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Variable Size Array Of Structs GLSL


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#1 BlueSpud   Members   -  Reputation: 447

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Posted 09 May 2014 - 12:52 PM

Hello,

I've been trying to work out a way to pass an array of structs to GLSL. I know that you can pass arrays, but they need to be a fixed size. On the CPU I store all my structs in a std::vector. I know there are uniform buffer objects that newer versions of GLSL have, but I want to keep compatibility for older systems. I've heard the idea of making an array with more than I could possibly ever need and filling it up with only a few, but I'm pretty sure that would allocate the memory for every one, even the not ones in use. If you could give me a tip or article, or even point me in the right direction, I would appreciate it. 

 

Thanks.



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#2 HappyCoder   Members   -  Reputation: 2880

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Posted 09 May 2014 - 01:54 PM

How much will the number of elements in your array vary? What is the average size?

 

If the number of elements doesn't vary much and there aren't that many of them. I would just allocate the array to be larger than what the average length is and just accept that there will be unused slots in that array.

 

If the size will generally be small but occasionally be larger, see if you could process the larger arrays in chunks. For example, say you allocate your shader to support up to 16 structures at once. When you get a batch of 48, you proccess three seperate batches, each of size 16. For more information on doing this, just look up mulit-pass rendering.



#3 BlueSpud   Members   -  Reputation: 447

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Posted 09 May 2014 - 02:01 PM

How much will the number of elements in your array vary? What is the average size?

 

If the number of elements doesn't vary much and there aren't that many of them. I would just allocate the array to be larger than what the average length is and just accept that there will be unused slots in that array.

 

If the size will generally be small but occasionally be larger, see if you could process the larger arrays in chunks. For example, say you allocate your shader to support up to 16 structures at once. When you get a batch of 48, you proccess three seperate batches, each of size 16. For more information on doing this, just look up mulit-pass rendering.

I was hoping to squeeze everything into one pass. Right now Im not passing many elements but potentially i'll probably have 64 structs which right now only need 2 vec3s. Even if I was to allocate that many, I still have no idea how to pass all the array in. I really don't want to have to manually pass in every single value using glUniform, Im sure theres a better way. 



#4 HappyCoder   Members   -  Reputation: 2880

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Posted 09 May 2014 - 02:14 PM

You can pass the entire array at once using glUniform3fv 
 
struct Data {
   Vector3 foo;
   Vector3 bar;
}

vector<Data> data;

// populate data

// 3 floats per vector
// 2 vectors per Data structure
float* floatBuffer = new float[data.size() * 3 * 2];

for (int i = 0; i < data.size(); ++i)
{
   floatBuffer[i * 6] = data[i].foo.x;
   floatBuffer[i * 6 + 1] = data[i].foo.y;
   floatBuffer[i * 6 + 2] = data[i].foo.z;
   floatBuffer[i * 6 + 3] = data[i].bar.x;
   floatBuffer[i * 6 + 4] = data[i].bar.y;
   floatBuffer[i * 6 + 5] = data[i].bar.z;
}

glUniform3fv(uniformLocation, data.size() * 2, floatBuffer);
This wont work if you ever mix in non floating point values into your glsl structure. In that case, you could just pass the uniforms in as seperate arrays instead of an array of structures.

Edited by HappyCoder, 09 May 2014 - 02:15 PM.


#5 BlueSpud   Members   -  Reputation: 447

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Posted 09 May 2014 - 02:46 PM

You can pass the entire array at once using glUniform3fv  

struct Data {
   Vector3 foo;
   Vector3 bar;
}

vector<Data> data;

// populate data

// 3 floats per vector
// 2 vectors per Data structure
float* floatBuffer = new float[data.size() * 3 * 2];

for (int i = 0; i < data.size(); ++i)
{
   floatBuffer[i * 6] = data[i].foo.x;
   floatBuffer[i * 6 + 1] = data[i].foo.y;
   floatBuffer[i * 6 + 2] = data[i].foo.z;
   floatBuffer[i * 6 + 3] = data[i].bar.x;
   floatBuffer[i * 6 + 4] = data[i].bar.y;
   floatBuffer[i * 6 + 5] = data[i].bar.z;
}

glUniform3fv(uniformLocation, data.size() * 2, floatBuffer);
This wont work if you ever mix in non floating point values into your glsl structure. In that case, you could just pass the uniforms in as seperate arrays instead of an array of structures.

How will I read that in the shader? Just define the strict the same way and make an array of them?

 

EDIT: I figured it out, it was as simple as creating an array on the other side. Thank you.


Edited by BlueSpud, 09 May 2014 - 04:45 PM.





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