How to specify the level value in isosurface drawing with TF3 and GL?

[fredo.faure]

Hello,
the following example shows that isosurface drawing with TF3 and GL seems to draw the surface level
f(x,y,z)=-0.2 whereas without GL options it draws the surface level f(x,y,z)=0.
Here i take for example f(x,y,z)=xyz.
Do i miss the correct way to specify the value of the function?
Thanks,
Frédéric Faure.

#include <TApplication.h>
#include <TCanvas.h>
#include <TStyle.h>
#include <TPaveLabel.h>
#include <TFormula.h>
#include <TF3.h>
#include <TText.h>
//-----------------------------
main ()
{
	TApplication theApp("App", nullptr, nullptr);

	//----- With GL ---------
    //.... prepare the 3D window
	gStyle->SetCanvasPreferGL(1);
	TCanvas *cnv = new TCanvas("glc", "TF3: Surface", 200, 10, 600, 600);
	TPad *tf3Pad = new TPad("box", "box",0,0,1,1);
	tf3Pad->Draw();
    //.... define and draw the 3D object

	TF3 *tf3 = new TF3("Surface","x*y*z+0.2", -1,1,-1,1,-1,1);
	
	tf3->SetFillColor(kRed);
	tf3Pad->cd();
	tf3->Draw("gl");

	TText *texte=new TText(0.2,0.9,"f(x,y,z)= x*y*z+0.2");
	texte->Draw();
	
	//----- Without GL ---------

	auto c2 = new TCanvas("c2","c2", 1000, 1000);

	TF3 *tf3bis = new TF3("Surface","x*y*z", -1,1,-1,1,-1,1);
	tf3bis->Draw("FBBB");

	//------------------------
	theApp.Run();
} 

__
ROOT Version: Not Provided
Platform: Not Provided
Compiler: Not Provided

---

[couet]

yes i can see the same with the following macro:

void tf3gl (int gl=1)
{
   if (gl==1) gStyle->SetCanvasPreferGL(true);
   auto c = new TCanvas("c", "TF3: Surface", 0, 0, 600, 600);

   if (gl==1) {
      TF3 *tf3 = new TF3("Surface","x*y*z+0.2", -1,1,-1,1,-1,1);
      tf3->Draw("glfbbb");
   } else {
      TF3 *tf3 = new TF3("Surface","x*y*z", -1,1,-1,1,-1,1);
      tf3->Draw("FBBB");
   }
}

[couet]

In the file graf3d/gl/src/TGLTF3Painter.cxx in the method Bool_t TGLTF3Painter::InitGeometry() I see the line:

   builder.BuildMesh(fF3, geom, &fMesh, 0.2);

if I replace 0.2 by 0. on that line then the gl and non gl version behave the same.

May be @tpochep (the author if this code) has an idea why there is an hardcoded 0.2 there ?

[couet]

I have done this change in the ROOT master.
The GL and non-GL version are the same now.
Thanks to have seen this mismatch.