Question

I am working on my own deffered rendering engine. I am rendering the scene to the g-buffer containing diffuse color, view space normals and depth (for now). I have implemented directional light for the second rendering stage and it works great. Now I want to render a point light, which is a bit harder.

I need the point light position for the shader in view space because I have only depth in the g-buffer and I can’t afford a matrix multiplication in every pixel. I took the light position and transformed it by the same matrix, by which I transform every vertex in shader, so it should align with verices in the scene (using D3DXVec3Transform). But that isn’t the case: transformed position doesn’t represent viewspace position nearly at all. It’s x,y coordinates are off the charts, they are often way out of the (-1,1) range. The transformed position respects the camera orientation somewhat, but the light moves too quick and the y-axis is inverted. Only if the camera is at (0,0,0), the light stands at (0,0) in the center of the screen. Here is my relevant rendering code executed every frame:

D3DXMATRIX matView;    // the view transform matrix
D3DXMATRIX matProjection;    // the projection transform matrix

D3DXMatrixLookAtLH(&matView,
                   &D3DXVECTOR3 (x,y,z),    // the camera position
                   &D3DXVECTOR3 (xt,yt,zt),    // the look-at position
                   &D3DXVECTOR3 (0.0f, 0.0f, 1.0f));    // the up direction

D3DXMatrixPerspectiveFovLH(&matProjection,
                           fov,    // the horizontal field of view
                           asp,    // aspect ratio
                           znear,    // the near view-plane
                           zfar);    // the far view-plane

D3DXMATRIX vysl=matView*matProjection;

eff->SetMatrix("worldViewProj",&vysl); //vertices are transformed ok ín shader

//render g-buffer

D3DXVECTOR4 lpos; D3DXVECTOR3 lpos2(0,0,0);

D3DXVec3Transform(&lpos,&lpos2,&vysl); //transforming lpos into lpos2 using vysl, still the same matrix
eff->SetVector("poslight",&lpos); //but there is already a mess in lpos at this time

//render the fullscreen quad with wrong lighting

Not that relevant shader code, but still, I see the light position this way (passing IN.texture is just me being lazy):

float dist=length(float2(IN.texture0*2-1)-float2(poslight.xy));
OUT.col=tex2D(Sdiff,IN.texture0)/dist;

I have tried to transform a light only by matView without projection, but the problem is still the same. If I transform the light in a shader, it’s the same result, so the problem is the matrix itself. But it is the same matrix as is transforming the vertices! How differently are vertices treated?

Can you please take a look at the code and tell me where the mistake is? It seems to me it should work ok, but it doesn’t. Thanks in advance.

Answer 1

You don’t need a matrix multiplication to reconstruct view position, here is a code snippet (from andrew lauritzen deffered light example)

tP is the projection transform, position screen is -1/1 pixel coordinate and viewspaceZ is linear depth that you sample from your texture.

float3 ViewPosFromDepth(float2 positionScreen,
                            float viewSpaceZ)
{
    float2 screenSpaceRay = float2(positionScreen.x / tP._11,
                               positionScreen.y / tP._22);

    float3 positionView;
    positionView.z = viewSpaceZ;
    positionView.xy = screenSpaceRay.xy * positionView.z;

    return positionView;
}