WorldPointPicker: get the right value if you zoom in

I use this example for a WorldPointPicker. It works fine, but if i zoom in, it doesn’t give the correct coordinates. How can i correct this?

Hi, Thomas,

I perform picking with this (simplified from

void v3dMouseInteractor::OnLeftButtonUp()
    // Don't pick during dragging.
    if( ! m_isDragging ){

        // Get pick position in 2D screen coordinates.
        int* clickPos = this->GetInteractor()->GetEventPosition();

        // Get the 3D object under the 2D screen coordinates (ray casting).
        vtkSmartPointer<vtkPropPicker>  picker = vtkSmartPointer<vtkPropPicker>::New();
        picker->Pick(clickPos[0], clickPos[1], 0, this->GetDefaultRenderer());

        // Get the cell of a 3D object under the 2D screen coordinates (ray casting).
        vtkSmartPointer<vtkCellPicker>  cellPicker = vtkSmartPointer<vtkCellPicker>::New();
        cellPicker->Pick(clickPos[0], clickPos[1], 0, this->GetDefaultRenderer());

        //if something was picked
        if( picker->GetActor() && cellPicker->GetCellId() != -1 ) {

            // Get the picked location in world coordinates.
            double* pos = picker->GetPickPosition();

            // Output the picked location.
            Application::instance()->logInfo( "Picked location (world coordinates): X="
                      + QString::number(pos[0]) + " Y=" + QString::number(pos[1])
                      + " Z=" + QString::number(pos[2]) );


    m_isLBdown = false;
    m_isDragging = false;

    // Forward the event to the superclass.

I hope this helps.



Hi Paulo,
thank you for your help, but your code doesn’t work, because “this->GetDefaultRenderer()” crashes. But now, i found the solution. In the example, I substitutet "this->Interactor->GetRenderWindow()->GetRenderers()->GetFirstRenderer() " and passed the Renderer as a parameter. Now it works.