Draw("colz") with more than 20 colors?

ROOT
1

Hello ROOT experts,

I’m using the rainbow palette one obtains with gStyle->SetPalette(1). When I draw a 2D histogram with “colz”, I’d like to have a smooth transition between colors, instead of the 20 distinct ones I get now.

I even tried setting my own palette with 500 different colors, following the example in TColor.cxx. If I iterate over the list of colors and dump out their hues, I indeed see that all 500 are distinct. But, when I view/save the image, the colors still get binned into about 20. This is especially apparent if I save the file as a SVG and count the unique colors:

>>> set([y.split('fill="')[1].split('"')[0] for y in open('test.svg').readlines() if 'fill="' in y])
set(['#66ff00', '#ff3300', '#ffff00', '#ccff00', '#ff9900', '#ffcc00', '#0099ff', 'black', '#0066ff', '#00ff33', '#99ff00', '#00ffff', '#0033ff', '#ff6600', '#00ff99', '#33ff00', '#ff0000', 'white', 'none', '#00ff66', '#00ffcc', '#00ff00', '#00ccff'])
>>> len(_)
23

Is there some binning in the color (z) axis that needs to be increased to get what I want?

Test script attached. I’m using ROOT 5.22/00a as installed on lxplus.

Thanks,
Jordan
colors.C (2.08 KB)

2

Here is an example creating a smooth palette:

{
   TCanvas *c  = new TCanvas("c","Contours",600,0,600,600);
   TF2 *f1 = new TF2("f2","0.1+(1-(x-2)*(x-2))*(1-(y-2)*(y-2))",1,3,1,3);

   Int_t Number = 5;
   Double_t Red[Number] = { 0.00, 0.09, 0.18, 0.09, 0.00 };
   Double_t Green[Number] = { 0.01, 0.02, 0.39, 0.68, 0.97 };
   Double_t Blue[Number] = { 0.17, 0.39, 0.62, 0.79, 0.97 };
   Double_t Stops[Number] = { 0.00, 0.34, 0.61, 0.84, 1.00 };


   Int_t nb=80;
   TColor::CreateGradientColorTable(Number,Stops,Red,Green,Blue,nb);
   f2->SetContour(nb);
   f2->Draw("colz");  
}

See also the HowTo here:
root.cern.ch/drupal/content/how- … or-palette

3

Apparently the key line is

f2->SetContour(nb);

as I had tried CreateGradientColorTable() before with the same results as my creating the palette manually.

Looking at TH1::SetContour, it is not immediately apparent that the number of contours also controls the number of colors, but the stated default of 20 agrees with the number of colors I was seeing. Perhaps the documentation could be improved to reflect this? Or perhaps a SetNumberOfColors(int nc)method could be added (it would simply call SetContour(nc), but would improve the “grammar” of the class)?

4

Just wanted to bump this thread so that perhaps the documentation would be improved.

5

I have improved the doc here:
root.cern.ch/root/html/THistPainter.html#HP14
I added a paragraph about SetContour. It will be visible as soon as the online documentation will be regenerated. Thanks for reporting this documentation weakness.