Extra vertical space on a canvas when CanvasPreferGL is set to true, reproducible via SSH

[YuryYury]

Check duplicate issues.

• Checked for duplicates

Describe the bug

When trying to draw a histogram with a transparent box on top of it, I noticed that whenever I set CanvasPreferGL to true, the canvas moves down, and the X-axis title is cut off.

What is the expected behaviour?

The only change I would expect is the transparency introduced to my box. Things are not supposed to be moved around only due to

        gStyle->SetCanvasPreferGL(true);

How to reproduce?

Consider this macro:

void transpar()
{
        gStyle->SetCanvasPreferGL(false);

        TH2F *h = new TH2F("h", "h;X;Y;Z", 10, 0., 10., 10, 0., 10.);
        h->Draw();

        TMarker *m = new TMarker(4., 7., 8);
        m->Draw();

        TBox *b = new TBox(1., 1., 8., 8.);
        printf("CanvasPreferGL is set to %s\n", gStyle->GetCanvasPreferGL() ? "true, the red box is expected to be transparent":"false, the red box is expected to be non-transparent");
        b->SetFillColorAlpha(kRed, 0.2);
        b->Draw();
}

Now if I run this as

root -l -n transpar.cxx

I get the following:

(no transparency is expected, so it is all good).
But when I change false to true at the third line

        gStyle->SetCanvasPreferGL(true);

and run again

root -l -n transpar.cxx

, I get

The box is now transparent, which is good, but there is too much empty space above the histogram name (“h”) and, as a consequence, the X-axis title is now cut off. Is there a way to make my box transparent and keep everything else where it was before?

Please note I’m using the -n option when running transpar.cxx, so it’s not due to something I have in my style file.

The original discussion at the ROOT forum revealed that:

• this is irreproducible if ran on local computers with Windows or Mac OS. The issue is observed only via SSH (from e.g. lxplus7)
• in principle, the issue can be mitigated by adding the following five lines at the very end of the macro (right after b->Draw();; the transparency remains; the +/- 0.1 offsets need a bit of a tuning of course):

        if (gStyle->GetCanvasPreferGL())
        {
                gPad->SetBottomMargin(gPad->GetBottomMargin() + 0.1);
                gPad->SetTopMargin(gPad->GetTopMargin() - 0.1);
        }

ROOT version

6.28/00

How did you install ROOT?

provided by ATLAS @ lxplus7

Which operating system are you using?

CentOS 7

Additional context

No response

[couet]

GL does not seem to work on lxplus. I get:

-bash-4.2$ root transpar.C 
   ------------------------------------------------------------------
  | Welcome to ROOT 6.24/08                        https://root.cern |
  | (c) 1995-2021, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for linuxx8664gcc on Sep 29 2022, 13:04:57                 |
  | From tags/v6-24-08@v6-24-08                                      |
  | With c++ (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44)                 |
  | Try '.help', '.demo', '.license', '.credits', '.quit'/'.q'       |
   ------------------------------------------------------------------

root [0] 
Processing transpar.C...
Fontconfig warning: ignoring UTF-8: not a valid region tag
Error in <TX11GLManager::CreateContext>: glXCreateContext failed

Error in <TRootCanvas::CreateCanvas>: GL context creation failed.
Info in <TCanvas::MakeDefCanvas>:  created default TCanvas with name c1
CanvasPreferGL is set to true, the red box is expected to be transparent
root [1] 

I will investigate

[YuryYury]

Hi Olivier,

GL does not seem to work on lxplus.

It works for me, both in ROOT 6.22/06 and 6.28/00 provided on lxplus by ATLAS.

[couet]

This is weird. I tried to connect specifically on lxplus7. That does not help. Even the standard app glxgears fails:

-bash-4.2$ glxgears
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  149 (GLX)
  Minor opcode of failed request:  3 (X_GLXCreateContext)
  Value in failed request:  0x0
  Serial number of failed request:  26
  Current serial number in output stream:  27
-bash-4.2$ 

Standard X11 ROOT display works though

[YuryYury]

Even the standard app glxgears fails

Again, glxgears works fine for me on lxplus7, even before I set up my usual ATLAS environment. FWIW, one difference I could notice is that I use zsh while you are using bash...

[couet]

I tried wich zsh . I still get the error with glxgears...

[dpiparo]

This has been fixed in the meanwhile, e.g. on lxplus for ROOT 6.32.06, the version currently installed there.

[YuryYury]

Agreed. It works fine even in ROOT 6.30/02.