Making a point light follow the camera

Question

I'm applying phong shading onto a single giant triangle, and I'd like the light's coordinates to coincide with the camera's coordinates in 3D space. In order to do this, whenever I update the camera's coordinates, I also update the light's coordinates. However, diffuse and specular lighting don't "focus" exactly at the camera position when I bring the camera close to the triangle, instead they do it a few units too far. The triangle is .5 units below the XZ plane and parallel to it. Here is a picture demonstrating this effect.

Also, here are my HLSL shaders; this is where all the transformations happen.

// Vertex Shader ////////////////////////////////////////

 float4x4    Matrix; // ViewProjection Matrix
 float4x4    Matrix2; // View matrix
 float4x4    Matrix3; // View matrix with translation zeroed out

 float4    lightPos;

struct VS_INPUT
{
    float4  Pos     : POSITION;
    float4  Normal  : NORMAL;
    float2  Tex0    : TEXCOORD0;
};

struct VS_OUTPUT
{
    float4  Pos     : POSITION;
    float4  Normal  : TEXCOORD1;
    float4  LightDir     : TEXCOORD2;
    float2  Tex0    : TEXCOORD0;
};

////////////////////////////////////////

VS_OUTPUT VShade(VS_INPUT In)
{
    VS_OUTPUT Out = (VS_OUTPUT) 0;

    Out.Tex0      = In.Tex0;

    //world space -> clip space transformation
    Out.Pos       = mul(In.Pos, Matrix);

    // world space -> camera space transformations
    Out.Normal    = normalize(mul(In.Normal, Matrix3));
    Out.LightDir  = normalize(mul(lightPos - In.Pos, Matrix3));

    return Out;
}

// Pixel Shader ////////////////////////////////////////

sampler   baseTex;

struct PS_INPUT
{
    float4  LightDir     : TEXCOORD2;
    float4  Normal  : TEXCOORD1;
    float2  Tex0    : TEXCOORD0;
};

struct PS_OUTPUT
{
    float4  Color   : COLOR;
};

////////////////////////////////////////

PS_OUTPUT PShade(PS_INPUT In)
{
    PS_OUTPUT Out = (PS_OUTPUT) 0;

    float4 Normal = normalize(In.Normal);
    float4 LightDir = normalize(In.LightDir);

    float di = saturate(dot(Normal, LightDir));

    float4 ambient = {0.1, 0.1, 0.1, 1};

    float4 Reflect = normalize(2 * di * Normal - LightDir); 
    float specular = pow(saturate(dot(Reflect, LightDir)), 2);

    Out.Color = ambient + tex2D(baseTex,In.Tex0) * di + specular;
    return Out;
}

EDIT

In.LightDir is computed for each pixel. It is first computed for each vertex in the vertex shader Out.LightDir = normalize(mul(lightPos - In.Pos, Matrix3));, then it is interpolated across the whole triangle. lightPos is just the camera's position (which coincides with the light):

if(handle = m_pVertexConstants->GetConstantByName(NULL,"lightPos"))
        m_pVertexConstants->SetVector(pDevice,handle,(D3DXVECTOR4*)m_camera.GetPosition());

Also, the world transformation matrix is just the identity, so I have not included it anywhere.

Answer 1

Thanks to everyone for their responses, I have finally figured out the problem. Turns out I had to interpolate the positions, as they are transformed in view space, instead of the light direction. Apparently light direction does not linearly interpolate properly. Here are my modified shaders:

// Vertex Shader ////////////////////////////////////////

float4x4    Matrix; // ViewProjection Matrix
float4x4    Matrix2; // View matrix
float4x4    Matrix3; // View matrix with translation zeroed out
float4x4    Matrix4; // World matrix

//float4    lightPos;

struct VS_INPUT
{
    float4  Pos     : POSITION;
    float4  Normal  : NORMAL;
    float2  Tex0    : TEXCOORD0;
};

struct VS_OUTPUT
{
    float4  Pos         : POSITION;

    float2  Tex0        : TEXCOORD0;
    float4  Normal      : TEXCOORD1;
    float4  Pos2        : TEXCOORD2;
};

////////////////////////////////////////

VS_OUTPUT VShade(VS_INPUT In)
{
    VS_OUTPUT Out = (VS_OUTPUT) 0;

    Out.Tex0      = In.Tex0;

    //world space -> clip space transformation
    Out.Pos       = mul( In.Pos, mul( Matrix4, Matrix ) );

    // world space -> camera space transformations
    Out.Normal    = normalize(mul(In.Normal, Matrix3));
    Out.Pos2      = mul( In.Pos, mul( Matrix4, Matrix3 ) );

    return Out;
}

// Pixel Shader ////////////////////////////////////////

sampler   baseTex;

float4x4    Matrix; // ViewProjection Matrix
float4x4    Matrix2; // View matrix
float4x4    Matrix3; // View matrix with translation zeroed out
float4x4    Matrix4; // World matrix

float4    lightPos;

struct PS_INPUT
{    
    float2  Tex0        : TEXCOORD0;
    float4  Normal      : TEXCOORD1;
    float4  Pos2        : TEXCOORD2;
};

struct PS_OUTPUT
{
    float4  Color       : COLOR;
};

////////////////////////////////////////

PS_OUTPUT PShade(PS_INPUT In)
{
    PS_OUTPUT Out = (PS_OUTPUT) 0;

    float4 Normal = normalize(In.Normal);
    float4 LightDir = normalize(mul(lightPos, Matrix3) - In.Pos2);

    float di = saturate(dot(Normal, LightDir));

    float4 ambient = {0.1, 0.1, 0.1, 1};

    float4 Reflect = normalize(2 * di * Normal - LightDir); 
    float specular = pow(saturate(dot(Reflect, LightDir)), 2);

    Out.Color = ambient + tex2D(baseTex,In.Tex0) * di + specular;
    return Out;
}