Introduction to Vertex Shaders - Introduction to 3D Game Programming with DirectX 9.0 - page 303

input.normal.w = 0.0f;

LightDirection = mul(LightDirection, ViewMatrix);

input.normal = mul(input.normal, ViewMatrix);

//
// Compute cosine of the angle between light and normal.
//
float s = dot(LightDirection, input.normal);

//
// Recall that if the angle between the surface and light
// is greater than 90 degrees the surface receives no light.
// Thus, if the angle is greater than 90 degrees we set
// s to zero so that the surface will not be lit.
//
if( s < 0.0f )
s = 0.0f;

//
// Ambient light reflected is computed by performing a
// component-wise multiplication with the ambient material
// vector and the ambient light intensity vector.
//
// Diffuse light reflected is computed by performing a
// component-wise multiplication with the diffuse material
// vector and the diffuse light intensity vector. Further
// we scale each component by the shading scalar s, which
// shades the color based on how much light the vertex received
// from the light source.
//
// The sum of both the ambient and diffuse components give
// us our final vertex color.
//

output.diffuse = (AmbientMtrl * AmbientLightIntensity) +
(s * (DiffuseLightIntensity * DiffuseMtrl));

return output;

}
Now that we have looked at the actual vertex shader code, let's shift
gears and look at the application code. The application has the following
relevant global variables:
IDirect3DVertexShader9* DiffuseShader = 0;
ID3DXConstantTable* DiffuseConstTable = 0;

ID3DXMesh* Teapot = 0;

D3DXHANDLE ViewMatrixHandle = 0;
D3DXHANDLE ViewProjMatrixHandle = 0;
D3DXHANDLE AmbientMtrlHandle = 0;

D3DXHANDLE DiffuseMtrlHandle = 0;

D3DXHANDLE LightDirHandle = 0;

D3DXMATRIX Proj;