XNA's ViewPort.Project() equivalent in DirectX 11

Question

     Hi Everybody!

     While migrating my XNA games to DirectX 11, I stumbled into a problem trying to draw a texture on the screen in the location where a 3D model is supposed to be standing. In XNA, the answer is simple:

Graphics.GraphicsDevice.Viewport.Project()

     I spent days trying to find the equivalent in DirectX 11, and although there is plenty of documentation about viewports available within the DeviceContext class, I couldn't find an equivalent method. Then, it hit me: The answer is back to the basics, back to math. The solution is rather simple, and even elegant:

XMVECTOR vecPixel = XMVectorSet(0, 0, 0, 0);
XMFLOAT3 vecThis = XMFLOAT3(0, 0, 0);


vecPixel = vecAPositionIn3DSpace;
vecPixel = XMVector4Transform(vecPixel, mtxWorldRotation);
vecPixel = XMVector4Transform(vecPixel, mtxView);
vecPixel = XMVector4Transform(vecPixel, mtxProjection);

XMStoreFloat3(&vecThis, vecPixel);
if (0 < vecThis.z)// *** <- No Div0!!! *** //
{
	lngDisplayPosX = (vecThis.x / vecThis.z) * (lngDisplayWidth / 2);
	lngDisplayPosY = -(vecThis.y / vecThis.z) * (lngDisplayHeight / 2);


...

     It works like a charm. Nothing like home-made code ;)

     I hope it helps!!!

     Tarh Ik

PS: This posting has been posted "AS IS"

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Tarh ik

Answer 1

See XMVector3Project which is a DirectXMath function for doing just this.

Note that if you are going to project more than a single point, you should combine mtxWorldRotation * mtxView * mtxProjection and then do the vector4*matrix multiply once.

Note: If you are porting XNA Game Studio code from C# to C++, you might want to look at SimpleMath in the DirectX Tool Kit.

Answer 2

     Hi Chuck,

     Cool, thanks!!! I never thought about looking for the answer in the vector functions... although now I understand why it's there.

     The idea about multiplying the three matrixes first and apply the result to all projections is a great one too, specially since I need to project about 600 instances per frame. Definitelly, that will boost performance.

     Thanks again!!!

     Tarh Ik.

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Tarh ik