QGraphicsView drawing artifacts

wawanbreton

Hi,

I am working on an application, based on a graph editor, which uses a QGraphicsView. Sometimes, when dragging the view, some strange artifacts appear, inside and even outside the widget :

alt text
It is quite annoying, and I don't see what I can do to remove them.
I tried to play with the render options, but I can see no change. I also tried to use a QGLWidget for the viewport, which seems to remove them, but it causes other troubles, and I don't want the opengl dependancy.

My items are quite standard, especially the one that seems to be the cause, the connection item : it is just a QGraphicsItemGroup with 3 QGraphicsPathItem as children. I also embed many QGraphicsProxyWidgets, but they seem to work fine after setting their bounding rect correctly.

Any idea on what I can do to fix this ?

If you want to test it, the whole code is on github :
https://github.com/wawanbreton/cvcomposer

Thanks !

SGaist

Hi and welcome to devnet,

Which version of Qt are you using ? On what version of Windows ?

wawanbreton

Hi,

I am using Qt 5.7.0 with mingw-32 on Windows 7.
I can try other versions of Qt it that may help, and even the Linux version.

SGaist

IIRC, there's 5.8 Beta that is available.
You can also test the latest 5.6 to see if it's a regression.

Just tested it on Mac and I'm not able to reproduce that problem.

What graphic card do you have on your machine ?

wawanbreton

Hi,

Sorry for the delay !
I tried today with a 5.5 version, provided mingw and same platform, and actually it works well ! Now I should try with 5.8 as you suggested, to see if the issue has been fixed in the meantime.

wawanbreton

Well, compiling Qt on Windows is quite a pain.. Can I find binaries of 5.8 somewhere ?

SGaist

The snapshot of the Beta can be found here.

wawanbreton

Thank you for the link, I will keep it somewhere :)
Well, the artifacts are still presents in 5.8 Beta (tag number 584), do you want me to test other versions, for example the msvc one ?

SGaist

One more thing you can test is forcing the use of Desktop OpenGL. See here for the variable to set and how (no need to re-build Qt though).

wawanbreton

Hi, I tried the various values of the QT_OPENGL variable, but nothing changes. Actually, I'm not using a QGLWidget for the QGraphicsView. I tried to : it does remove the artifacts, but generates other limitations, on antialiasing for example, so I'd like to keep the "non-opengl" implementation.

SGaist

Then it's likely a Windows specific issue, you should take a look at the bug report system to see if its something known.

wawanbreton

I can't find something similar, I will create a new bug.
Thank you for the support.

SGaist

Thanks ! Don't forget to share the link to the report, it will make it easier to find.

wawanbreton

Here is the bug link :
https://bugreports.qt.io/browse/QTBUG-56517

I think I will try to make a minimal project to reproduce the bug, I know it helps developpers :)

mrjj

@wawanbreton
A minimal project is very helpful ! - as it allows developers to easily reproduce
the bug, so please do! :)

wawanbreton

Hi,
I finally had time to add a project. The bug can be reproduced by starting the application and just play a bit with the mouse (I detailed the instructions in the bug report).
Now I hope someone has time to work on it :)

mrjj

@wawanbreton
Its a good idea to post link to bug report here as
people searching the forums can go vote for the issue.

sirhc

I had the same problem, it took a long and frustrating amount of time to try and fix, but I think that I have found a solution.
Inside my paint function for that graphicsItem I created a region, then set clip region.
QRegion r1(QRect(-40, -40, 400, 200));
painter->setClipRegion(r1);
This may cause some other problems later, but I haven’t come across them yet. And I’m not sure on the performance cost of doing this when you have a large number of items on the screen. I think that I will move the creation of the region to outside of paint to start with.