I m trying to convert the Screen Space Ambient Occlusion example from XNA 3.1 to XNA 4.0. I ve fixed all the problems in the source, except this strange problem in a shader file. I ve gone through and fixed all the obvious problems with the shader as guided by Shawn Hargreaves blog, but when it compiles it uses up 620 instruction slots, which is well over the 512 instruction slot limit. How could this have worked in XNA 3.1, but not in XNA 4.0?

The changes from the 3.1 copy of the file are very minimal, and only consisted of renaming a few functions. below is the full shader source in it s current form. I ll be very grateful for any help in reducing the number instruction slots this compiles to.

float sampleRadius;
float distanceScale;
float4x4 Projection;

float3 cornerFustrum;

struct VS_OUTPUT
{
    float4 pos              : POSITION;
    float2 TexCoord         : TEXCOORD0;
    float3 viewDirection    : TEXCOORD1;
}; 

VS_OUTPUT VertexShaderFunction(
    float4 Position : POSITION, float2 TexCoord : TEXCOORD0)
{
    VS_OUTPUT Out = (VS_OUTPUT)0;

    Out.pos = Position;
    Position.xy = sign(Position.xy);
    Out.TexCoord = (float2(Position.x, -Position.y) + float2( 1.0f, 1.0f ) ) * 0.5f;
    float3 corner = float3(-cornerFustrum.x * Position.x,
            cornerFustrum.y * Position.y, cornerFustrum.z);
    Out.viewDirection =  corner;

    return Out;
}


texture depthTexture;
texture randomTexture;

sampler2D depthSampler = sampler_state
{
    Texture = <depthTexture>;
    ADDRESSU = CLAMP;
    ADDRESSV = CLAMP;
    MAGFILTER = LINEAR;
    MINFILTER = LINEAR;
};

sampler2D RandNormal = sampler_state
{
    Texture = <randomTexture>;
    ADDRESSU = WRAP;
    ADDRESSV = WRAP;
    MAGFILTER = LINEAR;
    MINFILTER = LINEAR;
};

float4 PixelShaderFunction(VS_OUTPUT IN) : COLOR0
{
    float4 samples[16] =
    {
        float4(0.355512,    -0.709318,  -0.102371,  0.0 ),
        float4(0.534186,    0.71511,    -0.115167,  0.0 ),
        float4(-0.87866,    0.157139,   -0.115167,  0.0 ),
        float4(0.140679,    -0.475516,  -0.0639818, 0.0 ),
        float4(-0.0796121,  0.158842,   -0.677075,  0.0 ),
        float4(-0.0759516,  -0.101676,  -0.483625,  0.0 ),
        float4(0.12493,     -0.0223423, -0.483625,  0.0 ),
        float4(-0.0720074,  0.243395,   -0.967251,  0.0 ),
        float4(-0.207641,   0.414286,   0.187755,   0.0 ),
        float4(-0.277332,   -0.371262,  0.187755,   0.0 ),
        float4(0.63864,     -0.114214,  0.262857,   0.0 ),
        float4(-0.184051,   0.622119,   0.262857,   0.0 ),
            float4(0.110007,    -0.219486,  0.435574,   0.0 ),
        float4(0.235085,    0.314707,   0.696918,   0.0 ),
        float4(-0.290012,   0.0518654,  0.522688,   0.0 ),
        float4(0.0975089,   -0.329594,  0.609803,   0.0 )
    };

    IN.TexCoord.x += 1.0/1600.0;
    IN.TexCoord.y += 1.0/1200.0;

    normalize (IN.viewDirection);
    float depth = tex2D(depthSampler, IN.TexCoord).a;
    float3 se = depth * IN.viewDirection;

    float3 randNormal = tex2D( RandNormal, IN.TexCoord * 200.0 ).rgb;

    float3 normal = tex2D(depthSampler, IN.TexCoord).rgb;
    float finalColor = 0.0f;

    for (int i = 0; i < 16; i++)
    {
        float3 ray = reflect(samples[i].xyz,randNormal) * sampleRadius;

        //if (dot(ray, normal) < 0)
        //  ray += normal * sampleRadius;

        float4 sample = float4(se + ray, 1.0f);
        float4 ss = mul(sample, Projection);

        float2 sampleTexCoord = 0.5f * ss.xy/ss.w + float2(0.5f, 0.5f);

        sampleTexCoord.x += 1.0/1600.0;
        sampleTexCoord.y += 1.0/1200.0;
        float sampleDepth = tex2D(depthSampler, sampleTexCoord).a;

        if (sampleDepth == 1.0)
        {
            finalColor ++;
        }
        else
        {       
            float occlusion = distanceScale* max(sampleDepth - depth, 0.0f);
            finalColor += 1.0f / (1.0f + occlusion * occlusion * 0.1);
        }
    }

    return float4(finalColor/16, finalColor/16, finalColor/16, 1.0f);
}


technique SSAO
{
    pass P0
    {          
        VertexShader = compile vs_3_0 VertexShaderFunction();
        PixelShader  = compile ps_3_0 PixelShaderFunction();
    }
}