TCanvasWidget crashes

General Chat
1

I’m using ROOT version 6.26/10 on Suse- Linux tumbleweed.

Hi, all, I have been using the the Web-based TCanvasWidget embedded in my qt-based program.to draw markers, Lines Graphs etc for quite some time. I now want to draw objects on the screen which change repeatedly their position when I call the proper subroutine with a pushbutton. If I do this several times in sequence, the program crashes.This happens independently if I call gPad =KWidget->getCanvas(); (KWidget is my TCanvasWidget) only once in the calling routine or allways the inside the called subroutine. Unfortunartely I do not get useful information with the debugger. The crash seems to happen, when gPad->Update(); gPad->Modified(); is called.Someone experienced this and knows how to overcome the problem? Thank you very much in advance Best regards Karl

2

It is not easy to help without a reproducer. I found it weird you assign gPad yourself. gPad points to the current and usually is not set by the user program.

As your problem seems connected with GUI, I guess @bellenot might have some hints to propose to you.

3

Let’s ask @linev , the author of the TCanvasWidget class

1 Like
4

Hi,

As Olivier said, we need reproducer to understand the problem.

But main remark is - one have to be sure that any changes in canvas performed from the main thread where QApplication runs. Typically one do it with QTimer.

Regards,
Sergey

5

Thank you all for replying. @Linev:

Hi Sergey, I started from your ExampleWidget program. In the ExampleWidget constructor we have

gPad = fxTCanvasWidget->getCanvas();
Histo->Draw();

I do something similar, but since I want to draw things several times with with different parameters
I packed

{
.
.
 gPad = KWidget->getCanvas();
..
Draw something ....
..
something.Draw();
 ..
gPad->Modified()
gPad->Update()

}

in a Subroutine ExampleWidget::Doit() which is executed with different parameters each time I press my button.
The drawing looks fine the first few times (about 5) but the the program crashes.
I also tried to call the gPad = KWidget->getCanvas(); outside the subroutine only once and then used gPad->Clear() inside the subroutine before doing the next drawing because I thought that perhaps a repeated call of gPad = KWidget->getCanvas(); could cause the problem. However the result was the same: the program crashed after a few calls

6

So, very simply stated: What I want to do is:
Setup the canvas
Draw something
Change Datetime
with button: Redraw with drawing changed according to new datetime.
do this as often as I like

best regards Karl

7

Hi,

Strange, I just modify qt5web example with the method:

void ExampleWidget::DrawGeometryInCanvas()
{
   auto canv = fxTCanvasWidget->getCanvas();
   canv->Clear();

   int nth2points = 40;

   auto histo2 = new TH2I("test2", "Testing TH2 drawing in TCanvas", nth2points, -5, 5, nth2points, -5, 5);
   histo2->SetDirectory(nullptr);
   for (int n=0;n<nth2points;++n) {
      for (int k=0;k<nth2points;++k) {
         double x = 10.*n/nth2points-5.;
         double y = 10.*k/nth2points-5.;
         histo2->SetBinContent(histo2->GetBin(n+1, k+1), (int) (1000*TMath::Gaus(x)*TMath::Gaus(y)));
      }
   }
   canv->GetListOfPrimitives()->Add(histo2,"colz");

   canv->Modified();
   canv->Update();
}

And I can press button any number of times.

Regards,
Sergey

8

Hi Sergey, I investigated more the problem i am facing. As it seems now, it does not have to do with multiple calls to ui->KWidget->getCanvas(). My program was quite big so I broke it down to the essential parts and coded from scratch.

#include "mainwindow.h"
#include "./ui_mainwindow.h"
#include <iostream>
#include "TMarker.h"
#include "TCanvas.h"
#include "TEllipse.h"
#include "TMath.h"
#include <cmath>

#include "TCanvasWidget.h"


using namespace std;

double getProjectionRadius(double Dec)
{

    double Radius = 2*cos(Dec)/(1+sin(Dec));
    return Radius;

}


MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    setAttribute(Qt::WA_DeleteOnClose);
    ui->dateTimeEdit->setDisplayFormat("dd.MM.yyyy hh:mm:ss");
    ui->dateTimeEdit->setDateTime(QDateTime::currentDateTimeUtc()); //Visualizza UTC
    ui->dateTimeEdit->setCalendarPopup(true);


    DoDisplay();


}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::DoDisplay()
{
   auto canv= ui->KWidget->getCanvas();
   canv->Range(-5,-5,5,5);
   canv->Clear();

   /*
       for(int i=0;i<14;i++)
       {

           double ang =(90.-10*i)*TMath::DegToRad();
           double radius = getProjectionRadius(ang);

           cout << 90-10*i << " " << radius << " Ang Rad "<< endl;

           TEllipse e = TEllipse(0.,0.,radius,radius);
           e.SetFillStyle(0); // <-- Stroke only
           if(i==9)
               e.SetLineColor(4);
           else
               e.SetLineColor(2);
           e.SetLineWidth(1);
           e.Draw("L");

       }
   */
           for (int k=0;k<360;k++){


                double Dec =0.0;

                double azimut = k*TMath::DegToRad();

                double radius = 2.0;
                TMarker *ma = new TMarker();

                double circx=radius*TMath::Cos(azimut);
                ma->SetX(circx);
                double circy=radius*TMath::Sin(azimut);
                ma->SetY(circy);



                ma->SetMarkerStyle(20);
                ma->SetMarkerSize(1);
                ma->SetMarkerColor(2);
                ma->Draw();
            }

    canv->Update();
    canv->Modified();

}

This part of the program is for projecting points on the surface of a sphere to a plane. The program as I am posting it, works fine. It displays 360 points on the circumference of a circle. If however I uncomment the commented part which should draw a bunch of concentric circles, it crashes. when canv->Update(); is called. It seens that the crash is caused by a Resize event issued at the end of the subroutine:

Only if you are really convinced it is a bug in ROOT then please submit a
report at https://root.cern/bugs Please post the ENTIRE stack trace
from above as an attachment in addition to anything else
that might help us fixing this issue.
===========================================================
#5  0x0000000000000000 in  ()
#6  0x00007f7415c3127f in TPad::ResizePad(char const*) () at /home/karl/rootbin/lib/libGpad.so
#7  0x00007f7415c053cd in TCanvas::SetCanvasSize(unsigned int, unsigned int) () at /home/karl/rootbin/lib/libGpad.so
#8  0x0000000000408c1f in TCanvasWidget::resizeEvent(QResizeEvent*) (this=0x4a07110, event=0x7fffc16ec560) at /home/karl/startest/TCanvasWidget.cpp:114

I could send you the .ui file and the other files of the project as well as well, but i do not know how to attach them.
At the start of the program I also get this message, but I don’t know what I should do about it:

Please register the custom scheme ‘rootscheme’ via QWebEngineUrlScheme::registerScheme() before installing the custom scheme handler.

best regards Karl

9

As it seems, I am not even capable of posting the program code correctly, because the #include instructions with the “triangular brackets” (I dont’t know how to call them correctly) have the second part missing in my post

I apologise for that

karl

10

Hi,

Please try to use new TEllipse in your code like:

TEllipse *e = new TEllipse(0.,0.,radius,radius);
e->SetFillStyle(0); // <-- Stroke only
if(i==9)
  e->SetLineColor(4);
else
  e->SetLineColor(2);
e->SetLineWidth(1);
e->Draw("L");
11

Hi Sergey,
thank you very much indeed ! Your suggestion solves the problem. I’m sorry I overlooked the obvious. I should sleep more :wink:

Best regards Karl