(Demo) Box2D: Polygon and Chain Shapes


Box2D demo with chain and polygon shapes



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Box2D is a nice little physics engine and it’s fun to code physics simulation with it because it’s easy to use and it has some cool features such as polygon and chain shapes.

A polygon shape is a physics actor based on a convex polygon. Just add some vertices, build the polygon and you’re ready to play with your polygon shaped actor.

Let’s see an example with a simple polygon: the triangle. Here is a Lua code snippet (from the demo) that builds a triangle:

b2_triangle = gh_box2d.world_create_actor(b2_world, -3, 10,  "dynamic")
gh_box2d.actor_add_vertices(b2_triangle, -1,-1)
gh_box2d.actor_add_vertices(b2_triangle,  0, 1)
gh_box2d.actor_add_vertices(b2_triangle,  1,-1)
gh_box2d.actor_polygon_build(b2_triangle)

Now let’s see the second interesting feature: chain shapes.

A chain shape is simply a list of segments (or edges) connected together. A chain is an handy object to build static obstacles. Depending on the winding order of vertices, the edge normal will point outwards (counter-clockwise winding or CCW) or inwards (clockwise winding or CW).

Box2D chain winding order: CCW or CW

 
Here is the code to create a chain in CCW:

chain_vertices = {
  {x=10.0, y=-0.5},
  {x=7.0, y=-1.2},
  {x=6.0, y=-1.0},
  {x=5.0, y=-1.75},
  {x=4.0, y=-2.0},
  {x=3.0, y=-2.5},
  {x=2.0, y=-2.8},
  {x=1.0, y=-2.8},
  {x=0.0, y=-2.8},
  {x=-0.5, y=-2.0},
  {x=-1, y=-0.5},
  {x=-3, y=0},
  {x=-5, y=1},
  {x=-6, y=2},
}
 
b2_chain = gh_box2d.world_create_actor(b2_world, 0, 1,  "static")
local numverts = #chain_vertices
 
for i=1, numverts do
  local vertex = chain_vertices[i]
  gh_box2d.actor_add_vertices(b2_chain, vertex.x, vertex.y)
end
 
gh_box2d.actor_chain_build(b2_chain)

 
A demo that shows polygon and chain shapes in action is available in the OpenGL 2.1 demopack (geexlab-demopack-gl-21/d56-box2d/03/). This demo requires GeeXLab 0.37.0.

 

 
The d56-box2d/03/ demo is available in two versions: gl21 and gl32. The OpenGL 3.2 version can draw polygon shapes with more than four vertices. A helper library in Lua (polygon.lua) does all the dirty job.

Box2D demo with chain and polygon shapes

Box2D demo with chain and polygon shapes





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