# Rotation problem in game engine

In summary, the person is creating a simple game engine for a graphics course and is facing a problem with rotations of boxes. They are trying to get the boxes to rotate based on the location of forces applied to their corners. They are using a torque from the force to determine the angular acceleration and then using that to increment the angular velocity and rotation of the box. However, they are having difficulty with the rotation being defined by three scalars while the angular acceleration is only about one axis. They are seeking help in transforming the rotation to the correct form and are considering storing the entire rotation matrix for the object. They are also advised to use conservation laws to ensure the physics in their program is accurate.
Hey,

I am making a simple game engine for a university computer graphics course, and I'm running into a (simple?) problem with rotations of boxes. I am using bounding box collisions, where a vertex of the box which enters another box receives a force to push it away.

At the end of each frame, I have 8 force vectors for each box (one for each vertex) to apply to the velocity and position of the box. I could simply apply all forces to the center of the box, and this works fine. But, the boxes wouldn't rotate. I would like them to rotate depending on the location of the forces (which will always be in the 8 corners).

To achieve this, I am trying to get a torque from the force, and use that as the angular acceleration. Then, the angular acceleration is used to increment the angular velocity, which is then used to increment the rotation of the box.

force[8] = force vectors for each corner of the box
vertex[8] = vector from the center of the box to each corner
alpha = angular acceleration (i-component = angle about i-axis)
omega = angular velocity
rot = rotation of the box
pos = position of the box
vel = linear velocity of the box
dt = 0.01 timestep

// find torque and set alpha
for each vertex (i)
torque = force X vertex
alpha -= torque <-- why negative? positive doesn't work at all

vel += force[all] * dt / mass
pos += vel * dt
omega += alpha * dt / (mass*10)
rot += omega * dt

I don't want to have to worry about moment of inertia, since its constant for a box. When I run this, it seems that sometimes the box will rotate properly, while other times the box will spin out of control. I think the problem might be that some of the torques are too strong - am I forgetting something?

Is there a problem with my method? Remember, I'm just trying to get something simple that will run stable for collisions, not a perfectly accurate physics simulation.

I tried doing something similar once.. but if it's just boxes bouncing into one another, I'd recommend trying to use conservation of energy and angular momentum to determine the changes in rotation.

In the same manner, one uses conservation laws to determine how billiard balls respond after collision, rather than trying to integrate a force (since, unless your boxes are slightly squishy like in real life, your forces should all be infinite).

I nailed the problem down to the following:

In my engine, the rotation of an object is defined by 3 scalars: angle of rotation about the X axis, Y axis, and Z axis. To display the object, the rotations are applied in order:

glRotatef(rot[X], 1, 0, 0);
glRotatef(rot[Y], 0, 1, 0);
glRotatef(rot[Z], 0, 0, 1);

The problem is that [alpha] describes a rotation about one arbitrary axis, but [omega] and [rot] relate to three rotations designed to be executed in sequence. How can I transform the rotation to the second form?

I found a method online which supposedly does this: performing a rotation about an arbitrary axis by transforming the axis to one of x, y, or z, and then rotating. It didn't work for me, and I think it is because the method is designed to rotate a point - not an acceleration or velocity (if that makes any sense?). Or maybe this method is unrelated to my problem.

Also, I could probably (not tested) store the entire rotation matrix for the object and just apply the rotation each time a force is found, since glRotatef can accept an arbitrary axis. I don't know how I can store the velocity if I do it this way, though (another matrix?). Transforming the single rotation into three would probably be the best way to do it.

Please help if you can. After I solve this little problem, I'm finished with all the features of my engine.

you wrote:
torque = force X vertex

Assuming X means cross product.

but Torque=r X F which does not equal F X r

Torque should be a vector and it is this vector you would rotate about. But be careful as forces/torques are only there during a collision (unless the cubes are moving in a field that acts on them).

But as cesiumfrog said you should use conservation of energy, momentum and angular momentum to guarantee your program follows real life physics, otherwise you could end up with cubes accelerating out of control.

## 1. What is a rotation problem in a game engine?

A rotation problem in a game engine refers to an issue where an object or character in the game does not rotate properly or as expected. This can result in the object appearing distorted or moving in an unexpected manner.

## 2. What causes rotation problems in game engines?

Rotation problems in game engines can be caused by a variety of factors, including incorrect coding, hardware limitations, or conflicts with other game elements or mechanics. They can also be caused by bugs or glitches in the game engine itself.

## 3. How can rotation problems be fixed in a game engine?

The specific steps to fix rotation problems in a game engine will depend on the cause of the issue. In some cases, tweaking the game's code or adjusting settings may resolve the issue. In other cases, the game engine may need to be updated or a bug fix patch may need to be released.

## 4. Are there any common workarounds for rotation problems in game engines?

Yes, there are some common workarounds that can be used to mitigate rotation problems in game engines. These include limiting the use of rotation-heavy mechanics or effects, simplifying and optimizing object models, and using pre-made rotation scripts or tools.

## 5. Is it possible to prevent rotation problems in a game engine?

While it is not always possible to completely prevent rotation problems in a game engine, there are steps that can be taken to minimize the likelihood of experiencing them. This includes thorough testing and debugging during the development process, as well as regularly updating and maintaining the game engine and its components.

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