DirectX 11 Compute Shader - not writing all values - directx-11

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 ?

Answer 1

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.

Answer 2

I'm not sure but, shouldn't it be for BufferOut decl:
RWStructuredBuffer<BufType> BufferOut : register(u0);
instead of :
RWStructuredBuffer BufferOut : register(u0);
If you are only using a float4 write target, why not use just:
RWBuffer<float4> BufferOut : register (u0);
Maybe this could help.

Answer 3

After playing around today again, i ran into the same problem once again.
The following code produced all white output:
[numthreads(1, 1, 1)]
void Main( uint3 dispatchId : SV_DispatchThreadID )
{
float4 color = float4(1.0f,0.0f,0.0f,1.0f);
WriteResult(dispatchId,color);
}
The WriteResult method is a utility method from my hlsl standard library.
Long story short. After i upgraded from Driver version 192 to 195(beta) the problem went away.
Seems like the drivers have some definitive problems in compute shader support left, so beware.

Answer 4

from what ive seen, computer shaders are only useful if you need a more general computational model than the tradition pixel shader, or if you can load data and then share it between threads in fast shared memory. im fairly sure u would get better performance with a pixel shader for the mandelbrot shader.
on my setup (win7, feb 10 dx sdk, gtx480) my compute shaders have a punishing setup time of over 0.2-0.3ms (binding a SRV and a UAV and then calling dispatch()).
if u do a PS implementation please post your experiences.

Answer 5

I have no direct experience with DX compute shaders but...
Why are you setting alpha = 1.0?
IIRC, that makes the pixel 100% transparent, so your inside pixels are transparent red, and show up as whatever color was drawn behind them.
When alpha = 1.0, the RGB components are never used.