Scatter Plot Markers Not Rending in QQuickVTKItem in Qt QML Application

[Parker_Hranicky]

Hello!

I’ve been using QQuickVTKItem in our Qt QML application for line plots for a while now and its worked just fine. Though when I try to use markers to overlay them onto a line plot, or to create a scatter plot, the markers do not render (on the plot or the legend). The issue persists outside of our application when using the QtQuickChartsOn3DScene example. Attached is a simplified example (modified directly from QtQuickChartsOn3DScene) displaying just the plot demonstrating that a line plot can be rendered, but markers cannot. Also shows that the chart interactor can still hover over the invisible markers. I also was able to get Markers to properly display using this scatter plot example.

This image shows the unmodified QtQuickChartsOn3DScene example not rendering the markers:

This image shows the modified example rendering a line plot but not the markers and displaying that the invisible markers still have hover capability:

This image displays that the scatter plot example (with no Qt / QML) can render markers properly:

The issue here seems very similar to this discussion topic. Although QQuickVTKItem::setGraphicsApi looks like it properly sets up the default QSurfaceFormat through QVTKRenderWindowAdapter::defaultformat.

From what I’ve tried, the issue seems to be something with Qt/QML QSurfaceFormat. Though my knowledge of OpenGL / Rendering is limited.

Versions:

Qt: 6.9.1

VTK: v9.5.1

OS Specs: Windows 11 Pro Version 24H2

QtQuickChartsOn3DSceneModified.zip (3.0 KB)

Thanks!

[Parker_Hranicky]

I seem to have found the issue. The way the vtkOpenGLContextDevice2D seems to work when rendering markers is it requires that GL_PROGRAM_POINT_SIZE is disabled. Unless GL_ES_VERSION_3_0 is enabled, then cbo->Program->SetUniformf("pointSize", this->Pen->GetWidth()); will run and markers will render fine.

The difference in the Qt app is GL_PROGRAM_POINT_SIZE is enabled by default and disabled in the standalone vtk example. Explicitly disabling GL_PROGRAM_POINT_SIZE in the QSGNode in QQuickVTKItem fixes the issue.

Should VTK’s vtkOpenGLContextDevice2D be updated to always set the pointSize uniform (not just in the GL_ES_VERSION_3_0 branch) so it works consistently whether GL_PROGRAM_POINT_SIZE is enabled or not? Or is the intended contract that VTK’s chart code requires GL_PROGRAM_POINT_SIZE to be disabled, and it’s the embedding application’s responsibility to enforce that state?