Qt6 Support

[a17r]

Qt5 is now declared deprecated in Gentoo

At this point in time, Linux distributions are already tracking which packages are blocking a future Qt5 cleanup. Real Qt Company OSS support stopped back in 2020, when public availability of LTS releases started to be delayed for 1 year, stable branch repository access got closed down (with 5.15 branch repository receiving any new LTS tag one year after commercial release) and only Qt6-relevant bugs are being fixed (then backported) since then. The final release happened on 2025-05-19, which means a) we will not get it until 2026-06-19, b) any bugs discovered since last May will not be fixed and c) no bugs since 2020-12-08 that haven't also been affecting Qt6 were fixed at all.

At least Gentoo, Alpine and Ubuntu are entertaining Qt6 porting trackers by now, for Gentoo, Qt5 revdep cleanups are in full swing already.
https://bugs.gentoo.org/show_bug.cgi?id=qt5-removal
https://gitlab.alpinelinux.org/alpine/aports/-/issues/17114
https://ubuntu-archive-team.ubuntu.com/transitions/html/qt5-removal.html

The good news is that the porting effort to Qt6 is supposed to be much smaller than from Qt4 to Qt5. Links:
https://doc.qt.io/qt-6/portingguide.html
https://www.qt.io/blog/references-and-hints-about-porting-from-qt5-to-qt6
https://www.qt.io/blog/porting-from-qt-5-to-qt-6-using-qt5compat-library

[alemuntoni]

I am fully aware that Qt5 has been end-of-life for several years. I also know that, in theory, porting from Qt5 to Qt6 should be a straightforward task. Unfortunately, this has not been the case for MeshLab (at least, not for me).

As you can see from the various qopengl* branches, I have attempted to update the legacy QGLWidget to the new QOpenGLWidget. To put it mildly, the codebase in these areas is a mess. The QGLWidget class is used throughout MeshLab in several undocumented and non-standard ways, making it extremely difficult to understand what is happening and refactor the code correctly.

At this point, I have managed to get a version of MeshLab that builds with Qt6, but it has major issues:

• several plugins are disabled because they still need to be ported and are not functional (see qopengl_plugins* branches);
• the application is highly unstable, with frequent random crashes during use.

I tried, I failed and I gave up. Perhaps the task is simply beyond my abilities.
At this point, I believe that writing a new viewer from scratch would require less time and result in a more maintainable application in the long run.

If anyone in the community wants to take on the challenge of porting MeshLab to Qt6, pull requests are more than welcome.
Otherwise, MeshLab will have to remain on Qt5 for the foreseeable future, as we currently lack the resources to move forward with the port.

[ssh4net]

MESHLAB CODEBASE

Core Qt5 Incompatibilities

1. QGLWidget/QGLContext/QGLFormat (OpenGL module)
   The most critical blocker. Qt6 removed the entire QtOpenGL module's QGL* classes:
   • QGLWidget - used as base class for GLArea (main rendering widget), MLSceneGLSharedDataContext, MLPluginGLContext, and throughout vcglib
   • QGLContext - used extensively for multi-view context management
   • QGLFormat - used for OpenGL format configuration
   • QGL::* enums (e.g., QGL::SampleBuffers)
   Files affected:
   • src/meshlab/glarea.h - main rendering area inherits QGLWidget
   • src/common/ml_shared_data_context/ml_scene_gl_shared_data_context.h - inherits QGLWidget
   • src/common/ml_shared_data_context/ml_plugin_gl_context.h - inherits QGLContext
   • All render plugins and edit tools that interact with OpenGL contexts
   • vcglib wrappers: wrap/gui/rubberband.h, wrap/gui/coordinateframe.h, etc.
2. QSignalMapper
   Deprecated in Qt5 and removed in Qt6:
   • src/meshlab/mainwindow.h - window management
   • src/meshlabplugins/render_gdp/shaderDialog.h - shader parameter UI
   • Multiple unsupported plugins
   Replacement: Use lambdas with QObject::sender()
3. QRegExp
   Deprecated in Qt5, removed in Qt6:
   • Used in ~45 locations across the codebase
   • Common in file parsers, text processing, and UI validation
   Replacement: QRegularExpression
4. QString::SkipEmptyParts enum
   Moved to Qt::SkipEmptyParts in Qt6:
   • Used in ~33 locations, mainly in file parsers (X3D, XYZ, EXPE formats)
5. QDesktopWidget
   Removed in Qt6:
   • src/meshlabplugins/edit_align/AlignPairDialog.cpp
   • QApplication::desktop() calls
   Replacement: QScreen API
6. QTextCharFormat/text editing classes
   Some text editing APIs changed in Qt6 (minor impact, mostly in unsupported plugins)
   Migration Path
   The primary blocker is the QGLWidget/QGLContext architecture. Qt6 replacement requires:
7. Replace QGLWidget → QOpenGLWidget
8. Replace QGLContext → QOpenGLContext
9. Replace QGLFormat → QSurfaceFormat
10. Refactor multi-view context sharing (QOpenGLContext uses different sharing mechanisms)
11. Update all plugins and vcglib wrappers that interact with the OpenGL widget system
    The OpenGL refactoring is non-trivial due to:
    • Deep integration of QGLContext throughout the shared data context system
    • Multi-threaded rendering architecture relying on Qt5's context switching
    • vcglib dependency on QGLWidget in its GUI wrappers
    Secondary issues (QSignalMapper, QRegExp, QString enums) are straightforward to fix with find-replace operations.