Why is QLineWidth not taken into account? - qt

Question

In Qt3D certain properties of rendered objects are not just simply set on the renderer, but they are globally (per view) or locally (on the material of a rendered object) added to the renderPasses - or so is my comprehension at least. (I'm using PySide2 - but the code is almost the same in C++)
For example when adding a geometry-renderer and using its primitive type point (Qt3DRender.QGeometryRenderer.Point) instead of rendering triangle-faces it displays the points of the geometry.
Here is an example figure with the default type.
The same only showing the points (renderer.setPrimitiveType(Qt3DRender.QGeometryRenderer.Points))
Hard to guess, but here the point-size has been already been changed - using the following code:
material = Qt3DExtras.QPhongMaterial(e)
for t in material.effect().techniques():
for rp in t.renderPasses():
pointSize = Qt3DRender.QPointSize(rp)
pointSize.setSizeMode(Qt3DRender.QPointSize.SizeMode.Fixed)
pointSize.setValue(5.0)
rp.addRenderState(pointSize)
According to the documentation the same mechanism can be used to change the line-width when rendering the object with Lines (LineStrip) as primitive type. Adding
lineWidth = Qt3DRender.QLineWidth(rp)
lineWidth.setValue(5.)
lineWidth.setSmooth(True)
rp.addRenderState(lineWidth)
does not change the line-width.
Why? Where do I need to add QLineWidth? Is it the material I chose which ignores the QLineWidth-state?

Answer 1

I'm fighting with similar problems at the moment. I tried to reproduce the behaviour with Qt3D line width test. When setting format version to 4.6 with CoreProfile, the maximum of linewidth seems to be 1 (or equivalently width=3 displayed by the line test).
It might be possible that this is the maximum supported range.
See:
https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/glLineWidth.xhtml
opengl glLineWidth() doesn't change size of lines
Note: I deliberately chose version 4.6 as that is the supported openGL version on my environment.

Answer 2

I ran into the same issue. It appears the problem is caused by Qt3DExtras::Qt3DWindow, which constructs a QSurfaceFormat with an OpenGL core profile. The glLineWidth function is not supported in the core profile.
Unfortunately there is no way to pass a QSurfaceFormat to Qt3DWindow. Setting a new format after the window is created also does not work.
The only way around this is to write your own window class with a QSurfaceFormat in compatibility mode. For example:
setSurfaceType(QSurface::OpenGLSurface);
QSurfaceFormat format = QSurfaceFormat::defaultFormat();
format.setVersion(4, 3);
format.setProfile(QSurfaceFormat::CompatibilityProfile);
format.setDepthBufferSize(24);
format.setSamples(4);
format.setStencilBufferSize(8);
setFormat(format);
QSurfaceFormat::setDefaultFormat(format);
Fortunately Qt3DExtras::Qt3DWindow does not actually contain a lot of functionality and you can easily write a similar class with the QSurfaceFormat changes mentioned above.
You can find the original source here for reference:
https://code.woboq.org/qt5/qt3d/src/extras/defaults/qt3dwindow.cpp.html