Help understanding physics code.

[Akira__tyler]

Hello. I am writing a 3D physics engine for use in a game engine I'm creating with some friends. This is the first time I've written a 3D physics engine and I am having some difficulty understanding some of the math behind it all.

The section I am currently struggling with is collision resolution. At this point two bodies have collided and their velocities and penetration are being resolved. I am trying to do this with impulses, as guided by a book (Game Physics Engine Development). There are a few sections I do not understand fully.

In one section I try to find the amount of rotational velocity change for every unit of impulse, implemented something like this:

Vec3 impulsiveTorque = CrossProduct(relativeContact, contactNormal);
Vec3 angularVelocity = inverseInertiaTensor * impulsiveTorque;
Vec3 rotChangePerUnitImp = CrossProduct(angularVelocity, relativeContact);

The relativeContact is a vector from the CoM to the point of collision.
The contactNormal is the normal towards the other object from relativeContact.
The tensor is the inverse inertia tensor of the object in world space.

What I THINK the code is doing is finding how much of the relativeContact vector is not in the direction of the contact normal, changing that to angular velocity, and then ending up with the right result. I am missing a lot here and I'm having a lot of difficulty understanding why this does what it does.

Although it would be easiest for me to understand if someone just walked through this code in their own words, any help would be greatly appreciated. Thank you.

 

[AndyW]

Hello, thank you for reaching out for help with your physics engine. I understand that collision resolution can be a complex topic, especially when it comes to implementing it in a 3D environment. Let me try to break down the code you provided and explain it in simpler terms.

The first line of code, Vec3 impulsiveTorque = CrossProduct(relativeContact, contactNormal), is calculating the torque or rotational force that is applied to the object at the point of contact. The CrossProduct function is a mathematical operation that results in a vector that is perpendicular to both the relativeContact and contactNormal vectors. This torque is necessary for the object to rotate in response to the collision.

Next, the code is calculating the angular velocity of the object by multiplying the inverse inertia tensor with the impulsiveTorque vector. The inverse inertia tensor is a mathematical representation of an object's resistance to rotation, and by multiplying it with the torque, we can determine how much the object will rotate in response to the collision.

Finally, the last line of code, Vec3 rotChangePerUnitImp = CrossProduct(angularVelocity, relativeContact), is calculating the amount of rotational change that will occur for every unit of impulse applied. The CrossProduct function is used again to determine the direction of the rotational change, and by multiplying it with the angular velocity, we can get the final result.

In summary, this code is calculating the necessary rotational changes that need to occur in response to a collision. I hope this explanation helps you understand the code better. If you have any further questions, please don't hesitate to ask. Good luck with your physics engine!