Example showing how to produce a perceptual palette (root-project#6314)

tutorials/graphics/perceptualcolormap.C

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+/// \file
+/// \ingroup tutorial_graphics
+/// \notebook
+/// A “Perceptual” colormap explicitly identifies a fixed value in the data
+////
+/// On geographical plot this fixed point can be seen as the "sea level", and provides
+/// monotonic luminance variations above and below this fixed value. Unlike the rainbow
+/// colormap, this colormap provides  a faithful representation of the structures in the
+/// data.
+///
+/// This macro demonstrates how to produce the perceptual colormap shown on the figure 2
+/// in [this article](https://root.cern/blog/rainbow-color-map/).
+///
+/// The function `Perceptual_Colormap` take two parameters as input:
+///   1. `h`, the `TH2D` to be drawn
+///   2. `val_cut`, the Z value defining the "sea level"
+///
+/// Having these parameters this function defines two color maps: one above `val_cut` and one
+/// below.
+////
+/// \macro_image
+/// \macro_code
+///
+/// \author Olivier Couet
+
+void Perceptual_Colormap(TH2D *h, Double_t val_cut) {
+   Double_t max     = h->GetMaximum();         // Histogram's maximum
+   Double_t min     = h->GetMinimum();         // Histogram's minimum
+   Double_t per_cut = (val_cut-min)/(max-min); // normalized value of val_cut
+   Double_t eps     = (max-min)*0.00001;       // epsilon
+
+   // Definition of the two palettes below and above val_cut
+   const Int_t Number     = 4;
+   Double_t Red[Number]   = { 0.11, 0.19 , 0.30, 0.89};
+   Double_t Green[Number] = { 0.03, 0.304, 0.60, 0.91};
+   Double_t Blue[Number]  = { 0.18, 0.827, 0.50, 0.70};
+   Double_t Stops[Number] = { 0., per_cut, per_cut+eps, 1. };
+
+   Int_t nb= 256;
+   h->SetContour(nb);
+
+   TColor::CreateGradientColorTable(Number,Stops,Red,Green,Blue,nb);
+
+   // Histogram drawaing
+   h->Draw("colz");
+}
+
+void perceptualcolormap() {
+   TH2D *h = new TH2D("h","Perceptual Colormap",200,-4,4,200,-4,4);
+   h->SetStats(0);
+
+   Double_t a,b,c;
+   for (Int_t i=0;i<1000000;i++) {
+      c = gRandom->Rndm();
+      gRandom->Rannor(a,b);
+      h->Fill(a-1.5,b-1.5,0.1);
+      h->Fill(a+2.,b-3.,0.07);
+      h->Fill(a-3.,b+3.,0.05);
+      gRandom->Rannor(a,b);
+      h->Fill(a+1.5,b+1.5,-0.08);
+   }
+   Perceptual_Colormap(h, 0.);
+}