Monday, November 19, 2007

Benefits of Farseer 2D Physics Engine

[This was originally posted at]

I recently released Silverlight TruckWars v1.3, whose big enhancement was to incorporate the Farseer 2D Physics Engine. There are several benefits I like about Farseer:

  1. It just works - The collision algorithms just work. Before I had some code to do polygon detection. It was good code that I found online. And my use of it was okay when comparing two simple bodies, but it broke down with bigger things - like 5 units all pushing on each other. Because TruckWars is intensive on objects interacting with each other, this was important.

  2. Automatic pushing of objects - Before I had special code for the crate object (that you can push), but now, pushing objects is automatically handled.

  3. Collision categories - Not every object collides with every other object. For example, a fireball hovers over water, but a tank is blocked by it.

  4. Static objects - Farseer recognizes that sometimes, something is just impenetrable and unmovable - like the walls of a game board. Whereas in the real world, enough force will eventually push through something, I didn't want to allow that in the game world.

  5. Circular geometries - Farseer implements circular geometries via polygons, and lets you specify the number of sides for precision. My previous collision code only handled rectangles.

  6. Collision callback - You can add an extra method that fires on any collision. That method can then return true (to continue normal collision behavior) or false (to ignore collisions for that specific case).

  7. Body and Geometry have object tags, so I can associate them with a creature.

  8. API - The API is just clean, especially if you have any physics background. It both maps to standard physics knowledge, as well as provides desired method calls that you'd want to actually program something.

It took some extra effort, but it was well worth it. The engine feels much more solid now.


It's also worth noting, that Farseer almost makes you to do it correctly - by applying forces instead of directly setting positions. That's how the real phsycial world works. This also makes you appreciate what's involved in seemingly simple things - moving at a constant rate, having zero-turning radius, stopping a unit on demand (perfect breaks). You can achieve each of these by setting the correct properties. While it's tempting to just set the positions because then you can control everything, that becomes a nightmare with complex collisions.


I see a similar parallel to Enterprise development. It's tempting to just whip out some hack, but then as you scale up, that hack always comes back to haunt you.


I've open-sourced TruckWars on CodePlex - so you can see all the places Farseer is used. One place is from the MoveTowardTarget
 method in Tank\Tank.Test\Tank.Core\MovementStrategies.cs:

//Always clear angular velocity
c.Body.AngularVelocity = 0f;

//update direction and position
double dist = MovementUtilities.GetDistance(c.Position, c.TargetPosition);
if (dist > c.Size.Width / 2)

  Vector2 force = new Vector2();
  force.X = (float)(c.TargetPosition.X - c.Position.X);
  force.Y = (float)((c.TargetPosition.Y - c.Position.Y));
  float forceAmount = c.Thrust * PhysicsHelper.MovementForce;
  force = force.Normalize() * forceAmount;

  //Prevent sheering --> make LinearVelocity always be in direction of force
  c.Body.LinearVelocity = force.Normalize() * c.Body.LinearVelocity.Length();

  //You've reached your target, so apply the breaks

This clears the angular velocity to prevent spinning, normalizes the force to push you in  a straight direction, and incorporates applying & releasing breaks to allow for quick stopping. Here are the two break methods:

public static void ApplyBreaks(CreatureBase c)
  //Need to "release breaks" when moving again
  c.Body.LinearDragCoefficient = 4 * PhysicsHelper.LinearDragBreaks;
  c.Body.AngularVelocity = 0f;

private static void ReleaseBreaks(CreatureBase c)
  c.Body.LinearDragCoefficient = (float)(c.LinearDrag * PhysicsHelper.LinearDragMovement);

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