source: GTP/trunk/Lib/Vis/Preprocessing/src/SamplingStrategy.cpp @ 1968

#include "SamplingStrategy.h"
#include "Ray.h"
#include "Intersectable.h"
#include "Preprocessor.h"
#include "ViewCellsManager.h"
#include "AxisAlignedBox3.h"
#include "RssTree.h"

namespace GtpVisibilityPreprocessor {


//HaltonSequence SamplingStrategy::sHalton;

HaltonSequence ObjectBasedDistribution::sHalton;
HaltonSequence MixtureDistribution::sHalton;
HaltonSequence GlobalLinesDistribution::sHalton;
HaltonSequence SpatialBoxBasedDistribution::sHalton;
HaltonSequence ObjectDirectionBasedDistribution::sHalton;
HaltonSequence DirectionBasedDistribution::sHalton;
HaltonSequence HwGlobalLinesDistribution::sHalton;


SamplingStrategy::SamplingStrategy(Preprocessor &preprocessor): 
mPreprocessor(preprocessor), 
mRatio(1.0f),  
mTotalRays(0), 
mTotalContribution(0.0f)
{  
}


SamplingStrategy::~SamplingStrategy() 
{
}

int
SamplingStrategy::GenerateSamples(const int number,
                                                                  SimpleRayContainer &rays)
{
        SimpleRay ray;
        int samples = 0;
        int i = 0;
        const int maxTries = 20;
        // tmp changed matt. Q: should one rejected sample 
        // terminate the whole method?
        if (0)
        {
                for (; i < number; i++) {
                        if (!GenerateSample(ray))
                                return i;
                        rays.push_back(ray);
                }
        }
        else
        {
                for (; i < number; i++) 
                {
                        int j = 0;
                        bool sampleGenerated = false;

                        for (j = 0; !sampleGenerated && (j < maxTries); ++ j)
                        {
                                sampleGenerated = GenerateSample(ray);

                                if (sampleGenerated)
                                {               
                                        ++ samples;
                                        rays.push_back(ray);
                                }
                        }
                }
        }
        
        return samples;
}


/*********************************************************************/
/*            Individual sampling strategies implementation          */
/*********************************************************************/


bool ObjectBasedDistribution::GenerateSample(SimpleRay &ray)
{
  Vector3 origin, direction; 
  
  float r[5];
  sHalton.GetNext(5, r);
  
  mPreprocessor.mViewCellsManager->GetViewPoint(origin,
                                                                                                Vector3(r[2],r[3],r[4]));
  

  Vector3 point, normal;
  
  r[0] *= (float)mPreprocessor.mObjects.size() - 1;
  const int i = (int)r[0];
  
  Intersectable *object = mPreprocessor.mObjects[i];
  
  // take the remainder as a parameter over objects surface
  r[0] -= (float)i;
  
  object->GetRandomSurfacePoint(r[0], r[1], point, normal);
  
  direction = point - origin;
  
  const float c = Magnitude(direction);
  
  if (c <= Limits::Small) 
        return false;
  
  // $$ jb the pdf is yet not correct for all sampling methods!
  const float pdf = 1.0f;
  
  direction *= 1.0f / c;
  ray = SimpleRay(origin, direction, OBJECT_BASED_DISTRIBUTION, pdf);
  
  return true;
}


bool
ObjectDirectionBasedDistribution::GenerateSample(SimpleRay &ray)
{       
  Vector3 origin, direction; 


  float r[4];
  sHalton.GetNext(4, r);

  r[0] *= (float)mPreprocessor.mObjects.size() - 1;
  const int i = (int)r[0];
  
  Intersectable *object = mPreprocessor.mObjects[i];
  
  // take the remainder as a parameter over objects surface
  r[0] -= (float)i;

  Vector3 normal;

  object->GetRandomSurfacePoint(r[0], r[1], origin, normal);
  
  direction = Normalize(CosineRandomVector(r[2], r[3], normal));
  
  origin += 1e-2f*direction;
  
  // $$ jb the pdf is yet not correct for all sampling methods!
  const float pdf = 1.0f;
  
  ray = SimpleRay(origin, direction, OBJECT_DIRECTION_BASED_DISTRIBUTION, pdf);
  
  return true;
}


bool DirectionBasedDistribution::GenerateSample(SimpleRay &ray)
{       

  float r[5];
  sHalton.GetNext(5, r);

  Vector3 origin, direction; 
  mPreprocessor.mViewCellsManager->GetViewPoint(origin,
                                                                                                Vector3(r[2],r[3],r[4])
                                                                                                );
  
  direction = UniformRandomVector(r[0], r[1]);
  const float c = Magnitude(direction);
  
  if (c <= Limits::Small) 
        return false;
  
  const float pdf = 1.0f;
  
  direction *= 1.0f / c;
  ray = SimpleRay(origin, direction, DIRECTION_BASED_DISTRIBUTION, pdf);
  
  return true;
}


bool DirectionBoxBasedDistribution::GenerateSample(SimpleRay &ray)
{
        Vector3 origin, direction; 
        mPreprocessor.mViewCellsManager->GetViewPoint(origin);

        const float alpha = RandomValue(0.0f, 2.0f * (float)M_PI);
        const float beta = RandomValue((float)-M_PI * 0.5f, (float)M_PI * 0.5f);
        
        direction = VssRay::GetDirection(alpha, beta);
        
        const float c = Magnitude(direction);

        if (c <= Limits::Small) 
                return false;

        const float pdf = 1.0f;

        direction *= 1.0f / c;
        ray = SimpleRay(origin, direction, DIRECTION_BOX_BASED_DISTRIBUTION, pdf);

        return true;
}


bool SpatialBoxBasedDistribution::GenerateSample(SimpleRay &ray)
{
  Vector3 origin, direction; 

  float r[6];
  sHalton.GetNext(6, r);
  mPreprocessor.mViewCellsManager->GetViewPoint(origin, Vector3(r[0],r[1],r[2]));
  
  direction = mPreprocessor.mKdTree->GetBox().GetRandomPoint(Vector3(r[3],
                                                                                                                                         r[4],
                                                                                                                                         r[5])
                                                                                                                                         ) - origin;
  //cout << "z";
  const float c = Magnitude(direction);
  
  if (c <= Limits::Small) 
        return false;
  
  const float pdf = 1.0f;
  
  direction *= 1.0f / c;
  ray = SimpleRay(origin, direction, SPATIAL_BOX_BASED_DISTRIBUTION, pdf);
  
  return true;
}


bool ReverseObjectBasedDistribution::GenerateSample(SimpleRay &ray)
{
        Vector3 origin, direction; 

        mPreprocessor.mViewCellsManager->GetViewPoint(origin);

        Vector3 point;
        Vector3 normal;
        //cout << "y";
        const int i = (int)RandomValue(0, (float)mPreprocessor.mObjects.size() - 0.5f);

        Intersectable *object = mPreprocessor.mObjects[i];

        object->GetRandomSurfacePoint(point, normal);
        
        direction = origin - point;
        
        // $$ jb the pdf is yet not correct for all sampling methods!
        const float c = Magnitude(direction);
        
        if ((c <= Limits::Small) || (DotProd(direction, normal) < 0))
        {
                return false;
        }

        // $$ jb the pdf is yet not correct for all sampling methods!
        const float pdf = 1.0f;
        //cout << "p: " << point << " ";
        direction *= 1.0f / c;
        // a little offset
        point += direction * 0.001f;

        ray = SimpleRay(point, direction, REVERSE_OBJECT_BASED_DISTRIBUTION, pdf);
        
        return true;
}


bool ViewCellBorderBasedDistribution::GenerateSample(SimpleRay &ray)
{
        Vector3 origin, direction; 

        ViewCellContainer &viewCells = mPreprocessor.mViewCellsManager->GetViewCells();

        Vector3 point;
        Vector3 normal, normal2;
        
        const int vcIdx = (int)RandomValue(0, (float)viewCells.size() - 0.5f);
        const int objIdx = (int)RandomValue(0, (float)mPreprocessor.mObjects.size() - 0.5f);

        Intersectable *object = mPreprocessor.mObjects[objIdx];
        ViewCell *viewCell = viewCells[vcIdx];

        //cout << "vc: " << vcIdx << endl;
        //cout << "obj: " << objIdx << endl;

        object->GetRandomSurfacePoint(point, normal);
        viewCell->GetRandomEdgePoint(origin, normal2);

        direction = point - origin;

        // $$ jb the pdf is yet not correct for all sampling methods!
        const float c = Magnitude(direction);

        if ((c <= Limits::Small) /*|| (DotProd(direction, normal) < 0)*/)
        {
                return false;
        }

        // $$ jb the pdf is yet not correct for all sampling methods!
        const float pdf = 1.0f;
        //cout << "p: " << point << " ";
        direction *= 1.0f / c;
        ray = SimpleRay(origin, direction, VIEWCELL_BORDER_BASED_DISTRIBUTION, pdf);

        //cout << "ray: " << ray.mOrigin << " " << ray.mDirection << endl;

        return true;
}


#if 0
bool ObjectsInteriorDistribution::GenerateSample(SimpleRay &ray)
{
        Vector3 origin, direction; 

        // get random object
        const int i = RandomValue(0, mPreprocessor.mObjects.size() - 1);

        const Intersectable *obj = mPreprocessor.mObjects[i];

        // note: if we load the polygons as meshes, 
        // asymtotically every second sample is lost!
        origin = obj->GetBox().GetRandomPoint();

        // uniformly distributed direction
        direction = UniformRandomVector();

        const float c = Magnitude(direction);

        if (c <= Limits::Small) 
                return false;

        const float pdf = 1.0f;

        direction *= 1.0f / c;
        ray = SimpleRay(origin, direction, pdf);

        return true;
}

#endif


bool ReverseViewSpaceBorderBasedDistribution::GenerateSample(SimpleRay &ray)
{
        Vector3 origin, direction; 

        origin = mPreprocessor.mViewCellsManager->GetViewSpaceBox().GetRandomSurfacePoint();

        Vector3 point;
        Vector3 normal;
        //cout << "y";
        const int i = (int)RandomValue(0, (float)mPreprocessor.mObjects.size() - 0.5f);

        Intersectable *object = mPreprocessor.mObjects[i];

        object->GetRandomSurfacePoint(point, normal);
        
        direction = origin - point;
        
        // $$ jb the pdf is yet not correct for all sampling methods!
        const float c = Magnitude(direction);
        
        if ((c <= Limits::Small) || (DotProd(direction, normal) < 0))
        {
                return false;
        }

        // $$ jb the pdf is yet not correct for all sampling methods!
        const float pdf = 1.0f;
        //cout << "p: " << point << " ";
        direction *= 1.0f / c;
        // a little offset
        point += direction * 0.001f;

        ray = SimpleRay(point, direction, REVERSE_VIEWSPACE_BORDER_BASED_DISTRIBUTION, pdf);
        
        return true;
}


bool ViewSpaceBorderBasedDistribution::GenerateSample(SimpleRay &ray)
{
        Vector3 origin, direction; 

        origin = mPreprocessor.mViewCellsManager->GetViewSpaceBox().GetRandomSurfacePoint();

        Vector3 point;
        Vector3 normal;
        //cout << "w";
        const int i = (int)RandomValue(0, (float)mPreprocessor.mObjects.size() - 0.5f);

        Intersectable *object = mPreprocessor.mObjects[i];

        object->GetRandomSurfacePoint(point, normal);
        direction = point - origin;

        // $$ jb the pdf is yet not correct for all sampling methods!
        const float c = Magnitude(direction);

        if (c <= Limits::Small) 
                return false;

        // $$ jb the pdf is yet not correct for all sampling methods!
        const float pdf = 1.0f;
        
        direction *= 1.0f / c;

        // a little offset
        origin += direction * 0.001f;

        ray = SimpleRay(origin, direction, VIEWSPACE_BORDER_BASED_DISTRIBUTION, pdf);

        return true;
}


bool
GlobalLinesDistribution::GenerateSample(SimpleRay &ray) 
{
  Vector3 origin, termination, direction; 

  float radius = 0.5f*Magnitude(mPreprocessor.mViewCellsManager->GetViewSpaceBox().Size());
  Vector3 center = mPreprocessor.mViewCellsManager->GetViewSpaceBox().Center();

  const int tries = 1000;
  int i;
  for (i=0; i < tries; i++) {
        float r[4];
        sHalton.GetNext(4, r);
        
        origin = center + (radius*UniformRandomVector(r[0], r[1]));
        termination = center + (radius*UniformRandomVector(r[2], r[3]));
        
        direction = termination - origin;
        
        
        // $$ jb the pdf is yet not correct for all sampling methods!
        const float c = Magnitude(direction);
        if (c <= Limits::Small) 
          return false;
        
        direction *= 1.0f / c;

        // check if the ray intersects the view space box
        static Ray ray;
        ray.Init(origin, direction, Ray::LOCAL_RAY);    
        
        float tmin, tmax;
        if (mPreprocessor.mViewCellsManager->
                GetViewSpaceBox().ComputeMinMaxT(ray, &tmin, &tmax) && (tmin < tmax))
          break;
  }
  
  if (i!=tries) {
        // $$ jb the pdf is yet not correct for all sampling methods!
        const float pdf = 1.0f;
        
        
        ray = SimpleRay(origin, direction, GLOBAL_LINES_DISTRIBUTION, pdf);
        ray.mType = Ray::GLOBAL_RAY;
        return true;
  }
  
  return false;
}


  // has to called before first usage
void
MixtureDistribution::Init()
{
  for (int i=0; i < mDistributions.size(); i++) {
        // small non-zero value
        mDistributions[i]->mRays = 1;
        mDistributions[i]->mGeneratedRays = 1;
        // unit contribution per ray
        if (1 || mDistributions[i]->mType != RSS_BASED_DISTRIBUTION)
          mDistributions[i]->mContribution = 1.0f;
        else
          mDistributions[i]->mContribution = 0.0f;
  }
  UpdateRatios();
}

void
MixtureDistribution::Reset()
{
  for (int i=0; i < mDistributions.size(); i++) {
        // small non-zero value
        mDistributions[i]->mTotalRays = 0;
        // unit contribution per ray
        mDistributions[i]->mTotalContribution = 0.0f;
  }
  UpdateRatios();
}

// Generate a new sample according to a mixture distribution
bool
MixtureDistribution::GenerateSample(SimpleRay &ray)
{
  float r;
  sHalton.GetNext(1, &r);
  int i;
  // pickup a distribution
  for (i=0; i < mDistributions.size()-1; i++)
        if (r < mDistributions[i]->mRatio)
          break;

  bool result = mDistributions[i]->GenerateSample(ray);

  if (result)
        mDistributions[i]->mGeneratedRays++;
  
  return result;
}

  // add contributions of the sample to the strategies
void
MixtureDistribution::ComputeContributions(VssRayContainer &vssRays)
{
  int i;
  
  VssRayContainer::iterator it = vssRays.begin();

  for (i=0; i < mDistributions.size(); i++) {
        mDistributions[i]->mContribution = 0;
        mDistributions[i]->mRays = 0;
  }

  for(; it != vssRays.end(); ++it) {
        VssRay *ray = *it;
        for (i=0; i < mDistributions.size()-1; i++) {
          if (mDistributions[i]->mType == ray->mDistribution)
                break;
        }
 
        float contribution =
          mPreprocessor.mViewCellsManager->ComputeSampleContribution(*ray,
                                                                                                                                 true,
                                                                                                                                 false);

        mDistributions[i]->mContribution += contribution;
        mDistributions[i]->mRays ++;
        
        mDistributions[i]->mTotalContribution += contribution;
        mDistributions[i]->mTotalRays ++;
  }

  
  UpdateRatios();

}

void
MixtureDistribution::UpdateDistributions(VssRayContainer &vssRays)
{
  // now update the distributions with all the rays
  for (int i=0; i < mDistributions.size(); i++) {
        mDistributions[i]->Update(vssRays);
  }
}

#define RAY_CAST_TIME 0.9f
#define VIEWCELL_CAST_TIME 0.1f

void
MixtureDistribution::UpdateRatios()
{
  // now compute importance (ratio) of all distributions
  float sum = 0.0f;
  int i;
  for (i=0; i < mDistributions.size(); i++) {
        cout<<i<<": c="<<mDistributions[i]->mContribution<<
          " rays="<<mDistributions[i]->mRays<<endl;
        float importance = 0.0f;
        if (mDistributions[i]->mRays != 0) {
          //importance = pow(mDistributions[i]->mContribution/mDistributions[i]->mRays, 2);
          importance = mDistributions[i]->mContribution/
                (RAY_CAST_TIME*mDistributions[i]->mGeneratedRays +
                 VIEWCELL_CAST_TIME*mDistributions[i]->mRays);
        }
        mDistributions[i]->mRatio = importance;
        sum += importance;
  }

  
  
  if (sum == 0.0f)
        sum = Limits::Small;

  const float minratio = 0.01f;
  float threshold = minratio*sum;

  // recaluate the sum
  sum = 0.0f;
  for (i=0; i < mDistributions.size(); i++) {
        if (mDistributions[i]->mRatio < threshold) {
          mDistributions[i]->mRatio = threshold;
        }
        sum += mDistributions[i]->mRatio;
  }
  
  mDistributions[0]->mRatio /= sum;
  
  for (i=1; i < mDistributions.size(); i++) {
        float r = mDistributions[i]->mRatio / sum;
        mDistributions[i]->mRatio = mDistributions[i-1]->mRatio + r;
  }
  
  
  cout<<"ratios: ";
  float last = 0.0f;
  for (i=0; i < mDistributions.size(); i++) {
        cout<<mDistributions[i]->mRatio-last<<" ";
        last = mDistributions[i]->mRatio;
  }
  cout<<endl;
}



bool
MixtureDistribution::Construct(char *str)
{
  char *curr = str;

  while (1) {
        char *e = strchr(curr,'+');
        if (e!=NULL) {
          *e=0;
        }
        
        if (strcmp(curr, "rss")==0) {
          mDistributions.push_back(new RssBasedDistribution(mPreprocessor));
        } else
          if (strcmp(curr, "object")==0) {
                mDistributions.push_back(new ObjectBasedDistribution(mPreprocessor));
          } else
                if (strcmp(curr, "spatial")==0) {
                  mDistributions.push_back(new SpatialBoxBasedDistribution(mPreprocessor));
                } else
                  if (strcmp(curr, "global")==0) {
                        mDistributions.push_back(new GlobalLinesDistribution(mPreprocessor));
                  } else
                        if (strcmp(curr, "direction")==0) {
                          mDistributions.push_back(new DirectionBasedDistribution(mPreprocessor));
                        } else
                          if (strcmp(curr, "object_direction")==0) {
                                mDistributions.push_back(new ObjectDirectionBasedDistribution(mPreprocessor));
                          } else
                                if (strcmp(curr, "reverse_object")==0) {
                                  mDistributions.push_back(new ReverseObjectBasedDistribution(mPreprocessor));
                                } else
                                  if (strcmp(curr, "reverse_viewspace_border")==0) {
                                        mDistributions.push_back(new ReverseViewSpaceBorderBasedDistribution(mPreprocessor));
                                  } else
                                        if (strcmp(curr, "mutation")==0) {
                                          mDistributions.push_back(new MutationBasedDistribution(mPreprocessor));
                                        } 
        
        
        if (e==NULL)
          break;
        curr = e+1;
  }

  Init();
  return true;
}

int
MixtureDistribution::GenerateSamples(const int number,
                                                                         SimpleRayContainer &rays)
{
  for (int i=0; i < mDistributions.size(); i++) 
        mDistributions[i]->mGeneratedRays = 0;

  return SamplingStrategy::GenerateSamples(number, rays);
}

void
MutationBasedDistribution::Update(VssRayContainer &vssRays)
{
  //  for (int i=0; i < mRays.size(); i++)
  //    cout<<mRays[i].mSamples<<" ";
  //  cout<<endl;
  cerr<<"Muattion update..."<<endl;
  cerr<<"rays = "<<mRays.size()<<endl;
  if (mRays.size())
        cerr<<"Oversampling factor = "<<mRays[0].mSamples<<endl;
  for (int i=0; i < vssRays.size(); i++) {
        if (vssRays[i]->mPvsContribution) {
          if (mRays.size() < mMaxRays) {
                VssRay *newRay = new VssRay(*vssRays[i]);
                // add this ray
                newRay->Ref();
                mRays.push_back(RayEntry(newRay));
          } else {
                // unref the old ray
                *mRays[mBufferStart].mRay = *vssRays[i];
                mRays[mBufferStart].mSamples = 0;
                //              mRays[mBufferStart] = RayEntry(newRay);
                mBufferStart++;
                if (mBufferStart >= mMaxRays)
                  mBufferStart = 0;
          }
        }
  }
  cerr<<"Mutation update done."<<endl;

}

bool
MutationBasedDistribution::GenerateSample(SimpleRay &sray)
{
  float r[5];

  if (mRays.size() == 0) {
        // use direction based distribution
        Vector3 origin, direction;
        static HaltonSequence halton;
        
        halton.GetNext(5, r);
        mPreprocessor.mViewCellsManager->GetViewPoint(origin,
                                                                                                  Vector3(r[0], r[1], r[2]));
        

        direction = UniformRandomVector(r[3], r[4]);
        
        const float pdf = 1.0f;
        sray = SimpleRay(origin, direction, MUTATION_BASED_DISTRIBUTION, pdf);

        return true;
  } 
  
  //  int index = (int) (r[0]*mRays.size());
  //  if (index >= mRays.size()) {
  //    cerr<<"mutation: index out of bounds\n";
  //    exit(1);
  //  }

  // get tail of the buffer
  int index = (mLastIndex+1)%mRays.size();
  if (mRays[index].mSamples > mRays[mLastIndex].mSamples) {
        // search back for index where this is valid
        index = (mLastIndex - 1 + mRays.size())%mRays.size();
        for (int i=0; i < mRays.size(); i++) {
          if (mRays[index].mSamples > mRays[mLastIndex].mSamples)
                break;
          index = (index - 1 + mRays.size())%mRays.size();
        }
        // go one step back
        index = (index+1)%mRays.size();
  }
  
  VssRay *ray = mRays[index].mRay;
  mRays[index].mSamples++;
  mLastIndex = index;

  mRays[index].mHalton.GetNext(4, r);
  
  Vector3 v;
  // mutate the origin
  Vector3 d = ray->GetDir();
  if (d.DrivingAxis() == 0)
        v = Vector3(0, r[0]-0.5f, r[1]-0.5f);
  else
        if (d.DrivingAxis() == 1)
          v = Vector3(r[0]-0.5f, 0, r[1]-0.5f);
        else
          v = Vector3(r[0]-0.5f, r[1]-0.5f, 0);

  v=v*mOriginMutationSize;

  Vector3 origin = ray->mOrigin + v;
  
  // mutate the termination
  if (d.DrivingAxis() == 0)
        v = Vector3(0, r[2]-0.5f, r[3]-0.5f);
  else
        if (d.DrivingAxis() == 1)
          v = Vector3(r[2]-0.5f, 0, r[3]-0.5f);
        else
          v = Vector3(r[2]-0.5f, r[3]-0.5f, 0);

  AxisAlignedBox3 box = ray->mTerminationObject->GetBox();
  float size = 2.0f*Magnitude(box.Diagonal());
  if (size < Limits::Small)
        return false;
  
//   Vector3 nv;
  
//   if (Magnitude(v) > Limits::Small)
//      nv = Normalize(v);
//   else
//      nv = v;
  
//  v = nv*size + v*size;

  v = v*size;

  //  Vector3 termination = ray->mTermination + v;
  Vector3 termination = box.Center() + v;
  
  Vector3 direction = termination - origin;

  if (Magnitude(direction) < Limits::Small)
        return false;

  // shift the origin a little bit
  origin += direction*0.5f;
  
  direction.Normalize();
  
  // $$ jb the pdf is yet not correct for all sampling methods!
  const float pdf = 1.0f;
  
  sray = SimpleRay(origin, direction, MUTATION_BASED_DISTRIBUTION, pdf);
  return true;
}

MutationBasedDistribution::MutationBasedDistribution(Preprocessor &preprocessor
                                                                                                         ) :
  SamplingStrategy(preprocessor)
{
  mType = MUTATION_BASED_DISTRIBUTION;
  mBufferStart = 0;
  mMaxRays = 500000;
  mRays.reserve(mMaxRays);
  mOriginMutationSize = 10.0f;
  mLastIndex = 0;
  //  mOriginMutationSize = Magnitude(preprocessor.mViewCellsManager->
  //                                                              GetViewSpaceBox().Diagonal())*1e-3;
  
}


bool HwGlobalLinesDistribution::GenerateSample(SimpleRay &ray) 
{
        Vector3 origin, termination, direction; 

        float radius = 0.5f * 
                Magnitude(mPreprocessor.mViewCellsManager->GetViewSpaceBox().Size());

        Vector3 center = mPreprocessor.mViewCellsManager->GetViewSpaceBox().Center();

        const int tries = 1000;
        int i;
        for (i=0; i < tries; i++) 
        {
                float r[2];
                sHalton.GetNext(2, r);

                origin = center + (radius * UniformRandomVector(r[0], r[1]));
                termination = center;
                
                if (0)
                {
                        // add a small offset to provide some more randomness in the sampling
                        Vector3 offset(Random(radius * 1e-3f), 
                                                   Random(radius * 1e-3f), 
                                                   Random(radius * 1e-3f));
                        termination += offset;
                }

                direction = termination - origin;

                // $$ jb the pdf is yet not correct for all sampling methods!
                const float c = Magnitude(direction);

                if (c <= Limits::Small) 
                        return false;

                direction *= 1.0f / c;

                // check if the ray intersects the view space box
                static Ray ray;
                ray.Init(origin, direction, Ray::LOCAL_RAY);    

                float tmin, tmax;
                if (mPreprocessor.mViewCellsManager->
                        GetViewSpaceBox().ComputeMinMaxT(ray, &tmin, &tmax) && (tmin < tmax))
                        break;
        }

        if (i != tries) 
        {
                // $$ jb the pdf is yet not correct for all sampling methods!
                const float pdf = 1.0f;

                ray = SimpleRay(origin, direction, HW_GLOBAL_LINES_DISTRIBUTION, pdf);
                ray.mType = Ray::GLOBAL_RAY;
                return true;
        }

        return false;
}


}