Circle is not a circle

ROOT
1

I’m plotting a small circle into a canvas with TEllipse (see example macro below). When I look at the saved PDF file and zoom into the circle, the circle is actually not a circle but a polygon with 8 edges. Is there a draw option to increase the number of draw points?

Thank you,
Jochen

Root version 5.22/00 under cygwin

{
  TCanvas *c = new TCanvas("c","my canvas",0,0,400,400);
  c->Draw();
  TH1F *hframe = (TH1F*)c->DrawFrame(0, 0, 200, 200);

  TEllipse *e = new TEllipse(100,100,3,3);
  e->Draw();

  c->Print("canvas.pdf");
}
2

With the last root version I get the attached result. Seems to me I get the same on screen.
canvas.pdf (12.8 KB)

3

Thanks for the quick reply. Your attached PDF also shows an octagon. That was my point: TEllipse should give me a circle, not and octagon.

Jochen

4

Thanks for the quick reply. Your attached PDF also shows an octagon. That was my point: TEllipse should give me a circle, not and octagon.

Jochen[/quote]

I looked into the code of TEllipse.cxx and modified the ::PaintEllipse() method to improve the visual appeal of small ellipses by changing np from 200 to 2000.

I would like to suggest to add a method ::SetNPoints() to TEllipse (and related objects) so that a user can change the number of draw points.

Thank you,
Jochen

5

Dear All,

I’ve noticed that this exchange happened 10 years ago but the SetNPoints method nicely suggested by Jochen is still not in place and I am having the same problem again. I’ll use the hack that Jochen proposed but the better solution of ::SetNPoints would really be nice to have! (technically I guess the definite solution would be to export the arc as an arc and not bump up the number of points, but okay…)

Thank you and Best Regards,
David

6

Dear All,

Okay, quick reply to myself: I see the number of points has indeed been bumped up in the code but the problem still persists when drawing small ellipses. My use case is that I have a TCanvas that I’d like to export as an A4 page and I want to draw text-size ellipses which then look terrible. I’ll hack the code for now…

Thank you!
Best Regards,
David

7

Do you have a small macro reproducing your problem ?

8

Certainly: here goes… thank you for the quick reply!
DrawResponsePage01.C (1.6 KB)

9

Unfortunately one cannot change the number of point to draw the ellipse.
It is computed internally:
https://root.cern/doc/master/TEllipse_8cxx_source.html#l00526

10

may circular marker will be better for that wort of thing ?

11

Yes, that would be a possibility… though then I won’t have the option to adjust the thickness of the line, which is something I want to have if possible. In any case, my hacked version here produced the results I wanted, so I am happy for now :slight_smile: . Thanks a lot!

12

With hacking ROOT code you can also make a small macro drawing a circle with the number of points you want.