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akhin

random number generation based on time in HLSL

12 posts in this topic

hi,

i will be passing timeGetTime values from CPU to a variable
in my shader and based on that possible seed value ,

I want to implement a random number generation function in HLSL , but actually currently have no idea about algorithms

As starting point what would you advise ?

And also what about producing something with same quality with rand() of C/C++ ,
is it a hard job or can it be done with a reasonable effort ?

many thanks in advance...
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Why would you want to do that in HLSL? That's not really HLSL's job, you should pass in those kind of parameters to the shader if you need them.
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Because I want to implement main `Update` method of my particle system
in GPU with vertex shaders ( since I am using D3D9 and i cant use geometry shaders ,i still have code in C++ side except `Update` part which is the meat of the job actually)

so that is why I want to have random number generation function in my shader

in current implementation , in CPU side update method of my particle system , I am already passing random numbers to the shader variables , but I want to keep CPU side as low as possible
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You might be able to leverage noise. Though that's not really suited for that purpose.

Any actually good random number generator is going to burn through too much instruction count to be of any good.

You can also write a bunch of random numbers to a texture and look that up in the shader. That's probably the best answer if you really want random numbers in your shader.
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I guess this is what you're looking for =)



#define RANDOM_IA 16807
#define RANDOM_IM 2147483647
#define RANDOM_AM (1.0f/float(RANDOM_IM))
#define RANDOM_IQ 127773
#define RANDOM_IR 2836
#define RANDOM_MASK 123459876


int random_x;


float random ()
{
int k;
float ans;

random_x ^= RANDOM_MASK;
k = random_x / RANDOM_IQ;
random_x = RANDOM_IA * (random_x - k * RANDOM_IQ ) - RANDOM_IR * k;

if ( random_x < 0 )
random_x += RANDOM_IM;

ans = RANDOM_AM * random_x;
random_x ^= RANDOM_MASK;

return ans;
}


float random ( float low, float high )
{
float v = random();
return low * ( 1.0f - v ) + high * v;
}


float2 random ( float2 low, float2 high )
{
float2 v = float2( random(), random() );
return low * ( 1.0f - v ) + high * v;
}


float3 random ( float3 low, float3 high )
{
float3 v = float3( random(), random(), random() );
return low * ( 1.0f - v ) + high * v;
}


void seed ( int value )
{
random_x = value;
random();
}

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glSmurf , thanks very much

yeah it seems as what i am looking for :)

just a quick question , by using shader model 1.1 2.0 and 4.0 , i could not compile it because of '^' operator , how can I resolve this issue ?

thanks in advance...
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From msdn:
"Bitwise operators are defined to operate only on int and uint data types. Attempting to use bitwise operators on float, or struct data types will result in an error."

Maybe you changed 'int random_x' to 'float random_x' ?
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@glSmurf: Can you please tell us exactly how you got your code to work (shader model, compile options) and in what context (pixel/vertex shader, software shader even ?) because I have some doubts: One thing I'm concerned is the bitwise operations (only available on SM >= 4), the other that your PRNG uses a global that gets changed. You can't do that with shaders IMHO.
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First of all, it's not my own code. I got it from the NVidia DX10 SDK. I've used it in a vertex shader and it works just fine (shader model 4.0).
For more information check out the SDK.
File: ... / NVIDIA Direct3D SDK 10 / Source / Lightning / Lightning.fx
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Thanks, that clarifies the problem: akhin uses DX9 so only SM 3 (or less) is available, unfortunately.

@akhin: Bear in mind that if you now go for a texture lookup in the vertex shader, this is not possible with SM 2.
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thanks for clarification ,

I need to ask one last question since I am new to shader programming ,

I am not crystal clear sure how mechanics behind the scenes are so ,

As it has been said , it is Shader Model 3 with DX9 which is the latest SM I can use, what about using DX9 and NvidiaCG together instead of HLSL ?

So it wouldnt change anything would it ?

Actually I am after to learn how the graphics API itself is a bottleneck for the latest shader model we can use , if the shaders are being compiled externally and targeted for GPU
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Haven't explicitly checked myself, but I really doubt it (these profiles are from an old version of Cg, couldn't find a newer table). You still have to (want to?) load your shader in a DX 9 context, so you're limited to SM 3. By the way: Cg can compile HLSL code.

To get more features from your card you have to use a different API (DX10, OpenGL, or even OpenCL, CUDA).
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