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   "source": [
    "# hist056_TPolyMarker_contour\n",
    "Make a contour plot and get the first contour in a TPolyMarker.\n",
    "This macro generates a color contour plot by selecting entries\n",
    "from an ntuple file.\n",
    "The TGraph object corresponding to the first contour line is\n",
    "accessed and displayed into a separate canvas.\n",
    "\n",
    "\n",
    "\n",
    "\n",
    "**Author:** Rene Brun  \n",
    "<i><small>This notebook tutorial was automatically generated with <a href= \"https://github.com/root-project/root/blob/master/documentation/doxygen/converttonotebook.py\">ROOTBOOK-izer</a> from the macro found in the ROOT repository  on Tuesday, May 19, 2026 at 08:13 PM.</small></i>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "952299e0",
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "TString dir = gROOT->GetTutorialDir();\n",
    "dir.Append(\"/hsimple.C\");\n",
    "dir.ReplaceAll(\"/./\", \"/\");\n",
    "if (!gInterpreter->IsLoaded(dir.Data()))\n",
    "   gInterpreter->LoadMacro(dir.Data());\n",
    "TFile *file = (TFile *)gROOT->ProcessLineFast(\"hsimple(1)\");\n",
    "if (!file)\n",
    "   return;\n",
    "TTree *ntuple = (TTree *)file->Get(\"ntuple\");\n",
    "\n",
    "TCanvas *c1 = new TCanvas(\"c1\", \"Contours\", 10, 10, 800, 600);\n",
    "ntuple->Draw(\"py:px\", \"px*px+py*py < 20\", \"contz,list\");"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "d3ce84fd",
   "metadata": {},
   "source": [
    "we must call Update to force the canvas to be painted.  When\n",
    "painting the contour plot, the list of contours is generated\n",
    "and a reference to it added to the Root list of special objects"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "bb4c968e",
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "c1->Update();\n",
    "\n",
    "TCanvas *c2 = new TCanvas(\"c2\", \"First contour\", 100, 100, 800, 600);\n",
    "\n",
    "TObjArray *contours = (TObjArray *)gROOT->GetListOfSpecials()->FindObject(\"contours\");\n",
    "if (!contours)\n",
    "   return;\n",
    "TList *lcontour1 = (TList *)contours->At(0);\n",
    "if (!lcontour1)\n",
    "   return;\n",
    "TGraph *gc1 = (TGraph *)lcontour1->First();\n",
    "if (!gc1)\n",
    "   return;\n",
    "if (gc1->GetN() < 10)\n",
    "   return;\n",
    "gc1->SetMarkerStyle(21);\n",
    "gc1->Draw(\"alp\");"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "42cb7ac6",
   "metadata": {},
   "source": [
    "We make a TCutG object with the array obtained from this graph"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "fabc0eaf",
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "TCutG *cutg = new TCutG(\"cutg\", gc1->GetN(), gc1->GetX(), gc1->GetY());"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "26cc23b7",
   "metadata": {},
   "source": [
    "We create a polymarker object with npmax points."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "66bf7060",
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "const Int_t npmax = 50000;\n",
    "TPolyMarker *pm = new TPolyMarker(npmax);\n",
    "Int_t np = 0;\n",
    "while (1) {\n",
    "   Double_t x = -4 + 8 * gRandom->Rndm();\n",
    "   Double_t y = -4 + 8 * gRandom->Rndm();\n",
    "   if (cutg->IsInside(x, y)) {\n",
    "      pm->SetPoint(np, x, y);\n",
    "      np++;\n",
    "      if (np == npmax)\n",
    "         break;\n",
    "   }\n",
    "}\n",
    "pm->Draw();"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "0fd1e5d2",
   "metadata": {},
   "source": [
    "Draw all canvases "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "4269f4ee",
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "gROOT->GetListOfCanvases()->Draw()"
   ]
  }
 ],
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