VS_OUTPUT vs_main( VS_INPUT In )
{
   VS_OUTPUT Out;

   //Postions quad vertices at 4 corners of the screen..
   Out.screenPos = float4(sign(In.vertexPos.xy),0,1);

   //Gives the world position of the vertices of the quad.
   float4 worldPos = mul(Out.screenPos, worldViewProjInv);

   //Send the world position to the ps.
   Out.worldPos = worldPos;

   return Out;
}