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# [SOLVED] Hair simulation. Verlet/Constraints

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### #1AzraelilMeraz  Members

Posted 01 August 2012 - 01:51 PM

EDIT2: SOLVED, there is nothing wrong with the algorithm - I accidentally ran the constraint generation kernel every frame

Hello!
I've been toying with OpenGL and OpenCL trying to simulate Hair on the GPU.
However I've run into issues that are not OpenGL/OpenCL specific - I seem to be incapable of writing a correct solving algorithm for distance constraints.
I'm not sure where the issue is - maybe I'm generating the constraints incorrectly, maybe it's the solving.
That's how I'm generating the constraints:
[source lang="cpp"]// create constraints from original positions__kernel void buildConstraints(__global const float4* in, __global DistanceConstraint* out, const int verticesPerHair){ int id = get_global_id(0); if((id % verticesPerHair) != 0) { out[id].id1 = id-1; out[id].id2 = id; out[id].d = length(in[id-1]-in[id]); } else { out[id].id1 = id; out[id].id2 = id; out[id].d = 0; }}[/source]

(id % verticesPerHair) != 0 means that the hair is a root. All but the root is constrained to the preceding vertex in the hair. The root is constrained to itself so it won't move

Prior to constraining the vertices the verlet integration is computed on every vertex except the root:

[source lang="cpp"]// verletif(idl != 0){ float4 difference = localpos[idl] - oldpos[idx]; oldpos[idx] = localpos[idl]; localpos[idl] += (1-friction)*difference + force*pdt*pdt;}[/source]

Then to satisfy the constraints I run fixed number of iterations of this code:

[source lang="cpp"]barrier(CLK_LOCAL_MEM_FENCE);DistanceConstraint constraint = constraints[idx];for(int i = 0; i < dcIter; i++){ if((idl%2) == 1) { distanceConstraint(localpos, constraint.id1 % verticesPerHair, constraint.id2 % verticesPerHair, constraint.d); } barrier(CLK_LOCAL_MEM_FENCE); if((idl%2) == 0) { distanceConstraint(localpos, constraint.id1 % verticesPerHair, constraint.id2 % verticesPerHair, constraint.d); } barrier(CLK_LOCAL_MEM_FENCE);}[/source]

constraint is what was generated for this vertex in buildConstraints(). The barrier() function is there because the algorithm runs in parallel, so it solves the constraint for odd vertices then waits for all threads to complete and then solves the constraints for even vertices.

the function distanceConstraint() looks like this:
[source lang="cpp"]// satisfy distance constraintsvoid distanceConstraint(__local HairVertex* pos, int idx1, int idx2, const float conDist){ // vector from vertex 1 to vertex 2 float4 difference = pos[idx2].position-pos[idx1].position; // By what amount move which vertex float2 cFactors = constraintFactors(idx1, idx2); // length of the vector between the vertices float len = length(difference); // distance the vertices must move towards each other float distance = len-conDist; // no corruption through div by zero len = max(len,1e-7f); // move pos[idx1].position += cFactors.x*distance/len*difference; pos[idx2].position += cFactors.y*distance/len*difference;}[/source]

constraintFactors() returns factors that determine how to handle vertices depending on whether or not they are roots:
[source lang="cpp"]// for given indices id1 and id2 return constraint factorsfloat2 constraintFactors(int id1, int id2){ if(id1 != 0) { // first vertex is not a root if(id2 != 0) { // both vertices not a root return (float2)(0.5f, -0.5f); } else { // first vertex is not root, second is root return (float2)(0.0f, 0.0f); } } else { // first vertex is root if(id2 != 0) { // first vertex is root, second is not return (float2)(0.0f, -1.0f); } else { // both vertices are roots (shouldn't happen) return (float2)(0.0f, 0.0f); } }}[/source]

I have looked through NVIDIAs DirectX implementation of the simulation (NVIDIAs SDK 11) and the only deviation from the constraint code was that when the first vertex is not root and the second is, the factors were (1.0f,0.0f). I am however not certain how the constraints were generated, so I predict that my mistake is somewhere there.

The actual problem is that after a while the simulation goes full spaghetti mode... To be a bit more precise - the control vertices seem to move from the root towards the tip and the tip seems to move away in the direction the force is applied... Here's what it looks like in point mode:

http://sw-ores.de/pr...s/spaghetti.mkv

I have no clue what I'm doing wrong

Edited by AzraelilMeraz, 04 August 2012 - 08:52 AM.

### #2AzraelilMeraz  Members

Posted 02 August 2012 - 12:36 PM

I thought that maybe it's because of the parallel execution of the Distance constraints in the loop, so created a serialized version, where only one thread solves the constraints:
[source lang="cpp"] barrier(CLK_LOCAL_MEM_FENCE); if(idl == 0) { for(int i = 0; i < dcIter; i++) { for(int i = 1; i < verticesPerHair; i++) { DistanceConstraint constraint = localconst[i]; distanceConstraint(localpos, constraint.id1 % verticesPerHair, constraint.id2 % verticesPerHair, constraint.d); } } } barrier(CLK_LOCAL_MEM_FENCE);[/source]

This yielded the same results however :/. I've looked into several resources on the topic:

http://www.connellyb...let_Integration
http://dl.acm.org/ci...1837101.1837102
http://en.wikipedia....let_integration
http://www.gamasutra...acobson_pfv.htm

But I see nothing on the issue at hand. I'm using 20 iterations for the constraints, but the same happens if I do 200 or 2000 - just at a slower rate!

### #3AzraelilMeraz  Members

Posted 04 August 2012 - 08:49 AM

Nevermind... figured it out - there is nothing wrong with the algorithm - I just accidentally ran the constraint generation kernel every iteration...

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