# T5 Collision Detection with a Sphere
Search \[TODO 4] in ClothSim.cpp
Here we are using a sphere of radius 4.0 centred at (0, 4.0f, 0.0f).
```cpp
// Advanced: Test Sphere Intersection
float sphere_radius = 4.0f;
float sphere_friction = 0.8f;
glm::vec3 sphere_center = glm::vec3(0, 4.0f, 0.0f);
pos = mesh->vertices[v].pos;
// [TODO 4]: sphere intesection
// 4.1 replace false with checking if the vertex position falls into the sphere
if ( false ) {
// if it is true:
// push back the vertex position back to the sphere surface
// 4.2 calculate the outward vector direction
// from the sphere center to the vertex position pos
// glm::vec3 outDir = ???;
// 4.3 normalise the vector using glm::normalize(glm::vec3)
// glm::vec3 outDirUnit = ???;
// 4.4 push back the vertex position back to the sphere surface
// using sphere_center, outDirUnit and sphere radius
// mesh->vertices[v].pos = ???;
// 4.5 downscale the vertex velocity using sphere_friction (< 1)
// velocities[v] *= ???;
}
```
#### Check if your vertex position falls into the sphere
We can check that by calculating the distance between the vertex and the sphere center.
Hint: use glm::length(glm::vec3) to calculate the distance
#### If the vertex position falls into the sphere
Follow step 4.2 to 4.4 to recover the vertex position back to the sphere surface.
4.2 and 4.3: calculate direction from the sphere center to the current vertex postion
4.4 Move back the vertex to the sphere surface
4.5 Reduce the vertex velocity by a factor of sphere\_friction ( 0.8 in the code)
### Build and Run
Build and run your programme.
Pressing the **Space Bar**, you are going to see something like the following:
Press v to enable wind, you are going to see the following:
Change the ground boundary checking after TODO 3 from -5.0 to 0.0 leading to the following. You can play with different sphere radius and center position.
```cpp
if (mesh->vertices[v].pos.y < 0.0)
mesh->vertices[v].pos.y = 0.0;
```
