God Rays Shader [bugged]

sampler2D renderTarget;

#define SAMPLE_AMOUNT 64

Texture2D Tex;
float2 lightPosition;
float exposure;
float decay;
float weight;
float density;

SamplerState State = sampler_state
{
	Texture = <Tex>;
	MipFilter = Point;
	MinFilter = Linear;
	MagFilter = Linear;
	AddressU = Wrap;
	AddressV = Wrap;
};

float4x4 MatrixTransform;

//The default vertex shader of the spriteBatch, implemented manually so it can be compiled in 3_0
void SpriteVertexShader(inout float4 color    : COLOR0,
                        inout float2 texCoord : TEXCOORD0,
                        inout float4 position : SV_Position)
{
    position = mul(position, MatrixTransform);
}

//Copypasted from the tutorial, only changed some variable names
float4 main(float2 texCoord : TEXCOORD0) : COLOR0
{
  // Calculate vector from pixel to light source in screen space.
   half2 deltaTexCoord = (texCoord - lightPosition.xy);
  // Divide by number of samples and scale by control factor.
  deltaTexCoord *= 1.0f / SAMPLE_AMOUNT * density;
  // Store initial sample.
   half3 color = tex2D(State, texCoord);
  // Set up illumination decay factor.
   half illuminationDecay = 1.0f;
  // Evaluate summation from Equation 3 NUM_SAMPLES iterations.
   for (int i = 0; i < SAMPLE_AMOUNT; i++)
  {
    // Step sample location along ray.
    texCoord -= deltaTexCoord;
    // Retrieve sample at new location.
   half3 sample = tex2D(State, texCoord);
    // Apply sample attenuation scale/decay factors.
    sample *= illuminationDecay * weight;
    // Accumulate combined color.
    color += sample;
    // Update exponential decay factor.
    illuminationDecay *= decay;
  }
  // Output final color with a further scale control factor.
   return float4( color * exposure, 1);
}

technique Technique1
{
    pass Pass1
    {
	VertexShader = compile vs_3_0 SpriteVertexShader();
        PixelShader = compile ps_3_0 main();
    }
}