One of the fundamental aspects of Gamma is that anything that can be interacted with has a bounding box that is used for collision detection. For now, Gamma only supports rectangular bounding boxes, however this may change in the future.

To provide the functionality required for a bounding box, we have a a set of functions and properties that can be added to your object using gma.shapes.rectangle.


Gamma provides several entity classes out of the box that use gma.shapes.rectangle. Look at the descendants list on that api page for a full list of such classes.

Creating a rectangle

To specify a rectangle, you need to be able to determine its width, height and position using the available properties. Gamma allows you to specify this in several ways, as outlined below. If these aspects cant be determined, then the rectangle cant be made and gma.shapes.rectangle will return null.

Specifying width and horizontal position

Any of the following combinations will allow you to specify a width and horizontal position, where x is the horizontal co-ordinate of the centre of the rectangle, left and right are the horizontal extremes of the rectangle and width is its absolute width.

  • width and right
  • width and left
  • width and x
  • left and right
  • x and left
  • x and right

Specifying height and vertical position

Any of the following combinations will allow you to specify a height and vertical position, where y is the vertical co-ordinate of the centre of the rectangle, top and bottom are the vertical extremes of the rectangle and height is its absolute height.

  • height and bottom
  • height and top
  • height and y
  • top and bottom
  • y and top
  • y and bottom


// top, bottom, right and left
gma.platform({left:1, right:3, top:3, bottom:2});

// (top or bottom), (left or right), height and width
gma.platform({bottom:1, right:3, height:1, width:2});
gma.platform({bottom:1, left:1,  height:1, width:2});
gma.platform({top:2,    right:3, height:1, width:2});
gma.platform({top:2,    left:1,  height:1, width:2});

// (top or bottom), left, right and height
gma.platform({bottom:1, right:3, left:1, height:1});
gma.platform({top:2,    right:3, left:1, height:1});

// (left or right), top, bottom and width
gma.platform({left:1,  bottom:1, width:2});
gma.platform({right:3, bottom:1, width:2});

// centre, width and height
gma.platform({ centre:[2,1.5], width:2, height:1 });

// x, y, width and height
gma.platform({ x:2, y:1.5, width:2, height:1 });

Specifying Points

An alternative way of specifying a rectangle is to specify points that make up the corners of the rectangle. This is done by specifying points with an array of [x,y] co-ordinates. How many points you supply and in what order is up to you, as long as you have at least two diagonally opposite corners, then it will work.


For example, creating a gma.platform, which uses gma.shapes.rectangle.

// All four corners
gma.platform({ points:[[1,1],[3,1],[3,2],[1,2]] });

// 2 opposite corners
gma.platform({ points:[[1,1],[3,2]] });
gma.platform({ points:[[3,1],[1,2]] });