light pre-pass

Hello everyone,

I've been trying to implement light pre-pass recently, following this tutorial, i'm using Direct3D11 (shader model 4)
my problem is getting the pixel position in my pixel shader, here's the code (light map building shader):


float2 PostProjToScreen ( float4 position )
{
float2 screenPos = position.xy / position.w;
return 0.5f * ( float2 ( screenPos.x + 1, 1 - screenPos.y ) );
}

float2 HalfPixel ( void )
{
return float2 ( 0.5f / screenSize.x, 0.5f / screenSize.y );
}

VS_OUT VS ( VS_IN input )
{
VS_OUT output;
output.position = mul ( float4 ( input.position, 1.0f ), gWVP );
output.tex = ((output.position.xy + float2(1.0f,1.0f))/2.0f);
output.tex.y = 1.0f - output.tex.y;
return output;
}
float4 PS ( VS_OUT input ) : SV_TARGET
{
float2 texC = input.tex; //PostProjToScreen ( input.position ) + HalfPixel ( ); doesn't work

float z = depthMap.Sample ( gSampler, texC );
float x = texC.x * 2.0f - 1.0f;
float y = (1.0f - texC.y) * 2.0f - 1.0f;
float4 vProjectedPos = float4 ( x, y, z, 1.0f );
float4 vPositionVS = mul ( vProjectedPos, gProjMatrixInverse );

float3 pixelPosition = vPositionVS.xyz / vPositionVS.w; //littile problems with this variable also
float3 pixelNormal = normalMap.Sample ( gSampler, texC );

float3 lDir = -normalize ( gLightDir );
float nl = 1.0f / dot ( pixelNormal, lDir );

float3 camDir = normalize ( gEyePosW - pixelPosition );

// Calculate specular term
float3 h = normalize ( lDir + camDir );
float spec = pow ( saturate ( dot ( pixelNormal, h ) ), gSpecPower );

return float4 ( gLightColor * nl, spec );
}


so here's the whole thing: "PostProjToScreen ( input.position ) + HalfPixel ( )" line didn't work at all, it changed when my rendered shape changed which means it's not the real coords of the pixel (am i right? ), so i took it out and replaced it with the texcoords from the vertex shader, this only works when i render a fullscreen quad (to implement directional light, for example), but i wan't a more "general" solution so that it could work with other primitives/types of light

now the second thing is: the pixel z position is not being calculated correctly: i tested (with working texcoords from vertex shader) x and y and they seem to be fine, but the z seems to be buggy, im not sure how to debug this because im a little confused about it so i would appreciate some help.

note: the depth map is automatically generated by hardware, i didn't output it myself (i tried doing it myself, the depth map looked weird, alot of big flickers everywhere, even the model depth is screwed up)

Thank you for your time.