# Converting coordinates to rotations.

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I am looking for a formula that takes a normalized vector indicating an up-direction, and uses it to rotate a camera to the specified orientation. I am using Java, but I understand a number of computer languages, so feel free to write in whatever language you are comfortable with. In my initial attempt, I came up with the following formula (I tried to translate this to pseudo code so everyone can follow):
distance = lengthOf( previousUpVector - newUpVector );
angle = acos( (2 - (distance*distance)) / 2 );
cross = crossProduct( previousUpVector, newUpVector );
normalize( cross );

rotateCameraAroundAxis( cross, angle );

The idea here was to use the law of cosines to calculate the angle between the two vectors:
cos(A) = (b*b + c*c - a*a) / (2*b*c)

So:
A = acos( (b*b + c*c - a*a) / (2*b*c) )

Since b and c are normalized (length = 1):
A = acos( (2 - a*a) / 2 )

And then calculate the cross-product to calculate what axis to rotate around. This formula doesn't seem to be working though (I'm getting sporatic rotations when tring to use this formula to match a camera's orientation to an object's). Any ideas what might be wrong with my formula, or does anyone know of one that works?

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1. Normalise v1 and v2 (in your case previousUpVector and upVector)
2. Compute the dot product of v1 and v2
3. Get the acos of the result obtained in step 2
4. Compute the cross product of v1 and v2
5. Rotate by angle obtained in step 3 around the vector obtained in step 4 (you might need to normalise the cross product vector as well depending on how your angle around axis rotation method works).

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Awsome, thanks for the quick reply! I will try this out right now.

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Unfortunately, this formula does not appear to be working (possibly there is a problem elsewhere in my program, so I will check into that as well). In the mean time, can anyone think of another formula to use for this?

Here is an applet demonstrating the problem I am getting:

http://www.paulscode.com/source/CameraOrbit/

Arrow keys orbit the camera around the tree (the tree is stationary, only the camera is moving). The new up-vector I am passing to the formula is printed out to the Java console for every call.

The problem seems to be after rotations around the x-axis. You can rotate around the y-axis all day with no problem.

If you need further information/source, let me know. Thanks in advance!

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I think my problem is that I am trying to solve look-direction and up-direction seperately.

Any ideas about how to take both a look-direction vector and an up-direction vector and convert that into rotations?

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Quote:
 Original post by paulscodeI think my problem is that I am trying to solve look-direction and up-direction seperately.Any ideas about how to take both a look-direction vector and an up-direction vector and convert that into rotations?
If you have a look direction and an up direction, then their cross-product gives you the "right" direction, which gives you 3 orthonormal basis vectors with which you can build a rotation matrix.

HERE: Scroll down to "Constructing a camera frame". Basically a 4x4 matrix of [RIGHTVEC UPVEC -DIRECTIONVEC Origin] gives the transformation for orientation.

EDIT: Changed the sign on the direction vector in the matrix... sorry about that.

[Edited by - smitty1276 on October 15, 2008 7:55:26 PM]

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Ok, I'm tracking the first part of "Constructing Camera Frame". I have a couple of questions before I'll understand what to do with the information, though.

After taking a closer look, I've figured out the U, V, and W direction vectors and the "e" position vector (correct me if I am wrong here):

U = (-up.x, -up.y, -up.z)V = look CROSS upW = (-look.x, -look.y, -look.z)e = position of camera in "world space"

Next, we have two 4X4 matrices:

U.x, U.y, U.z, 0.0V.x, V.y, V.z, 0.0W.x, W.y, W.z, 0.00.0, 0.0, 0.0, 1.0

And the second 4X4 matrix:

1.0, 0.0, 0.0, e.x0.0, 1.0, 0.0, e.y0.0, 0.0, 1.0, e.z0.0, 0.0, 0.0, 1.0

From here, the author loses me. Maybe I am not understanding the terminology, so I don't know which of the various formulas apply to what I am tring to do. I want to create a rotation matrix, but I dont know what to do with those original two matrices to accomplish this?

[Edited by - paulscode on October 16, 2008 7:55:47 PM]

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DOH! I am dumb. I didn't pay attention to what you said, that [RIGHTVEC UPVEC DIRECTIONVEC Origin] IS a rotation matrix! Sorry about that. Yes, this works beautifully. Thanks a million!!

Here is the working applet:

http://www.paulscode.com/source/CameraOrbit/Vectors2Matrix/

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Ah, good to see its working... did you use the negative direction vector?

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Quote:
 Original post by smitty1276Ah, good to see its working... did you use the negative direction vector?

I had to play around a bit to get it to work properly. I ended up reversing the cross product for "right" and not using the negative of the look directon. The following is the result (this is in Java using the jPCT engine):

    public void setOrientation( Camera camera, SimpleVector look, SimpleVector up )    {        SimpleVector right = up.calcCross( look ).normalize();        Matrix m = new Matrix();                m.set( 0, 0, right.x );        m.set( 1, 0, right.y );        m.set( 2, 0, right.z );        m.set( 3, 0, 0.0f );                m.set( 0, 1, up.x );        m.set( 1, 1, up.y );        m.set( 2, 1, up.z );        m.set( 3, 1, 0.0f );                m.set( 0, 2, look.x );        m.set( 1, 2, look.y );        m.set( 2, 2, look.z );        m.set( 3, 2, 0.0f );                m.set( 0, 3, 0.0f );        m.set( 1, 3, 0.0f );        m.set( 2, 3, 0.0f );        m.set( 3, 3, 1.0f );                camera.setBack( m );    }

I could not have solved this problem without understanding what a rotation matrix is, so thanks again!

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