Question

I have a Fast Moving Body(A) which is dynamic. It is supposed to collide with another Body(B). A collides with B, but sometimes it passes the Body B without collision. This is totally random behavior. I must have that collision. Kindly guide why it is acting like this, randomly.

Answer 1

The effect of one object passing through another due to large movement in a single timestep is called tunneling.

Box2D uses Continuous Collision Detection between dynamic and static objects to solve this problem. However, your case (dynamic v.s. dynamic) isn’t automatically handled, so it’s just a random dice throw whether your objects happen to be in colliding positions at the exact moment the collisions are evaluated.

From the Box2d Manual:

Normally CCD is not used between dynamic bodies. This is done to keep
performance reasonable. In some game scenarios you need dynamic bodies
to use CCD. For example, you may want to shoot a high speed bullet at
a stack of dynamic bricks. Without CCD, the bullet might tunnel
through the bricks.

Fast moving objects in Box2D can be labeled as bullets. Bullets will
perform CCD with both static and dynamic bodies. You should decide
what bodies should be bullets based on your game design. If you decide
a body should be treated as a bullet, use the following setting.

bodyDef.bullet = true;

The bullet flag only affects dynamic bodies.

Box2D performs continuous collision sequentially, so bullets may miss
fast moving bodies.