Volume rendering using Cg:compile error:semantic "POSITION" not visible in this profile

I am now coding to implement direct ray-casting volume rendering using OpenGL,Cg and C++.My computer's system is Win7 and my graphics is nVidia GTX570. Actually,I have implement basic functions of direct ray-casting volume rendering sucessfully.But now I want to add illumination on it and I use local illumination model.After I add several lines of code for illumination,I get Cg compiler error.Before adding,it runs well. The Cg profile I use is CG_PROFILE_VP40 and CG_PROFILE_FP40. Now I post my Cg code: ``` #define kNumberOfRaySteps 800 // Define interface between the application and the vertex program struct app_vertex { float4 Position : POSITION; float4 TexCoord : TEXCOORD1; float4 Color : COLOR0; }; // Define the interface between the vertex- and the fragment programs struct vertex_fragment { float4 Position : POSITION; // For the rasterizer float4 TexCoord : TEXCOORD0; float4 Color : TEXCOORD1; float4 Pos : TEXCOORD2; }; struct fragment_out { float4 Color : COLOR0; }; //Blinn-Phong illumination float3 shading(float3 N, float3 V, float3 L) { // material properties float3 Ka = float3(0.1, 0.1, 0.1); // ambient float3 Kd = float3(0.6, 0.6, 0.6); // diffuse float3 Ks = float3(0.2, 0.2, 0.2); // specular float n = 100.0; // shininess // light properties float3 lightColor = float3(1.0, 1.0, 1.0); float3 ambientLight = float3(0.3, 0.3, 0.3); // Calculate halfway vector float3 H= normalize(L + V); // Compute ambient term float3 ambient = Ka * ambientLight; // Compute the diffuse term float diffuseLight = max(dot(L, N), 0); float3 diffuse = Kd * lightColor * diffuseLight; // Compute the specular term float specularLight = pow(max(dot(H, N), 0), n); if (diffuseLight <= 0) specularLight = 0; float3 specular = Ks * lightColor * specularLight; return ambient + diffuse + specular; } // Raycasting vertex program implementation vertex_fragment vertex_main( app_vertex IN ) { vertex_fragment OUT; // Get OpenGL state matrices float4x4 ModelView = glstate.matrix.modelview[0];//model view matrix 0 float4x4 ModelViewProj = glstate.matrix.mvp;//modelview-projection matrix // Transform vertex OUT.Position = mul( ModelViewProj, IN.Position ); OUT.Pos = mul( ModelViewProj, IN.Position ); OUT.TexCoord = IN.TexCoord; OUT.Color = IN.Color; return OUT; } // Raycasting fragment program implementation fragment_out fragment_main( vertex_fragment IN, uniform sampler2D tex, //back face uniform sampler2D preint_table,//pre-intergration classification uniform sampler3D volume_tex, //volume data uniform float stepsize, //add local illumination uniform float3 lightPosition,//the external light position uniform float3 eyePosition,//the eye position uniform sampler3D normal_vec//normal vectors ) { fragment_out OUT; // find the right place to lookup in the backside buffer //normalize the spatial position to range [0,1] float2 texc = ((IN.Pos.xy / IN.Pos.w) + 1) / 2; // the start position of the ray is stored in the texturecoordinate float4 start = IN.TexCoord; float4 back_position = tex2D(tex, texc); float3 dir = float3(0,0,0); dir.x = back_position.x - start.x; dir.y = back_position.y - start.y; dir.z = back_position.z - start.z; // the length from front to back is calculated and used to terminate the ray float len = length(dir.xyz); float3 norm_dir = normalize(dir); float delta = stepsize; float3 delta_dir = norm_dir * delta; float delta_dir_len = length(delta_dir); float3 vec1 = start.xyz;//position of back point float3 vec2 = start.xyz;//position of front point float4 col_acc = float4(0,0,0,0); float alpha_acc = 0;//accumulation of alpha value(opacity) float length_acc = 0; float4 color_sample; float alpha_sample; float4 lookup = float4(0.0,0.0,0.0,0.0); //set background color to blue float4 backgroundColor = float4(1.0,1.0,1.0,0.0); //loops:sample points for(int i = 0; i < kNumberOfRaySteps; i++) { vec2 += delta_dir; lookup.x = (tex3D(volume_tex,vec1)).x;//scalar value of back point lookup.y = (tex3D(volume_tex,vec2)).x;//scalar value of front point color_sample = tex2D(preint_table,lookup.xy); alpha_sample = color_sample.a; //add code for illumination float3 N = tex3D(normal_vec,start.xyz).xyz;//normal vector //calculate light and viewing directions float3 L = normalize(lightPosition - IN.Position.xyz); float3 V = normalize(eyePosition - IN.Position.xyz); col_acc += (1.0 - alpha_acc) * color_sample * alpha_sample * 2; //add ilumination color col_acc += float4(shading(N,V,L),0.0); alpha_acc += (1.0 - alpha_acc) * alpha_sample; vec1 += delta_dir;//next sample point length_acc += delta_dir_len;//accumulation of ray length in the cube //change the background color from black to other color if(length_acc >= len) col_acc += (1.0 - alpha_acc) * backgroundColor; // terminate if opacity > 1 or the ray is outside the volume if(length_acc >= len || alpha_acc > 0.99) break; } OUT.Color = col_acc; return OUT; } ``` The information about Cg error: I found that if I erase the line ``` col_acc += float4(shading(N,V,L),0.0); ``` Then the program runs well,surely I cannot get the illumination effect. So I guess there is something wrong with the function shading of calculating illumination effect. But I cannot figure it out. Could someone tell me how to solve this problem?