source: GTP/trunk/App/Demos/Illum/Ogre/Media/materials/programs/GameTools_HPS_Smoke_S.hlsl @ 1336

struct VS_OUT
{
        float4 hPosition : POSITION;
        float4 cPosition : TEXCOORD0;
        float  radius    : TEXCOORD2;
        float4 texCoord  : TEXCOORD1;
};

VS_OUT VP(float4 mPosition : POSITION,
                  uniform float4x4 worldView,
                  uniform float4x4 Proj,
                  uniform float4x4 worldViewProj
                  float2 texCoord :TEXCOORD0)
{
 VS_OUT Out;
 Out.texCoord.xy = texCoord; 
 Out.texCoord.zw = (texCoord * 2.0) - 1.0; 
 
 float2 offset = texCoord.zw;
 float4 cPosition = mul(worldView, mPosition);
 cPosition.xy += offset;
   
 Out.hPosition = mul( Proj, cPosition );
 Out.cPosition = cPosition;
 Out.radius = abs(offset.x);
 
  return OUT;
}

float4 FP(VS_OUT In,
                  uniform float4x4 Proj,
                  uniform sampler2D Texture):COLOR
{
        float4 color = 1;
        float front = 0;
        float back = 1;
        
        float4 tex = tex2D(Texture, In.texCoord.xy);
        float d = length(In.TexCoord.zw);
        float w = sqrt(In.radius * In.radius - d * d);
        if(d < r)
        {
                float4 positionF; float4 positionB;
                positionF = positionB = In.cPosition;
                positionF.z -= w;
                positionB.z += w;
                positionF = mul(Proj, positionF);
                positionB = mul(Proj, positionB);
                positionF = (positionF / positionF.w + 1.0) / 2.0;
                positionB = (positionB / positionB.w + 1.0) / 2.0;
        }
        color.rgb = positionF.z;
        
        return color;
}