vtkPolygon concave rendering bug

Development
1

A concave vtkPolygon has rendering issues. I have tested this with multiple different shapes; the example I provide below is minimalistic. The bug propagates into other filters as well; for example a vtkGeometryFilter on the concave polygon will result in the same triangle shape as indicated by the rendering example.

2 
#include <vtkActor.h>
#include <vtkCamera.h>
#include <vtkDataSetMapper.h>
#include <vtkIdList.h>
#include <vtkNew.h>
#include <vtkPolyData.h>
#include <vtkProperty.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkRenderer.h>

int main(int, char*[])
{
    vtkNew<vtkPoints> points;
    vtkNew<vtkPolyData> polyData;
    polyData->SetPoints(points);
    polyData->Allocate(1);
    vtkNew<vtkIdList> list;
    list->InsertNextId(points->InsertNextPoint(0.0, 0.0, 0.0));
    list->InsertNextId(points->InsertNextPoint(3.0, 0.0, 0.0));
    list->InsertNextId(points->InsertNextPoint(3.0, 2.0, 0.0));
    list->InsertNextId(points->InsertNextPoint(2.0, 2.0, 0.0));
    list->InsertNextId(points->InsertNextPoint(2.0, 1.0, 0.0));
    list->InsertNextId(points->InsertNextPoint(1.0, 1.0, 0.0));
    list->InsertNextId(points->InsertNextPoint(1.0, 2.0, 0.0));
    list->InsertNextId(points->InsertNextPoint(0.0, 2.0, 0.0));
    polyData->InsertNextCell(VTK_POLYGON, list);

    vtkNew<vtkDataSetMapper> mapper;
    mapper->SetInputData(polyData);

    vtkNew<vtkActor> wireActor;
    wireActor->SetMapper(mapper);
    wireActor->GetProperty()->SetRepresentationToWireframe();
    wireActor->GetProperty()->SetColor(1.0, 0, 0);
    wireActor->GetProperty()->SetLineWidth(5);

    vtkNew<vtkActor> surfaceActor;
    surfaceActor->SetMapper(mapper);
    surfaceActor->GetProperty()->SetRepresentationToSurface();
    surfaceActor->GetProperty()->SetColor(1.0, 1.0, 1.0);

    vtkNew<vtkRenderer> renderer;
    vtkNew<vtkRenderWindow> renderWindow;
    renderWindow->AddRenderer(renderer);
    vtkNew<vtkRenderWindowInteractor> iren;
    iren->SetRenderWindow(renderWindow);

    renderer->AddActor(surfaceActor);
    renderer->AddActor(wireActor);
    renderWindow->SetSize(1500, 1500);
    renderWindow->SetWindowName("testPolygon");

    renderWindow->Render();

    iren->Start();

    return EXIT_SUCCESS;
}
3

Results in:

image
image1500Ɨ1564 54.7 KB

Note the shape should be ā€˜Uā€™ shaped. The bug fills in the part in the upper middle. The red wire outline is the correct shape.

4

Not supported by VTK afaik.

5

Graphics hardware uses a primitive method to tessellate polygons, and it assumes the polygons are convex. Use vtkTriangleFilter to fix the rendering issues.

1 Like
6

Thanks! Looking further at it appears you may be right, and I got fooled by it, i.e. vtkGeometryFilter appears to be working as intended as well.