A new tutorial to demonstrate radial gradient fill with OpenGL.

tutorials/gl/radialgradients.C

+//This tutorial demonstrates how to use radial gradients,
+//custom colors, transparency.
+//Requires OpenGL: either set OpenGL.CanvasPreferGL to 1
+//in the $ROOTSYS/etc/system.rootrc,
+//or use gStyle->SetCanvasPreferGL(kTRUE).
+
+
+//Includes for ACLiC:
+#include <cassert>
+#include <cstdlib>
+
+#include "TColorGradient.h"
+#include "TVirtualPad.h"
+#include "TVirtualX.h"
+#include "TEllipse.h"
+#include "TRandom.h"
+#include "TCanvas.h"
+#include "TStyle.h"
+#include "Rtypes.h"
+#include "TError.h"
+
+//Aux. functions.
+#include "customcolorgl.h"
+
+namespace {
+
+//Just some colors (rgba) to build our
+//fancy gradients from.
+const Double_t basicColors[][4] =
+{
+{1., 0.,  0., 1.},
+{1., 0.3, 0., 1.},
+{0., 0.,  1., 1.},
+{1., 1.,  0., 1.},
+{1., 0.,  1., 1.},
+{0., 1.,  1., 1.},
+{0., 1.,  0., 1.},
+{0., 0.5,  0., 1.},
+//transparent colors:
+{1., 0.,  0., 0.5},
+{1., 0.3, 0., 0.5},
+{0., 0.,  1., 0.5},
+{1., 1.,  0., 0.5},
+{1., 0.,  1., 0.5},
+{0., 1.,  1., 0.5},
+{0., 1.,  0., 0.5},
+{0., 0.5,  0., 0.5},
+//and even more transparent:
+{1., 0.,  0., 0.2},
+{1., 0.3, 0., 0.2},
+{0., 0.,  1., 0.2},
+{1., 1.,  0., 0.2},
+{1., 0.,  1., 0.2},
+{0., 1.,  1., 0.2},
+{0., 1.,  0., 0.2},
+{0., 0.5,  0., 0.2}
+};
+
+const unsigned nBasicColors = sizeof basicColors / sizeof basicColors[0];
+
+//______________________________________________________________________
+Color_t CreateRandomGradientFill()
+{
+   const Double_t * const fromRGBA = basicColors[(std::rand() % (nBasicColors / 2))];
+   //With odd number of colors the last one is never selected :)
+   const Double_t * const toRGBA = basicColors[nBasicColors / 2 + (std::rand() % (nBasicColors / 2))];
+
+   const Double_t locations[] = {0., 1.};
+   const Double_t rgbas[] = {fromRGBA[0], fromRGBA[1], fromRGBA[2], fromRGBA[3],
+                             toRGBA[0], toRGBA[1], toRGBA[2], toRGBA[3]};
+
+   Color_t idx[1] = {};
+   if (ROOT::GLTutorials::FindFreeCustomColorIndices(idx) != 1)
+      return -1;
+
+   TRadialGradient * const grad = new TRadialGradient(idx[0], 2, locations, rgbas);
+   //
+   grad->SetRadialGradient(TColorGradient::Point(0.5, 0.5), 0.5);
+
+   return idx[0];
+}
+
+//______________________________________________________________________
+bool add_ellipse(const Double_t xC, const Double_t yC, const Double_t r)
+{
+   assert(gPad != nullptr && "add_ellipse, no pad to add ellipse");
+
+   const Color_t newColor = CreateRandomGradientFill();
+   if (newColor == -1) {
+      ::Error("add_ellipse", "failed to find a new color index for a gradient fill");
+      return false;
+   }
+
+   TEllipse * const newEllipse = new TEllipse(xC, yC, r, r);
+   
+   newEllipse->SetFillColor(newColor);
+   newEllipse->Draw();
+   
+   return true;
+}
+
+}
+
+//______________________________________________________________________
+void radialgradients()
+{
+   gStyle->SetCanvasPreferGL(kTRUE);
+   
+   TCanvas * const cnv = new TCanvas("radial gradients", "radial gradients", 1200, 1200);
+   if (!cnv->UseGL()) {
+      ::Error("radialgradients", "this demo OpenGL");
+      delete cnv;
+      return;
+   }
+
+   for (unsigned i = 0; i < 100; ++i)
+      if (!add_ellipse(gRandom->Rndm(), gRandom->Rndm(), 0.5 * gRandom->Rndm()))
+         break;
+   
+   cnv->Modified();
+   cnv->Update();
+}