TLines on Spawned Canvases/Pads are drawn to the wrong Pad

[jbertaux]

Hello,

I have a code designed to show a 2-dimensional histogram. I have a TExec that updates a pad/text to show which bin the user’s mouse is over, and on click creates another canvas with more information. Ideally I would like to draw lines on a pad on the spawned canvas, but they appear in the wrong place (somewhere on the original canvas) despite calls to TPad::cd().

I’ve made a small example which reproduces the bug:

RootDrawingBug.C (3.5 KB)

My original code is a mix of macro and compiled code and I can update the pad/text with mouse position. However, the example is a pure macro and the pad/text for the mouse position doesn’t update until the second canvas is drawn, despite calling TPad::Update() in the same way as my original code.

As far as where the TLines appear, they usually appear in the most recently updated pad (e.g., the pad where I show the text with mouse position information). However, I added the TExec to the pad with some original TLines, and now they appear there.

[jbertaux]

I’m still curious as to why the pad isn’t updated in the example macro–so if someone identifies why that is, I would like to hear it.

However, I found the solution that solves my original problem, which is that each TLine must be allocated to the heap and drawn individually.

Before I had something like

//Draw vertical lines
TLine line;
for(double d = 0.25; d < 1.0; d += 0.25)
{
  line.SetX1(d);
  line.SetY1(0.1);
  line.SetX2(d);
  line.SetY2(0.9);

  line.DrawClone();
}

This needed to be

//Same as above, but the lines will be drawn to the gPad
for(double d = 0.25; d < 1.0; d += 0.25)
{
  TLine* line = new TLine(d, 0.1, d, 0.9);
  line->Draw();
}

[Wile_E_Coyote]

In your “original” code, try: gROOT->SetSelectedPad(gPad); line.DrawClone();

[jbertaux]

I tried that and it also works.

TLine::DrawClone() just uses TObject::DrawClone() and that draws to gROOT->GetSelectedPad().
TObject::Draw() calls TObject::AppendPad() and that draws to gPad.

It’s weird that they’re not the same but it is what it is. Thank you for that solution.