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Reference Guide
perceptualcolormap.C
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1/// \file
2/// \ingroup tutorial_graphics
3/// \notebook
4/// A “Perceptual” colormap explicitly identifies a fixed value in the data
5///
6/// On geographical plot this fixed point can, for instance, the "sea level". A perceptual
7/// colormap provides a monotonic luminance variations above and below this fixed value.
8/// Unlike the rainbow colormap, this colormap provides a faithful representation of the
9/// structures in the data.
10///
11/// This macro demonstrates how to produce the perceptual colormap shown on the figure 2
12/// in [this article](https://root.cern/blog/rainbow-color-map/).
13///
14/// The function `Perceptual_Colormap` takes two parameters as input:
15/// 1. `h`, the `TH2D` to be drawn
16/// 2. `val_cut`, the Z value defining the "sea level"
17///
18/// Having these parameters this function defines two color maps: one above `val_cut` and one
19/// below.
20///
21/// \macro_image
22/// \macro_code
23///
24/// \author Olivier Couet
25
26void Perceptual_Colormap(TH2D *h, Double_t val_cut) {
27 Double_t max = h->GetMaximum(); // Histogram's maximum
28 Double_t min = h->GetMinimum(); // Histogram's minimum
29 Double_t per_cut = (val_cut-min)/(max-min); // normalized value of val_cut
30 Double_t eps = (max-min)*0.00001; // epsilon
31
32 // Definition of the two palettes below and above val_cut
33 const Int_t Number = 4;
34 Double_t Red[Number] = { 0.11, 0.19 , 0.30, 0.89};
35 Double_t Green[Number] = { 0.03, 0.304, 0.60, 0.91};
36 Double_t Blue[Number] = { 0.18, 0.827, 0.50, 0.70};
37 Double_t Stops[Number] = { 0., per_cut, per_cut+eps, 1. };
38
39 Int_t nb= 256;
40 h->SetContour(nb);
41
42 TColor::CreateGradientColorTable(Number,Stops,Red,Green,Blue,nb);
43
44 // Histogram drawing
45 h->Draw("colz");
46}
47
48void perceptualcolormap() {
49 TH2D *h = new TH2D("h","Perceptual Colormap",200,-4,4,200,-4,4);
50 h->SetStats(0);
51
52 Double_t a,b;
53 for (Int_t i=0;i<1000000;i++) {
54 gRandom->Rannor(a,b);
55 h->Fill(a-1.5,b-1.5,0.1);
56 h->Fill(a+2.,b-3.,0.07);
57 h->Fill(a-3.,b+3.,0.05);
58 gRandom->Rannor(a,b);
59 h->Fill(a+1.5,b+1.5,-0.08);
60 }
61 Perceptual_Colormap(h, 0.);
62}
#define b(i)
Definition: RSha256.hxx:100
#define h(i)
Definition: RSha256.hxx:106
int Int_t
Definition: RtypesCore.h:45
double Double_t
Definition: RtypesCore.h:59
R__EXTERN TRandom * gRandom
Definition: TRandom.h:62
static Int_t CreateGradientColorTable(UInt_t Number, Double_t *Stops, Double_t *Red, Double_t *Green, Double_t *Blue, UInt_t NColors, Float_t alpha=1.)
Static function creating a color table with several connected linear gradients.
Definition: TColor.cxx:2234
2-D histogram with a double per channel (see TH1 documentation)}
Definition: TH2.h:292
virtual void Rannor(Float_t &a, Float_t &b)
Return 2 numbers distributed following a gaussian with mean=0 and sigma=1.
Definition: TRandom.cxx:500
auto * a
Definition: textangle.C:12