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The final project of the course. It's a course assignment of a Computer Aided Design couse in CMU. The detailed instruction is in addtionalMaterial.pdf and README. The source edited files are src/physics/collision*, physics*, spherebody*, spring*.


CollisionCollision StressDamping
Spring RotationNewtons CradleRotation

Implement Details

  • Collision Detection

    • Sphere & Sphere: A collision would happen when and .
    • Sphere & Plane: d<r and sphere moves towards the plane.
    • Sphere & Triangle: When the projection of the sphere center on the triangle's plane falls in the triangle ( i.e., parallel with each other), same as the plane case. Otherwise, consider the projection of P on the three edges' lines (if falls outside, the select the nearest vertex). Calculate the distance of the three selected points to the sphere center respectively; one of them is the nearest distance of the two bodies, which is also the collision point if collision happens.
  • Spring Force Apply

    • The step() function applies forces and torques to the attached bodies.
    • The force is applied every frame, so we should clear the force applied in the previous frame.
  • SphereBody Integration

    • The force varies with velocity and position. So we need to use 2-degree RK4 in Physics and call the Spring::step to update force (acceleration). The motion_damping used in SphereBody's step functions also need to be updated by Spring, which is a little troublesome. I just see force as a constant in every frame.
    • I was confused by the instructions, so I didn't know where to implement the RK4. The time is limited to complete this task, so I used my friends' method ---
  • Physics Move Forward

    • The position and angular_position of a SphereBody::Sphere has already been updated in SphereBody, so in Physics the only thing to do is to call them. The gravity is applied in the initialization.
    • Fisrt, check collisions by traverse the sphere list and check them with the other bodies (including sphere).
    • Traverse the spring list, apply forces.
    • Traverse all the sphere and update them.

Environment & Dependencies

  • MacOS Mojave
  • OpenGL and other framework
  • SDL1.2 (not 2.0)
  • libpng-1.2.57 (this version only)


make # compile with make
./physics scenes/collision_stress.scene # or other scene in scenes dir

Unsolved Issues

  • Integrate with Eurler's method would cause energy gaining in the spring system. But I also didn't realize a RK4.
  • The OpenGL version is too old and every time I run a scene, I should minimize the window and reopen it. Just as my issued after updated to Mojave, but in that case resizing or moving window works:). I even had to slow down the process to record some scenes.


  • The force may be added during initializing, so do not reset force during the loop.
  • The spring attached points are body1->position + body1_offset and body2->position - body2_offset . The offset should also rotate with the body. Moreover, I used to rotate the offset vector by body1_offset = Vector3(body1->orientation * body1_offset);, which can cause a huge deviation and as a result, the ball flies out of the plane.
  • Collision detection between two spheres should avoid check whether they are approaching first, or more spheres in newtows_cradle scene would move.