DirectX 11 Compute Shader - not writing all values

Question

I am trying some experiments in fractal rendering with DirectX11 Compute Shaders. The provided example runs on a FeatureLevel_10 device.

My RwStructured output buffer has a data format of R32G32B32A32_FLOAT

The problem is that when writing to the buffer, it seems that only the ALPHA ( w ) value gets written nothing else.... Here is the shader code:

struct BufType
{
    float4 value;
};

cbuffer ScreenConstants : register(b0)
{
  float2 ScreenDimensions;
  float2 Padding;
};

RWStructuredBuffer<BufType> BufferOut : register(u0);

[numthreads(1, 1, 1)]
void Main( uint3 DTid : SV_DispatchThreadID )
{
    uint index = DTid.y * ScreenDimensions.x + DTid.x;
    
    float minRe = -2.0f;
    float maxRe = 1.0f;
    float minIm = -1.2;
    float maxIm = minIm + ( maxRe - minRe ) * ScreenDimensions.y / ScreenDimensions.x;
    float reFactor = (maxRe - minRe ) / (ScreenDimensions.x - 1.0f);
    float imFactor = (maxIm - minIm ) / (ScreenDimensions.y - 1.0f);
    float cim = maxIm - DTid.y * imFactor;
    
    uint maxIterations = 30;
    
    float cre = minRe + DTid.x * reFactor;
    float zre = cre;
    float zim = cim;
    
    bool isInside = true;
    uint iterationsRun = 0;
    
    for( uint n = 0; n < maxIterations; ++n )
    {
        float zre2 = zre * zre;
        float zim2 = zim * zim;
        
        if ( zre2 + zim2 > 4.0f )
        {
            isInside = false;
            iterationsRun = n;
        }
        
        zim = 2 * zre * zim + cim;
        zre = zre2 - zim2 + cre;
    }
    
    if ( isInside )
    { 
        BufferOut[index].value = float4(1.0f,0.0f,0.0f,1.0f);
    }
}

The code actually produces in a sense the correct result ( 2D Mandelbrot set ) but it seems somehow only the alpha value is touched and nothing else is written, although the pixels inside the set should be colored red... ( the image is black & white )

Anybody has a clue what's going on here ?

you're only showing the HLSL code generating the buffer. What about the c code actually rendering the buffer ? How do you know for sure your buffer does not have red component ? — Bahbar
Did you check the generated Asm Shader for potential compiler errors ? — Stringer

Gluber Gluber · Accepted Answer · 2009-11-21T17:07:51

After some fiddling around i found the problem. I have not found any documentation from MS mentioning this, so it could also be a Nvidia specific driver issue.

Apparently you are only allowed to write ONCE per Compute Shader Invocation to the same element in a RWSructuredBuffer. And you also HAVE to write ONCE.

I changed the code to accumulate the correct color in a local variable, and write it now only once at the end of the shader.

Everything works perfectly now in that way.

DirectX 11 Compute Shader - not writing all values

5 Answers