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Different behaviour between opencl c99 and c++ with the same implementation of a realtime voxel raycaster


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#1 drowne   Members   -  Reputation: 101

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Posted 11 June 2012 - 07:55 AM


I am working with opencl to develop a voxel raycasting engine. I am trying to do something similar to Gigavoxels by Crassin (http://maverick.inria.fr/Publications/2011/Cra11/). In this paper, they are using an octree to store the voxel data. For the moment I am just trying to descend inside the octree until I reach a leaf that contains rendering data.

I have made two implementations: one in OpenCl on the GPU and another C++ on the CPU. The problem I am experiencing is that on the GPU the algorithm is going through a wrong number of levels until reaching a leaf inside the octree. The CPU version gives correct results. The algorithm for both versions is the same and the code is almost similar.

Do you guys know what could be the problem? Could it be a hardware problem, an OpenCl problem or am I doing something wrong? I am experiencing the same result on three different nVidia GPUs.


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#2 coderchris   Members   -  Reputation: 207

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Posted 11 June 2012 - 08:37 AM

You said the code is similar, but I'm guessing not exactly the same. I'd bet the problem lies somewhere in the minor differences...
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#3 drowne   Members   -  Reputation: 101

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Posted 11 June 2012 - 09:08 AM

You said the code is similar, but I'm guessing not exactly the same. I'd bet the problem lies somewhere in the minor differences...



Here is the C++ Code:
// Calculate actual ray stepping position
glm::vec4 pos = eyeRay_o + eyeRay_d * t;

uint offset = 0;
//check if root is leaf
uint leafFlag = GetLeafBit(octreeNodes[0]);
//get children address of root
uint childrenAddress = GetChildAddress(octreeNodes[0]);

while (iterations < 30) {  
	iterations++;

	// Calculate subdivision offset
	offset = (uint)(pos.x * 2) + (uint)(pos.y * 2) * 2 + (uint)(pos.z * 2) * 4;
	
	if (leafFlag == 1) {
		 //return some colour and exit the loop
		 break;
	}
	else
	{
		 glm::uvec4 off = glm::uvec4(pos.x * 2, pos.y * 2, pos.z * 2, pos.w * 2);
		 pos.x = 2 * pos.x - off.x;
		 pos.y = 2 * pos.y - off.y;
		 pos.z = 2 * pos.z - off.z;
		 pos.w = 2 * pos.w - off.w;  
	}

	// Extract node data from the children
	finalAddress = childrenAddress + offset;	
	leafFlag = GetLeafBit(nodes[finalAddress]);
	childrenAddress = GetChildAddress(nodes[finalAddress]);
}  

Here is the OpenCL Code:


// Calculate actual ray stepping position
float4 position = rayOrigin + rayDirection * t;
uint offset = 0;
//check if root is leaf
uint leafFlag = extractOctreeNodeLeaf(octreeNodes[0]);
//get children address of root
uint childrenAddress = extractOctreeNodeAddress(octreeNodes[0]);

//position will be in the [0, 1] interval
//size of octree is 1
while (iterations < 30) {  
	iterations++;

	//calculate the index of the next child based on the position in the current subdivision
	offset = (uint)(position.x * 2) + (uint)(position.y * 2) * 2 + (uint)(position.z * 2) * 4;
	
	if (leafFlag == 1) {
		//return some colour and exit the loop
		break;
	}
	else
	{
		 //transform the position inside the parent
		 //to the position inside the child subdivision
		 //size of child will be considered to be 1
		 uint4 off;
		 off.x = floor(position.x * 2);
		 off.y = floor(position.y * 2);
		 off.z = floor(position.z * 2);
		 off.w = floor(position.w * 2);
		 position = 2 * position - off;  
	}
	
	// Extract node data from the children
	finalAddress = childrenAddress + offset;
	leafFlag = extractOctreeNodeLeaf(octreeNodes[finalAddress]);
	//each node has an index to an array of 8 children - the index points to the first child
	childrenAddress = extractOctreeNodeAddress(octreeNodes[finalAddress]);
}

Edited by drowne, 11 June 2012 - 09:10 AM.


#4 Erik Rufelt   Crossbones+   -  Reputation: 3478

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Posted 11 June 2012 - 09:34 AM

Probably the error is in extractOctreeNodeLeaf or extractOctreeNodeAddress or similar. Try posting the entire code files, and perhaps someone will spot an error.

Also, floating point calculations are often not guaranteed to yield exactly the same results when run on different devices.

#5 drowne   Members   -  Reputation: 101

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Posted 11 June 2012 - 09:41 AM

Probably the error is in extractOctreeNodeLeaf or extractOctreeNodeAddress or similar. Try posting the entire code files, and perhaps someone will spot an error.

Also, floating point calculations are often not guaranteed to yield exactly the same results when run on different devices.



Both functions just do some bit operations:

OpenCL version:

inline char extractOctreeNodeLeaf(uint value) {
 value = value >> 1;
 return value & 1;
}

inline uint extractOctreeNodeAddress(uint value) {
 return value >> 2;
}

C++ version:

inline byte GetLeafBit(uint value)
{
 value = value >> 0x1;
 return value & 0x1;
}

inline uint GetChildAddress(uint value)
{
 return value >> 0x2;
}


#6 coderchris   Members   -  Reputation: 207

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Posted 11 June 2012 - 11:22 AM

You should try verifying that the "octreeNodes" array is the same (byte for byte) on the GPU version as it is on the CPU version.

If thats true, also make sure your test is using the same values for rayOrigin, rayDirection, and t.

And like Erik said, it could just be floating point playing tricks on you.

It probably wont make a difference, but you could try changing the opencl version from using floor() inside the else to casting (like you do in the CPU version).
Check out my stuff: http://www.chrisacoder.com/




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