Transparent TEllipse

[Viesturs]

In this example:

 TCanvas c("c","transparent canvas",200,10,700,500);
  TH2F hist("hist", "hist", 10, -5, 5, 10, -5, 5);
  hist.Draw();
  TEllipse ell1(0.0, 0.0, 1.0, 1.0);
  ell1.SetFillColor(kRed);
  ell1.Draw();
  TEllipse ell2(0.5, 0.5, 1.0, 1.0);
  ell2.SetFillStyle(4000);
  ell2.SetLineStyle(kDashed);
  ell2.Draw();
  c.SaveAs("ellipses.png");
  c.SaveAs("ellipses.eps");
  c.SaveAs("ellipses.pdf");

I used SetFillStyle(4000) on ell2 to make it transparent.

.png shows the result that is expected:

while in .pdf and .eps ell2 became opaque:

What is happening and what can we do to obtain the desired result in .pdf and .eps formats?

Also there is an old similar post begging to be answered - https://root-forum.cern.ch/t/error-in-saving-the-figure-made-by-tarc-or-tellipse/18336

[ksmith]

@https://root.cern.ch/doc/master/classTAttFill.html#F2

Conventions for fill styles:

• 0 : hollow
• 1001 : Solid
• 3000+pattern_number (see below)
• For TPad only:
  • 4000 :the window is transparent.
  • 4000 to 4100 the window is 100% transparent to 100% opaque.

The pad transparency is visible in binary outputs files like gif, jpg, png etc … but not in vector graphics output files like PS, PDF and SVG. This convention (fill style > 4000) is kept for backward compatibility. It is better to use the color transparency instead.

https://root.cern.ch/doc/master/classTAttFill.html#F2

According to the above documentation you should use SetFillStyle(0), the note at the bottom may also be true for TEllipse.

[Viesturs]

using SetFillStyle(0) on TEllipse worked.

[couet]

Note also that with a solid filling it is possible (depending on which platform you are running) to have transparent color as explained here.