#include "TStyle.h"
#include "TList.h"
#include "TH3.h"
#include "TF1.h"

namespace {

Double_t my_transfer_function(const Double_t *x, const Double_t * /*param*/)
{
   // Bin values in our example range from -2 to 1.
   // Let's make values from -2. to -1.5 more transparent:
   if (*x < -1.5)
      return 0.008;

   if (*x < -0.5)
      return 0.015;

   if (*x < 0.)
      return 0.02;

   if (*x < 0.5)
      return 0.03;

   if (*x < 0.8)
      return 0.04;

   return 0.05;
}

} // namespace

void glvox2()
{
   // Create and fill TH3.
   const UInt_t nX = 30;
   const Double_t xMin = -1., xMax = 1., xStep = (xMax - xMin) / (nX - 1);

   const UInt_t nY = 30;
   const Double_t yMin = -1., yMax = 1., yStep = (yMax - yMin) / (nY - 1);

   const UInt_t nZ = 30;
   const Double_t zMin = -1., zMax = 1., zStep = (zMax - zMin) / (nZ - 1);

   TH3F *hist = new TH3F("glvoxel", "glvoxel", nX, -1., 1., nY, -1., 1., nZ, -1., 1.);

   // Fill the histogram to create a "sphere".
   for (UInt_t i = 0; i < nZ; ++i) {
      const Double_t z = zMin + i * zStep;

      for (UInt_t j = 0; j < nY; ++j) {
         const Double_t y = yMin + j * yStep;

         for (UInt_t k = 0; k < nX; ++k) {
            const Double_t x = xMin + k * xStep;

            const Double_t val = 1. - (x * x + y * y + z * z);
            hist->SetBinContent(k + 1, j + 1, i + 1, val);
         }
      }
   }

   // Now, specify the transfer function.
   TList *lf = hist->GetListOfFunctions();
   if (lf) {
      TF1 *tf = new TF1("TransferFunction", my_transfer_function, -2., 1.);
      lf->Add(tf);
   }

   gStyle->SetCanvasPreferGL(true);

   hist->Draw("glcol");
}