Improper behavior of TCanvas::Divide

[rlalik]

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Description

Documentation for TCanvas::Divide says:

Automatic pad generation by division.

• The current canvas is divided in nx by ny equal divisions (pads).
• xmargin is the space along x between pads in percent of canvas.
• ymargin is the space along y between pads in percent of canvas.
• color is the color of the new pads. If 0, color is the canvas color.

But setting margins to 0 results in:

1. Not setting up colors of subpards
2. Adding top-right padding to the main canvas.

The documentation also says that:

Note3: in case xmargin <=0 and ymargin <= 0, there is no space between pads. The current pad margins are recomputed to optimize the layout.

Not sure what the second sentence means, what pad margins are recomputed?

Reproducer

Here a couple of examples of proper and improper behaviors:

1. This works as expected - OK

{
    TCanvas * can = new TCanvas("can", "can", 800, 800);
    can->Divide(2, 2, 0.01, 0.01, 2);
}

2. This gives white pads whereas they should be red - NOT OK

{
    TCanvas * can = new TCanvas("can", "can", 800, 800);
    can->Divide(2, 2, 0.0, 0.0, 2);
}

3. After putting colours manually, we see odd top-right margin - NOT OK

{
    TCanvas * can = new TCanvas("can", "can", 800, 800);
    
    can->Divide(2, 2, 0.0, 0.0, 2);
    
    can->cd(1);
    gPad->SetFillColor(30);
    gPad->Modified();
    gPad->Update();
    
    can->cd(2);
    gPad->SetFillColor(38);
    gPad->Modified();
    gPad->Update();
    
    can->cd(3);
    gPad->SetFillColor(41);
    gPad->Modified();
    gPad->Update();
    
    can->cd(4);
    gPad->SetFillColor(42);
    gPad->Modified();
    gPad->Update();
    
    can->cd(0);
    gPad->SetFillColor(2);
    gPad->Modified();
    gPad->Update();
}

ROOT version

ROOT v6.36.04
Built for linuxx8664gcc on Sep 05 2025, 08:28:36
From tags/6-36-04@6-36-04
With 
Binary directory: /usr/bin

Installation method

Gentoo, build from sources

Operating system

Gentoo

Additional context

No response

[rlalik]

After the merged PR, I made tests with following macro:

{
    TCanvas * can_0 = new TCanvas("can_0", "can_0", 200, 200, 200, 200);
    can_0->Divide(2, 2, 0.1, 0.1, 2);
    can_0->Draw();

    TCanvas * can_1 = new TCanvas("can_1", "can_1", 408, 200, 200, 200);
    can_1->Divide(2, 2, 0.0, 0.1, 2);
    can_1->Draw();

    TCanvas * can_2 = new TCanvas("can_2", "can_2", 616, 200, 200, 200);
    can_2->Divide(2, 2, 0.1, 0.0, 2);
    can_2->Draw();

    TCanvas * can_3 = new TCanvas("can_3", "can_3", 824, 200, 200, 200);
    can_3->Divide(2, 2, 0.0, 0.0, 2);
    can_3->Draw();

    TCanvas * can_4 = new TCanvas("can_4", "can_4", 200, 432, 200, 200);
    can_4->Divide(2, 2, -0.1, -0.1, 2);
    can_4->Draw();

    TCanvas * can_5 = new TCanvas("can_5", "can_5", 408, 432, 200, 200);
    can_5->Divide(2, 2, 0.1, -0.1, 2);
    can_5->Draw();

    TCanvas * can_6 = new TCanvas("can_6", "can_6", 616, 432, 200, 200);
    can_6->Divide(2, 2, -0.1, 0.1, 2);
    can_6->Draw();

    TCanvas * can_7 = new TCanvas("can_7", "can_7", 824, 432, 200, 200);
    can_7->Divide(2, 2, 0.1, 0.1, 2);
    can_7->Draw();
}

Canvases 4...6 still seems to behave odd with negative margin value. I didn't yet implement my feature, this is the version after this PR merge.

I will try to implement my negative margins.

[rlalik]

In the code there is:

// special case when xmargin < 0 or ymargin < 0

and

/// __Note3:__  in case xmargin < 0 or ymargin < 0, there is no space
///             between pads. The current pad margins are recomputed to
///             optimize the layout.

What is this special case? What does it mean margins are recomputed and layout optimized? Can we break this with my idea of negative margins?

[github-actions]

Hi @linev, @couet,

It appears this issue is closed, but wasn't yet added to a project. Please add upcoming versions that will include the fix, or 'not applicable' otherwise.

Sincerely,
🤖

[couet]

The discussion of this issue is now in: #20209