TH2 not drawn if only TH2::AddBinContent is called (Entries=0). For TH1 it works.

[ferdymercury]

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Description

Assume one only uses TH2::AddBinContent to fill a 2D histogram.
Then you draw it with COLZ.
Resulting canvas is empty.
In contrast, if you do the same with TH1::AddBinContent, it works without problems.

Reproducer

auto h2 = new TH2I("h2", "h2", 10, 0, 1, 2, 0, 2);
h2->AddBinContent(16,5);
h2->Draw("COLZ");

Nothing happens now, canvas is empty.

You are forced to:

h2->SetEntries(1)
gPad->Modified();
gPad->Update();

In contrast, with TH1, all works fine without calling SetEntries:

auto h = new TH1I("h", "h", 10, 0, 1);
h->AddBinContent(3,5)
h->Draw()

ROOT version

ROOT v6.30/02
Built for linuxx8664gcc on Nov 27 2023, 19:50:38
From tags/v6-30-02@
With c++ (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Binary directory: /opt/root/bin

Installation method

Binary

Operating system

Ubuntu 22.04

Additional context

No response

[couet]

Well, I do not think it is a graphics issue. Nothing is drawn simply because the number of entries is 0:

root [0] auto h2 = new TH2I("h2", "h2", 10, 0, 1, 2, 0, 2);
root [1] h2->AddBinContent(16,5)
root [2] h2->GetEntries()
(double) 0.0000000

May be @lmoneta may know.

[ferdymercury]

@couet But in the case of TH1, the number of entries is also 0, and it is drawn

root [4] auto h = new TH1I("h", "h", 10, 0, 1);
root [5] h->AddBinContent(3,5)
root [6] h->GetEntries()
(double) 0.0000000
root [7] h->Draw()

[couet]

See:

root/hist/histpainter/src/THistPainter.cxx

Line 9656 in e46ffe8

} else if (fH->GetEntries() != 0 && Hoption.Axis<=0) {

[ferdymercury]

Good catch, thanks.

Alternatively, I would suggest adding in the Doxygen documentation of all the "SetBinContent" and "AddBinContent" functions, that you need to set the "entries" per hand later on, so that the result is drawn.

[couet]

Yes that can be done I guess.