perceptualcolormap.C

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/// \file
/// \ingroup tutorial_graphics
/// \notebook
/// \preview A “Perceptual” colormap explicitly identifies a fixed value in the data.
///
/// On geographical plot this fixed point can, for instance, the "sea level". A perceptual
/// colormap provides a 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` takes 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 drawing
   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;
   for (Int_t i = 0; i < 1000000; i++) {
      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.);
}