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# Trigonometry question

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### #1belfegor  Members   -  Reputation: 1096

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Posted 19 April 2012 - 10:28 AM

If i had a 3D arrow model pointing upwards (0, 1, 0) and i set this:
```D3DXMATRIX world, rot;
D3DXMatrixIdentity(&world);
D3DXMatrixRotationAxis(&rot, &D3DXVECTOR3(1.0f, 0.0f, 0.0f), 0.3f);
world = rot * world;
...
```

How can i obtain new direction arrow is pointing?

### #2winsrp  Members   -  Reputation: 218

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Posted 19 April 2012 - 02:03 PM

make a new vector and use the translate function.

### #3belfegor  Members   -  Reputation: 1096

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Posted 19 April 2012 - 02:23 PM

I don't see how's that gonna help? I need to obtain direction vector it is "pointing at" after rotation.

### #4Bacterius  Crossbones+   -  Reputation: 3516

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Posted 19 April 2012 - 03:04 PM

Make a vector pointing (0, 1, 0), i.e. the same direction as your arrow model, and rotate it in the same way. The rotated vector will give you the direction of the arrow model after rotation...

Putting it a different way, choose two vertices on your original arrow model, a vertex on the "bottom" of the arrow and a vertex of the "top" (the pointy end) of the arrow. These two points define a vector, which points in the arrow's direction. Once your model is rotated, find those two same vertices, they define another vector, which still represents the arrow's direction (obviously). The conclusion trivially follows...

"The best comment is a deleted comment."

### #5belfegor  Members   -  Reputation: 1096

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Posted 19 April 2012 - 03:11 PM

```D3DXMatrixRotationAxis(&rot, &D3DXVECTOR3(1.0f, 0.0f, 0.0f), 0.1f);

D3DXVECTOR4 orgDir(0.0f, 1.0f, 0.0f, 0.0f);
D3DXVECTOR4 outDir;
D3DXVec4Transform(&outDir, &orgDir, &rot);
```

Is this what you mean?

### #6Bacterius  Crossbones+   -  Reputation: 3516

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Posted 19 April 2012 - 03:18 PM

Yes, although you want to use

`D3DXMatrixRotationAxis(&rot, &D3DXVECTOR3(1.0f, 0.0f, 0.0f), 0.3f);`

as the rotation matrix (says 0.1 in your last post but 0.3 in the first one)

"The best comment is a deleted comment."

### #7belfegor  Members   -  Reputation: 1096

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Posted 19 April 2012 - 03:20 PM

It was my mistake, It was a typo. Thanks for your help.

### #8Trienco  Members   -  Reputation: 1298

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Posted 19 April 2012 - 10:03 PM

Or, since you already know that your model was pointing along one of the coordinate system axes, you could just read out the second column of the world matrix. (Transformation) matrices aren't magic, they are just four vectors: right, up, forward, position. (depending on your handedness and as what you define your x,y,z axes).
f@dzhttp://festini.device-zero.de

### #9belfegor  Members   -  Reputation: 1096

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Posted 20 April 2012 - 02:54 AM

I don't see how that would be correct?
I tried it and its almost similar, apart that 'z' axis is negative if i extract it from second column of world.

### #10Trienco  Members   -  Reputation: 1298

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Posted 20 April 2012 - 08:49 AM

Read the values from your world matrix before applying the rotation, just to be sure you are looking at the correct column. My point is that after setting it to identity, the second column must be (0,1,0,0). I see no reason why applying the same transformation to the same vectors should return different results just because one is transforming the whole matrix and the other a standalone vector.

Though I notice you are transforming orgDir by rot instead of the world matrix. How would that help if your world matrix has already acumulated several rotations? I can't imagine you want to keep track of (and update) a ton of vectors every time you apply a rotation to the world matrix.
f@dzhttp://festini.device-zero.de

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