[Octave-bug-tracker] [bug #55735] QGLWidget not detected with Qt 4, so Qt graphics toolkit is not compiled

[Mike Miller]

Follow-up Comment #10, bug #55735 (project octave):

Thanks for taking a detailed look at this, I think your observations are
correct and very helpful. I think that Octave uses Qt5OpenGL to support
versions of Qt earlier than 5.4, when QOpenGLWidget was added. In Qt 5.4 and
later, this library is unnecessary overlinking.

You're right, in Octave 4.4.1 it was included in QT_MODULES, so it was
required. It was factored out in 332be8be16eb
(https://hg.savannah.gnu.org/hgweb/octave/rev/332be8be16eb) as part of the
change to make the qt graphics toolkit module separate from the Qt GUI
library.

So from my re-reading of that change, the reason for factoring it out wasn't
to make it optional, but to make sure that the GUI is not overlinked with
libQtOpenGL, only the loadable '__init_qt__' module is linked with it. Adding
it back into QT_MODULES would add it back into QT_LIBS, and overlink
liboctgui. Does that help explain why QT_OPENGL_LIBS is a separate variable?

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