I'm using OpenGL for a top-down 2.5 game. The player is a ball and I need to rotate it based on the direction it's moving. I have created a Matrix class which has a SetRotationYawPitchRoll() method but rotation on an axis will mess up the rotation on another axis.

I've been searching on here for some ideas and I came across the following:

1. Store the orientation of the ball as a quaternion or matrix
2. For each update, if the speed of the ball is greater than a specified epsilon:
3. The axis of rotation is the normalized cross product of the ball's velocity vector and the current 'up' vector
4. The angle of rotation (in radians) is the distance traveled by the ball during that update divided by the ball's radius
5. Apply this axis-angle rotation to the ball's orientation
6. Re-normalize the orientation to prevent drift (normalization for quaternions, orthogonalization for matrices)

Based on this I have created the following methods

In my Matrix class
inline void Matrix3D::SetRotation(Vector3D axis, float angle)
{
Matrix3D& matrix = *this;

float nx = axis.x;
float ny = axis.y;
float nz = axis.z;
float c = cosf(angle);
float s = fastSinf(angle);

matrix(0) = c+nx*nx*(1.0f-c);
matrix(1) = nx*ny*(1.0f-c)-nz*s;
matrix(2) = nx*nz*(1.0f-c)+ny*s;
matrix(3) = matrix(7) = matrix(11) = matrix(12) = matrix(13) = matrix(14) = 0.0f;
matrix(4) = ny*nx*(1.0f-c)+nz*s;
matrix(5) = c+ny*ny*(1.0f-c);
matrix(6) = ny*nz*(1.0f-c)-nx*s;
matrix(8) = nz*nx*(1.0f-c)-ny*s;
matrix(9) = nz*ny*(1.0f-c)+nx*s;
matrix(10) = c+nz*nz*(1.0f-c);
matrix(15) = 1.0f;
}

And my function to rotate the ball
static inline bool getBallRotationMatrix(Vector3D velocity, Vector3D up, Matrix3D& matrix)
{
Vector3D axis = Cross(velocity, up).Normalize();

float distance = sqrtf(velocity.x * velocity.x + velocity.y * velocity.y);
float angle = distance;

matrix.SetRotation(axis, angle);

return true;
}

Finally my code to apply the matrix to the model

Matrix3D matrix = Model->GetMatrix();

static Vector3D velocity = Vector3DEmpty();
velocity.x += position.x - m_lastPosition.x;
velocity.y += position.y - m_lastPosition.y;

if(getBallRotationMatrix(velocity, Vector3DMake(0, 0, 1), matrix))
Model->SetMatrix(matrix);

I have excluded the "distance traveled by the ball during that update divided by the ball's radius" part for now as I just want to have the ball roll in the direction it's moving for now.

Unfortunately it seems no different to SetRotationYawPitchRoll() in that one rotation axis seems to screw up the another.

What is wrong with my code? I don't quite understand how to "orthogonalize the matrix" at the end. Would this explain the results?
[/quote]
It is quite easy. First you need to understand that the orientation of an object could be represented by an orthogonalized matrix, this are in fact just three normalized vectors and the angel between two vectors is orthogonal = 90 degree. This is often represented by a coordination system consisting of x,y,z vectors seen in modelling tools for example.

The next step is to create a "look-at" matrix. Let's say your objects looks along the z-axis, then your first vector is given: lookat = normalize(ball.velocity)
Now you have to create an orthogonal matrix from this vector (assumption for sake of simplicity: ball can never move up):

vec lookat = normalize(ball.velocity)
// at first we need a valid reference vector, take a up vector pointing along the y-axis
vec up = vec(0,1,0)
// construct the vector pointing to the right
vec right = crossProduct( lookat,up)
// now we need to correct our up vector
up = crossProduct(lookat,right)

// now all vectors are normalized and orthogonal, construct transformation matrix from these vectors
matrix transform = matrix(right,up,lookat)

// no spin the ball around the right axis
matrix rotationMatrix = rotationMatrix( right, angle)

// rotate transformation
matrix final = transform * rotationMatrix


That's it. There might be some misstakes, but this would be the basic approach.

Thanks for your reply. I've tried to translate what you posted into my own function but it's still not quite working. My ball moves along the x and y axis and the up vector is z (ie. 0, 0, 1). If you can imagine it's a top down perspective game.

Here is the function based on your code

static inline bool getBallRotationMatrix(Vector3D velocity, Vector3D up, Matrix3D& matrix)
{
Vector3D lookAt = velocity.Normalize();
Vector3D right = Cross(lookAt, up);
up = Cross(lookAt, right);

Matrix3D transformMatrix;
transformMatrix(M11) = right.x; transformMatrix(M12) = right.y; transformMatrix(M13) = right.z; transformMatrix(M14) = 0;
transformMatrix(M21) = up.x; transformMatrix(M22) = up.y; transformMatrix(M23) = up.z; transformMatrix(M24) = 0;
transformMatrix(M31) = lookAt.x; transformMatrix(M32) = lookAt.y; transformMatrix(M33) = lookAt.z; transformMatrix(M34) = 0;
transformMatrix(M41) = 0; transformMatrix(M42) = 0; transformMatrix(M43) = 0; transformMatrix(M44) = 1;

static float angle = 0;
angle += 1.0f;

Matrix3D rotationMatrix;
rotationMatrix.SetRotation(right, angle);

matrix = transformMatrix * rotationMatrix;

return true;
}

Right now the angle is just incrementing so the ball should roll in the direction based on the velocity but it's not; it seems to be spinning all over the place.

So it's really the left and right movement that seems to be off. If I swap the x and y values for the "right" vector when passed into SetRotation then left and right is now on the y axis and up and down rolls on the z axis.