Question

I am looking to billboard a sun image in my 3d world (directx 9).

Creating a D3DXSPRITE is great in some cases, but it is only a 2d object and can not exist in my "world" as a 3d object. What is an alternative method for billboarding, similar to d3dxsprite? How can I implement it?

The only alternative I have currently found is this link: http://www.two-kings.de/tutorials/dxgraphics/dxgraphics17.html which does not seem to work

Answer 1

Taking the center of your object vCenter. The object has a width and height of (w,h).

Firstly you need your camera to billboard vector. This is calculated as vCamToCen = normalise( vCamera - vCenter ).

You then need an appropriate rough up vector. This can be extracted from the view matrix (handily described here, ie the second column). You can then calculate the side vector by doing vSide = vCamToCen x vUp. Then calculate the REAL up vector by doing vUp = vCamToCen x vSide. Where x is a cross product.

You now have all the info you need to do your billboarding.

You can then form your 4 verts as follows.

const float halfW = w / 2.0f;
const float halfH = h / 2.0f;

const D3DXVECTOR3 vHalfSide = vSide * halfW;
const D3DXVECTOR3 vHalfUp = vUp  * halfH;

vertex[0].pos = vCenter;
vertex[1].pos = vCenter;
vertex[2].pos = vCenter;
vertex[3].pos = vCenter;

vertex[0].pos -= vHalfSide;
vertex[0].pos -= vHalfUp;

vertex[1].pos += vHalfSide;
vertex[1].pos -= vHalfUp;

vertex[2].pos += vHalfSide;
vertex[2].pos += vHalfUp;

vertex[3].pos -= vHalfSide;
vertex[3].pos += vHalfUp;

Build your 2 triangles out of those verts and pass it through your pipeline as normal (ie with your normal view and projection matrices).

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