image2hist.C

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/// \file
/// \ingroup tutorial_image
/// Create a 2-D histogram from an image.
///
/// \macro_image
/// \macro_code
///
/// \author Olivier Couet

void image2hist()
{
   TASImage image("$ROOTSYS/tutorials/image/rose512.jpg");
   UInt_t yPixels = image.GetHeight();
   UInt_t xPixels = image.GetWidth();
   UInt_t *argb   = image.GetArgbArray();

   TH2D* h = new TH2D("h","Rose histogram",xPixels,-1,1,yPixels,-1,1);

   for (int row=0; row<xPixels; ++row) {
      for (int col=0; col<yPixels; ++col) {
         int index = col*xPixels+row;
         float grey = float(argb[index]&0xff)/256;
         h->SetBinContent(row+1,yPixels-col,grey);
      }
   }

   gStyle->SetPalette(kCubehelix);
   h->Draw("colz");
}