Matrix Rotate
Author
How do i set up a matrix and multiply a rotation so that it rotates around a certain point? What i'm trying to do is have a Sprite class and i have a float Rotation variable. in the rendering stage, the rotation of every sprite is then calculated but it rotates around the point (0,0,0)... i want it to rotate around the center of the sprite
As you've obviously noted, rotation is defined about the origin [0,0,0]. The trick is to do the following:
Translate by [-SpriteX,-SpriteY]
Rotate by [theta]
Translate by [SpriteX,SpriteY]
Basically you move the sprite to the origin before rotating, and then "undo" the move operation immediately afterwards [smile]
In the above example replace the "-SpriteX" and "-SpriteY" with the negative of whatever of the center point of your sprite is.
hth
Jack
Translate by [-SpriteX,-SpriteY]
Rotate by [theta]
Translate by [SpriteX,SpriteY]
Basically you move the sprite to the origin before rotating, and then "undo" the move operation immediately afterwards [smile]
In the above example replace the "-SpriteX" and "-SpriteY" with the negative of whatever of the center point of your sprite is.
hth
Jack
Author
doesn't that seem... un.. professional? (is that the word i'm looking for?) and take up memory? there has to be some way i can just pick an axis to rotate around...
Thats basic matrix mathematics for you [wink]
Go to any matrix-maths course and they'll explain it properly for you if you wish.
For performance, there might be a quicker way... D3DXMatrixRotationAxis() might be able to do what you want.
I would be surprised if you found matrix composition as a bottleneck in your pipeline, but if you did, it's quite simple to multiply them out on paper and then hard-code a simplified version...
It was a while back, but in my VB6 days I wrote this article on 2D/3D transforms. Hand multiplying the SRT matrix wasn't particularly entertaining, but it allowed me to eliminate a large number of common terms and improve the "function" performance by 2.5x (or thereabouts).
hth
Jack
Go to any matrix-maths course and they'll explain it properly for you if you wish.
For performance, there might be a quicker way... D3DXMatrixRotationAxis() might be able to do what you want.
I would be surprised if you found matrix composition as a bottleneck in your pipeline, but if you did, it's quite simple to multiply them out on paper and then hard-code a simplified version...
It was a while back, but in my VB6 days I wrote this article on 2D/3D transforms. Hand multiplying the SRT matrix wasn't particularly entertaining, but it allowed me to eliminate a large number of common terms and improve the "function" performance by 2.5x (or thereabouts).
hth
Jack
Author
hmmm.. that vector, how would i specify that i want it at something like (100,200,0) and rotated on the Z axis?
Author
can someone explain the D3DXVECTOR3 parameter? i tried putting the point it's at but it's not rotating on the Z axis at the point...
D3DXMATRIX *WINAPI D3DXMatrixRotationAxis(
D3DXMATRIX *pOut,
CONST D3DXVECTOR3 *pV,
FLOAT Angle
);
D3DXMATRIX *WINAPI D3DXMatrixRotationAxis(
D3DXMATRIX *pOut,
CONST D3DXVECTOR3 *pV,
FLOAT Angle
);
The D3DXVECTOR3 parameters is the normalized vector you wish to rotate around.For example, a vector (0.f, 1.f, 0.f) will make a rotation basically like a rotation in the y axis of the world. a vector (1.f, 0.f, 0.f) will make a rotation like one made around the x axis and so on....
Author
ok, how's this? is this efficient? but i know it's not correct b/c it's not rotating about the center, it looks like it rotating in the bottom right axis...
[source lang=cpp] D3DXMatrixRotationZ(&RotateM, m_Pos.Radian); // Rotate about the center of sprite D3DXMatrixTranslation( &TranslationM, -1 * (Position.x + Center.x), -1 * (Position.y + Center.y), 0.0f); D3DXMatrixMultiply(&OutM, &TranslationM, &RotateM); D3DXMatrixTranslation( &TranslationM, Position.x + Center.x, Position.y + Center.y, 0.0f); D3DXMatrixMultiply(&OutM, &OutM, &TranslationM); // Scaling D3DXMatrixScaling( &ScaleM, m_fScaleFactorX, m_fScaleFactorY, 1.0f ); D3DXMatrixMultiply(&OutM, &OutM, &ScaleM); // Set and draw sprite W_Graphics.GetSpriteCOM()->SetTransform( &OutM );
Quote:hmmm.. that vector, how would i specify that i want it at something like (100,200,0) and rotated on the Z axis?Jollyjeffers gave the answer here:
Quote:Translate by [-SpriteX,-SpriteY]In DirectX terms (pseudocode - I don't know the DirectX function names):
Rotate by [theta]
Translate by [SpriteX,SpriteY]
D3DXMatrix T1, R, T2, M;SetTranslationMatrix(T1, -100, -200, 0);SetRotationMatrixZ(R, angleInRadians);SetTranslationMatrix(T2, 100, 200, 0);M = T1 * R * T2;I'll also say that there may be better ways to do this. Generally it's a good idea to keep your objects - models, sprites, or whatever - defined in local space (usually centered at the origin and aligned with the world axes). Then when you're ready to draw, just apply the appropriate transform matrix, usually M = R*T. That avoids all the above complications with rotating an object about some other point than the origin.
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