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DarkRonin

Calculate screen co-ords from a point in 3D space

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Hi Guys,

I have a function where I can calculate the point in 3D space given x & y screen co-ordinates below;

void Graphics::ScreenToWorld(float mousePosX,float mousePosY,float nDepth)
{
// Get matrices and the viewport.
D3DXMATRIX pm; // projection matrix
D3DXMATRIX vm; // view matrix
D3DXMATRIX wm; // world matrix
D3DVIEWPORT9 vp; // viewport
md3dDevice->GetTransform(D3DTS_PROJECTION,&pm);
md3dDevice->GetTransform(D3DTS_VIEW,&vm);
D3DXMatrixIdentity(&wm); // make the world matrix an identity matrix
md3dDevice->GetViewport(&vp);
// Call the D3DXVec3Unproject function twice to determine two points
// in our world space. Having two points we will be able to find
// a line in the world space. One point is for the z-near plane
// (which is usually z=0.0f) and another point is for z-far plane
// (which is usually z=1.0f). We grab the z-planes from the viewport.
D3DXVECTOR3 p1;
D3DXVec3Unproject(&p1, &D3DXVECTOR3(mousePosX,mousePosY, vp.MinZ), &vp, &pm, &vm, &wm);
D3DXVECTOR3 p2;
D3DXVec3Unproject(&p2, &D3DXVECTOR3(mousePosX,mousePosY, vp.MaxZ), &vp, &pm, &vm, &wm);
// We calculate an intersection of the line found previously
// with a plane z=0.0f. Equations of a line in 3-D space can
// be found on the following websites:
// http://mathforum.org...html#threelines
// http://mathforum.org...view/65721.html
// Here, wx, wy, and wz represent coordinates in the world space.
fWorldZ=nDepth;
fWorldX=((fWorldZ-p1.z)*(p2.x-p1.x))/(p2.z-p1.z)+p1.x;
fWorldY=((fWorldZ-p1.z)*(p2.y-p1.y))/(p2.z-p1.z)+p1.y;

// w is the point in the world space corresponding to the p point
// in the screen coordinates.
//D3DXVECTOR3 w(wx, wy, wz);
}


How would I do the opposite? If I had a point in the word x,y,z - how would I get the screen co-ordinates for that point?

Any help would be greatly appreciated. cool.png

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Simply transform your 3d position by view projection matrix .

md3dDevice->GetTransform(D3DTS_PROJECTION,&pm);
md3dDevice->GetTransform(D3DTS_VIEW,&vm);
D3DXMATRIX vp;
D3DXMatrixMultiply( &vp, &vm, &pm );
D3DXVECTOR4 posBase;
D3DXVec4Transform(&posBase,&D3DXVECTOR4(m_positionVal.x,m_positionVal.y,m_positionVal.z,1.0f),&vp);
posBase.x/=posBase.w; // Screen space X pos
posBase.y/=posBase.w; // Screen space Y pos
posBase.z/=posBase.w;

Edited by stc.5421

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Awesome! I will give that a try when I get back in front of my dev rig tomorrow night.

Matrices are not my strong point. I'll let you know how it goes.

Thanks again!

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Hi stc.5421,

I turned your code snippet into a function as below, but the values for fScreenX and fScreenY remain at zero, no matter what the inputs are for world X,Y, & Z.


void Graphics::WorldToScreen(float fPosX,float fPosY,float fPosZ)
{
D3DXMATRIX pm;
D3DXMATRIX vm;
D3DXMATRIX vp;
md3dDevice->GetTransform(D3DTS_PROJECTION,&pm);
md3dDevice->GetTransform(D3DTS_VIEW,&vm);
D3DXMatrixMultiply(&vp,&vm,&pm);
D3DXVECTOR4 posBase;
//D3DXVec4Transform(&posBase,&D3DXVECTOR4(m_positionVal.x,m_positionVal.y,m_positionVal.z,1.0f),&vp);
D3DXVec4Transform(&posBase,&D3DXVECTOR4(fPosX,fPosY,fPosZ,1.0f),&vp);
fScreenX=(posBase.x/=posBase.w); // Screen space X pos
fScreenY=(posBase.y/=posBase.w); // Screen space Y pos
posBase.z/=posBase.w;
}


Any help would be awesome laugh.png Edited by lonewolff

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I just sorted it out cool.png

It turns out the return values were relative. So, I modified the last couple of lines to get it back into pixels

fScreenX=((posBase.x/=posBase.w)*1600+1600)/2; // Screen space X pos
fScreenY=-((posBase.y/=posBase.w)*900-900)/2; // Screen space Y pos


Thanks again for your help biggrin.png Edited by lonewolff

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