vtkPolyData rendered upside down and reversed, why?

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1

I’m using VTK-8.2.0 on ubuntu 18.04.5.
I’ve implemented rendering of VTK data with QtQuick via QQuickFramebufferObject, using the C++ approach described here; I subclass QQuickFramebufferObject (I call it QVtkItem) and QQuickFramebufferOject::Renderer (I call it QVtkRenderer). QVtkRenderer creates a VTK pipeline which loads a vtkPolyData object and communicates with a QVtkItem; the QVtkItem is specified in QML. But for some reason my code renders the VTK data reveresed and upside down. Here is a test case adapted from the VTK TextOrigin example. This case loads vtkPolyData from 3D text and maps it to a vtkFollower, and the original example displays it like this:
image
Here is the result of my software, where the same pipeline logic is implemented by my QVtkRenderer:

image
image968×760 3.85 KB

Here’s code from the VTK TextOrigin example:

int main(int, char*[])
{
  vtkNew<vtkNamedColors> colors;

  // Create the axes and the associated mapper and actor.
  vtkNew<vtkAxes> axes;
  axes->SetOrigin(0, 0, 0);
  vtkNew<vtkPolyDataMapper> axesMapper;
  axesMapper->SetInputConnection(axes->GetOutputPort());
  vtkNew<vtkActor> axesActor;
  axesActor->SetMapper(axesMapper);

  // Create the 3D text and the associated mapper and follower (a type of
  // actor).  Position the text so it is displayed over the origin of the
  // axes.
  vtkNew<vtkVectorText> atext;
  atext->SetText("Origin");
  vtkNew<vtkPolyDataMapper> textMapper;
  textMapper->SetInputConnection(atext->GetOutputPort());
  vtkNew<vtkFollower> textActor;
  textActor->SetMapper(textMapper);
  textActor->SetScale(0.2, 0.2, 0.2);
  textActor->AddPosition(0, -0.1, 0);
  textActor->GetProperty()->SetColor(colors->GetColor3d("Peacock").GetData());

  // Create the Renderer, RenderWindow, and RenderWindowInteractor.
  vtkNew<vtkRenderer> renderer;
  vtkNew<vtkRenderWindow> renderWindow;
  renderWindow->AddRenderer(renderer);
  renderWindow->SetSize(640, 480);

  vtkNew<vtkRenderWindowInteractor> interactor;
  interactor->SetRenderWindow(renderWindow);

  vtkNew<vtkInteractorStyleTrackballCamera> style;
  interactor->SetInteractorStyle(style);

  // Add the actors to the renderer.
  renderer->AddActor(axesActor);
  renderer->AddActor(textActor);
  renderer->SetBackground(colors->GetColor3d("Silver").GetData());

  // Zoom in closer.
  renderer->ResetCamera();
  renderer->GetActiveCamera()->Zoom(1.6);

  // Reset the clipping range of the camera; set the camera of the
  // follower; render.
  renderer->ResetCameraClippingRange();
  textActor->SetCamera(renderer->GetActiveCamera());

  interactor->Initialize();
  renderWindow->SetWindowName("TextOrigin");
  renderWindow->Render();
  interactor->Start();

  return EXIT_SUCCESS;
}

And here is the “equivalent” code from my QVtkRenderer:

bool QVtkRenderer::assembleTestPipeline(
                                    vtkRenderer *renderer,
                                    vtkGenericOpenGLRenderWindow *renderWindow,
                                    vtkGenericRenderWindowInteractor *interactor,
                                    PickerInteractorStyle *style) {


  vtkNew<vtkNamedColors> colors;

  // Create the axes and the associated mapper and actor.
  vtkNew<vtkAxes> axes;
  axes->SetOrigin(0, 0, 0);
  vtkNew<vtkPolyDataMapper> axesMapper;
  axesMapper->SetInputConnection(axes->GetOutputPort());
  vtkNew<vtkActor> axesActor;
  axesActor->SetMapper(axesMapper);

  // Create the 3D text and the associated mapper and follower (a type of
  // actor).  Position the text so it is displayed over the origin of the
  // axes.
  vtkNew<vtkVectorText> atext;
  atext->SetText("Origin");
  vtkNew<vtkPolyDataMapper> textMapper;
  textMapper->SetInputConnection(atext->GetOutputPort());
  vtkNew<vtkFollower> textActor;
  textActor->SetMapper(textMapper);
  textActor->SetScale(0.2, 0.2, 0.2);
  textActor->AddPosition(0, -0.1, 0);
  textActor->GetProperty()->SetColor(colors->GetColor3d("Peacock").GetData());

  renderWindow->AddRenderer(renderer);
  renderWindow->SetSize(640, 480);

  interactor->SetRenderWindow(renderWindow);

  interactor->SetInteractorStyle(style);
  
  // Per QtVTK example
  interactor->EnableRenderOff();
  
  // Add the actors to the renderer.
  renderer->AddActor(axesActor);
  renderer->AddActor(textActor);
  renderer->SetBackground(colors->GetColor3d("Silver").GetData());

  // Zoom in closer.
  renderer->ResetCamera();
  renderer->GetActiveCamera()->Zoom(1.6);

  // Reset the clipping range of the camera; set the camera of the
  // follower; render.
  renderer->ResetCameraClippingRange();
  textActor->SetCamera(renderer->GetActiveCamera());

  // Initialize the OpenGL context for the renderer
  renderWindow->OpenGLInitContext();

  return true;
  
}

What can cause this? I’ve been trying to solve this for a while, with no luck yet. How can I debug this? Any suggestions are greatly appreciated!

2

Hello,

It seems that the handedness (3D rendering can be right- or left-handed) of your scene is not what you expected. Please, take a look at this: Change coordinate system from right to left-handed (change direction of Z axis).

I hope this helps,

Paulo

1 Like
3

Interesting, thanks!
I’m wondering how my coordinate system is reversed, since my app uses the same pipeline logic as the VTK Text Origin example, displaying the polydata of a vtkVectorText object, which no explicit coordinate rotation. Any common ways that the coordinate system could be “inadvertently” rotated? Thanks!

4

Judging from your secreen captures, it’s not rotation, but handedness. These are two different concepts. A 3D coordinate system can either be right-handed or left-handed:

image

Either this or there is something ill-coded in your QVtkItem class. It is possible that you’re not properly handling the fact that the origin of screen coordinates is in the top left corner of the widget. Check the mirrorVertically property of your class: QQuickFramebufferObject Class | Qt Quick 5.15.6

1 Like
5

Thanks! Wasn’t able to identify the bug in my code, but I set QVtkItem::mirrorVertically = true and now it works.

1 Like