be able to collide, but not the monsters. This can be done with masking bits. For example:

 Scene {
   EntityBase {
     entityType: "player"

     PolygonCollider {
       categories: Circle.Category1
       // collide with enemies and powerups
       collidesWith: Circle.Category2 | Circle.Category3
     }
   }

   EntityBase {
     entityType: "enemy"

     PolygonCollider {
       categories: Circle.Category2
       // collide with players
       collidesWith: Circle.Category1
     }
   }

   EntityBase {
     entityType: "powerup"

     PolygonCollider {
       categories: Circle.Category3
       // collide with players
       collidesWith: Circle.Category1
     }
   }
 }

The groupIndex can also be used to choose fixtures that collide with each other: Fixtures with the same positive group index will always collide, regardless of their categories or collidesWith settings. Fixtures with the same negative groupIndex will never collide, regardless of categories or collidesWith.

Note: Only dynamic bodies collide with others. So 2 static bodies or 2 kinematic bodies can never collide with each other. When you use Joints to connect 2 fixtures, you can enable or disable collisions between these connected fixtures.

See also Fixture::categories, Fixture::collidesWith, and Fixture::groupIndex.

See the categories property documentation.

This property holds the density in kg/pixel^2. The fixture density is used to compute the mass properties of the parent body. The density can be 0 or positive. You should generally use similar densities for all your fixtures. This will improve stacking stability.

The default value is 0. The masses of all fixtures of a Body get added for dynamic bodies. If none of the fixtures of a body has a density set, the default body mass is set to 1kg.

Consider that static and kinematic bodies internally do not have a mass, so setting the density for them is useless.

If you want to make objects fall down faster, increase the PhysicsWorld::gravity property.

See also Fixture::density.

This property alias allows access to the Circle physics shape, which is called a fixture in Box2D.

Usually, you will not directly need to access this property, because you can access all fixture properties by the provided alias properties.

Friction is used to make objects slide along each other realistically. It is usually in the range [0,1]. The default value is 0.2. A friction value of 0 turns off friction and a value of 1 makes the friction strong. If any of 2 colliding shapes has 0 friction, the resulting friction is 0 as the friction values get multiplied.

See also Fixture::friction.

See the categories property documentation.

Restitution is used to make objects bounce, so to simulate elastic objects like a rubber ball. It is usually in the range [0,1]. The default value is 0. Consider dropping a ball on a table. A value of zero means the ball won't bounce. This is called an inelastic collision. A value of one means the ball's velocity will be exactly reflected. This is called a perfectly elastic collision. The resulting restitution of 2 colliding shapes will be the maximum value.

See also Fixture::restitution.

Set this property if you do not want the shape to react to collisions. This is useful if you only want to know that a collision happened, but not physically respond to it. The default value is based on the collisionTestingOnlyMode property - if it is set to true, the sensor flag also is set to true, otherwise to false.

See also Fixture::sensor.

Contains a list of 3 or more (up to 8) vertices that make up the shape in a counter clockwise winding. The vertices must define a convex polygon. See here for the definition of a convex polygon: http://en.wikipedia.org/wiki/Polygon#Convexity_and_types_of_non-convexity.

You can set the vertices like in this example:

 EntityBase {
   x: 30
   entityId: "trapezoid1"
   entityType: "trapezoidEntity"

   PolygonCollider {

     vertices: [
       Qt.point(-30, 0), // top left
       Qt.point(0, 100), // bottom left
       Qt.point(130, 100), // bottom right
       Qt.point(100, 0) // top right
     ]

   }
 }

The 0/0 position of the vertices is the top left point, thus when the entity is positioned at 30/0 the PolygonCollider physics shape will look like the following image: