%%cpp -d

#include "TColor.h"
#include "TImage.h"
#include "TImageDump.h"
#include "TVirtualPad.h"
#include "TROOT.h"
#include "TFrame.h"

UInt_t color2rgb(TColor *col)
{
   // returns RGB value of color

   return ((UInt_t(col->GetRed()*255) << 16) +
           (UInt_t(col->GetGreen()*255) << 8) +
            UInt_t(col->GetBlue()*255));
}

Bool_t batch = gROOT->IsBatch();

gROOT->SetBatch(kTRUE);

gROOT->Macro("$ROOTSYS/tutorials/graphs/graph.C");

TImageDump dmp("dummy.png");
TImage *fore = dmp.GetImage();  // image associated with image_dump

gPad->SetCanvasSize(400, 300);
gPad->Paint(); // paint gPad on fore image associated with TImageDump

TImage *back = TImage::Open("$ROOTSYS/tutorials/image/rose512.jpg");

TColor *bk1 = gROOT->GetColor(gPad->GetFillColor());
TColor *bk2 = gROOT->GetColor(gPad->GetFrame()->GetFillColor());
UInt_t rgb1 = color2rgb(bk1);
UInt_t rgb2 = color2rgb(bk2);

UInt_t *argb = fore->GetArgbArray();
UInt_t w = fore->GetWidth();
UInt_t h = fore->GetHeight();

for (UInt_t i = 0; i < h; i++) {
   for (UInt_t j = 0; j < w; j++) {
      Int_t idx = i*w + j;

      // RGB part of ARGB color
      UInt_t col = argb[idx] & 0xffffff;

      // 24..31 bits define transparency of the color in the range 0 - 0xff
      // for example, 0x00000000 - black color with 100% transparency
      //              0xff000000 - non-transparent black color

      if ((col == rgb1) || (col == rgb2)) { //
         argb[idx] = 0; // 100% transparent
      } else {
         argb[idx] = 0xff000000 + col;  // make other pixels non-transparent
      }
   }
}

back->Merge(fore, "alphablend", 20, 20);

back->WriteImage("trans_graph.png");
printf("*************** File trans_graph.png created ***************\n");

delete back;

if (!batch) gROOT->SetBatch(kFALSE);

gROOT->GetListOfCanvases()->Draw()