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Rotating a Sprite Around Its Center Directx11

Started by MatthewDiaz May 2, 2012 at 6:08 PM 1 reply 2.8k views

Original Post

May 2, 2012 at 6:08 PM

Here is my current WorldMatrix function


 XMMATRIX GameSprite::GetWorldMatrix( )
 {
 //position is the pixel position of the sprite on screen
 XMMATRIX translation = XMMatrixTranslation( position_.x, position_.y, 0.0f );
 XMMATRIX rotationZ = XMMatrixRotationZ(rotation_ );
 XMMATRIX scale = XMMatrixScaling( scale_.x, scale_.y, 1.0f );
 return translation * rotationZ * scale;
 }

When changing the radians for the rotation_ my sprite does rotate but around the bottom left of the screen.

I was guessing that I would need to use XMMatrixRotationNormal() to get the results I want but through experimentation I still come up with the same results.

For example I tried


 XMMATRIX rotationZ = XMMatrixRotationNormal(tempvector, rotation_ );
 //with defiintions like these
 XMVECTOR tempvector = {0.0f, 0.0f, 1.0f};//Same as RotationZ()
 XMVECTOR tempvector = {position_.y, position_.x, 1.0f};//Wierd results

Could anyone tell me how to move the origin for the z-axis to the center of my sprite?

SeiryuEnder

May 3, 2012 at 1:20 AM

It's been a while since I've done work with matrices but...

Try changing:


 return translation * rotationZ * scale;

To:


 return rotationZ * translation * scale;

It seems like you're rotating then translating in the direction of the rotation.
You want to translate it to its intended position first, then rotate it to the desired orientation.

Could be wrong, but it's pretty quick to test... I'm also not sure about the ordering on the scalar matrix. It might be fine where it is, I'd have to dig up some old code to verify.

MatthewDiaz

Author

MatthewDiaz

May 3, 2012 at 2:04 AM

It's been a while since I've done work with matrices but...

Try changing:
return translation * rotationZ * scale;

To:
return rotationZ * translation * scale;

It seems like you're rotating then translating in the direction of the rotation.
You want to translate it to its intended position first, then rotate it to the desired orientation.

Could be wrong, but it's pretty quick to test... I'm also not sure about the ordering on the scalar matrix. It might be fine where it is, I'd have to dig up some old code to verify.

Problem solved, thank you. That worked.

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