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[DX9]: SSAO not working

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I tried getting SSAO to work, however it is not really working. Here is what my implementation looks like:

float random_size = 64.0f;
float g_sample_rad = 1.0f;
float g_intensity = 0.5f;
float g_scale = 0.1f;
float g_bias = 0.01f;

float2 vTex0 : TEXCOORD0;


o.vPos = i.vPos;

o.vTex0 = i.vTex0;

return o;

float3 getPosition(in float2 uv)
return tex2D(ScenePositionSampler,uv).xyz;

float3 getNormal(in float2 uv)
return normalize(tex2D(SceneNormalSampler, uv).xyz * 2.0f - 1.0f);

float2 getRandom(in float2 uv)
return normalize(tex2D(RandomSampler, g_screen_size * uv / random_size).xy * 2.0f - 1.0f);

float doAmbientOcclusion(in float2 tcoord,in float2 uv, in float3 p, in float3 cnorm)
float3 diff = getPosition(tcoord + uv) - p;
const float3 v = normalize(diff);
const float d = length(diff)*g_scale;
return max(0.0,dot(cnorm,v)-g_bias)*(1.0/(1.0+d))*g_intensity;

float4 psAmbient(PS_INPUT_LIGHT i) : COLOR0
float4 vDiffuseMaterial;
vDiffuseMaterial.rgb = tex2D(SceneMaterialSampler, i.vTex0).rgb;
vDiffuseMaterial.rgb *= LightAmbient;

vDiffuseMaterial.a = 1.0;

const float2 vec[4] = {float2(1,0),float2(-1,0),

float3 p = mul(float4(getPosition(i.vTex0), 1.0f), View);
float3 n = normalize(mul(float4(getNormal(i.vTex0), 0.0f), View));
float2 rand = getRandom(i.vTex0);

float ao = 0.0f;
float rad = g_sample_rad/p.z;

int iterations = 4;
for (int j = 0; j < iterations; ++j)
float2 coord1 = reflect(vec[j],rand)*rad;
float2 coord2 = float2(coord1.x*0.707 - coord1.y*0.707,
coord1.x*0.707 + coord1.y*0.707);

ao += doAmbientOcclusion(i.vTex0,coord1*0.25, p, n);
ao += doAmbientOcclusion(i.vTex0,coord2*0.5, p, n);
ao += doAmbientOcclusion(i.vTex0,coord1*0.75, p, n);
ao += doAmbientOcclusion(i.vTex0,coord2, p, n);

return vDiffuseMaterial * ao;

However, my results looks like this (depending on the view angel, sometimes I get only black..):


I really don't know whats going wrong. Can somebody help me? I was quite sure my implementation was valid, however the results aren't what I expected.. Thanks in advance for any hints!

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