gradients.C: This macro requires OpenGL:

//Author: Timur Pocheptsov, 19/03/2014
//This macro requires OpenGL:
//either call gStyle->SetCanvasPreferGL(kTRUE) before
//a canvas is created, or set OpenGL.CanvasPreferGL to 1 in
//$ROOTSYS/etc/system.rootrc.


//Features:
//1. Radial and linear gradients
//2. Transparent/semitransparent colours.


//Includes for ACLiC:
#include "TColorGradient.h"
#include "TCanvas.h"
#include "TStyle.h"
#include "TError.h"
#include "TText.h"
#include "TPie.h"

//Aux. functions:
#include "customcolors.h"

void gradients()
{
   //Find free colour indices in the ROOT's palette for:
   //1. A radial gradient for TPie;
   //2. A linear gradient for TCanvas
   //3. A fully transparent fill color for a nested pad.

   //I have to hardcode them, but can also look for free indices in the ROOT's color palette.
   const Color_t radialFill = FindFreeCustomColorIndex(1000);//Start a lookup from 1000.
   if (radialFill == -1) {
      ::Error("gradients", "failed to allocate custom color");
      return;
   }
   
   const Color_t linearFill = FindFreeCustomColorIndex(radialFill + 1);
   if (linearFill == -1) {
      ::Error("gradients", "failed to allocate custom color");
      return;
   }
   
   const Color_t transparentFill = FindFreeCustomColorIndex(linearFill + 1);
   if (transparentFill == -1) {
      ::Error("gradients", "failed to allocate custom color");
      return;
   }
   
   gStyle->SetCanvasPreferGL(kTRUE);

   TCanvas * const c = new TCanvas("cpie","Gradient colours demo", 700, 700);
   //Before we allocated any new colour or created any object:
   if (!c->UseGL()) {
      ::Error("gradients", "This macro requires OpenGL");
      delete c;
      return;
   }

   c->cd();

   //Linear gradient is defined by: 1) colors (to interpolate between them),
   //2) coordinates for these colors along the gradient axis [0., 1.] (must be sorted!).
   //3) Start and end points for a gradient, you specify them in some NDC rect ([0,0 - 1,1]),
   //and this rect is either: bounding rect of your polygon/object to fill
   //(gradient->SetCoordinateMode(TColorGradient::kObjectBoundingMode))
   //or bounding rect of a pad (gradient->SetCoordinateMode(TColorGradient::kPadMode)).
   //kObjectBoundingMode is the default one.


   //Colour positions in the gradient's palette (here I place colors at the
   //ends of 0-1):
   const Double_t locations[] = {0., 1.};
   //Linear gradient fill (with an axis angle == 45):
   const Double_t rgbaData1[] = {0.2, 0.2, 0.2, 1.,/*gray*/
                                 0.8, 1., 0.9, 1.  /*pale green*/};
   TLinearGradient * const gradientFill1 = new TLinearGradient(linearFill, 2, locations, rgbaData1);
   //45 degrees:
   gradientFill1->SetStartEnd(TColorGradient::Point(0., 0.), TColorGradient::Point(1., 1.));
   //Set as a background color in the canvas:
   c->SetFillColor(linearFill);

   //Draw a text in the canvas (the object above the text will be
   //semi-transparent):
   TText * const t = new TText(0.05, 0.7, "Can you see the text?");
   t->Draw();
   
   //We create a nested pad on top to render a TPie in,
   //this way we still have a text (below) + TPie with
   //a fancy colour on top.
   TPad * const pad = new TPad("p", "p", 0., 0., 1., 1.);
   
   //TPad itself is fully transparent:
   new TColor(transparentFill, 1., 1., 1., "transparent_fill_color", 0.);
   pad->SetFillColor(transparentFill);
   //Add our pad into the canvas:
   pad->Draw();
   pad->cd();


   //Radial gradient fill for a TPie object:
   const Double_t rgbaData2[] = {/*opaque orange at the start:*/1., 0.8, 0., 1.,
                                 /*transparent red at the end:*/1., 0.2, 0., 0.6};

   TRadialGradient * const gradientFill2 = new TRadialGradient(radialFill, 2,
                                                               locations, rgbaData2);
   //Parameters for a gradient fill:
   //the gradient is 'pad-related' - we calculate everything in a pad's
   //space and consider it as a NDC (so pad's rect is (0,0), (1,1)).
   gradientFill2->SetCoordinateMode(TColorGradient::kPadMode);
   //Simple radial gradient - center and radius:
   gradientFill2->SetRadialGradient(TColorGradient::Point(0.5, 0.5), 0.4);

   const UInt_t nSlices = 5;
   //Values for a TPie (non-const, that's how TPie's ctor is declared):
   Double_t values[nSlices] = {0.8, 1.2, 1.2, 0.8, 1.};
   Int_t colors[nSlices] = {radialFill, radialFill, radialFill,
                            radialFill, radialFill};

   TPie * const pie = new TPie("pie", "TPie:", nSlices, values, colors);
   //One slice is slightly shifted:
   pie->SetEntryRadiusOffset(2, 0.05);
   //Move labels to the center (to fit the pad's space):
   pie->SetLabelsOffset(-0.08);
   //
   pie->SetRadius(0.4);
   pie->Draw("rsc");
}