// This is C7E3v_refraction from "The Cg Tutorial" (Addison-Wesley, ISBN
// 0321194969) by Randima Fernando and Mark J. Kilgard.  See page 187.

void C7E3v_refraction(float4 position : POSITION,
                      float2 texCoord : TEXCOORD0,
                      float3 normal   : NORMAL,
   
                  out float4 oPosition  : POSITION,
                  out float2 oTexCoord  : TEXCOORD0,
                  out float3 T          : TEXCOORD1,

              uniform float etaRatio,
              uniform float3 eyePositionW,
              uniform float4x4 modelViewProj, 
              uniform float4x4 modelToWorld)
{
  oPosition = mul(modelViewProj, position);
  oTexCoord = texCoord;

  // Compute position and normal in world space
  float3 positionW = mul(modelToWorld, position).xyz;
  float3 N = mul((float3x3)modelToWorld, normal);
  N = normalize(N);
  
  // Compute the incident and refracted vectors
  float3 I = normalize(positionW - eyePositionW);
  T = refract(I, N, etaRatio);
}