System information (version)

• OpenCV => master (latest)
• Operating System / Platform => ubuntu 20.04
• Compiler => gcc 9.3.0

Detailed description

Code below is from cmake/OpenCVFindLibsGUI.cmake

The condition to decide whether to check opengl support is

if(WITH_WIN32UI OR (HAVE_QT AND QT_QTOPENGL_FOUND) OR HAVE_GTKGLEXT)
    find_package (OpenGL QUIET)
  ...

and the condition for gtkglext is here

if(WITH_OPENGL AND NOT HAVE_GTK3)
  ocv_check_modules(GTKGLEXT gtkglext-1.0)
  if(HAVE_GTKGLEXT)
    ocv_append_build_options(HIGHGUI GTKGLEXT)
  endif()
endif()

Thus, If I have GTK3 installed, gtkglext will not be checked, and opengl will not be checked too.

I'm not familiar with both GTK3 and gtkglext. Is this behaviour intentional? Or just a bug that opengl support is disabled when GTK3 is installed?

Issue submission checklist

• I report the issue, it's not a question

• I checked the problem with documentation, FAQ, open issues,
  forum.opencv.org, Stack Overflow, etc and have not found solution

• I updated to latest OpenCV version and the issue is still there

• There is reproducer code and related data files: videos, images, onnx, etc