strange result when show polygon using vtkPolyData

Support
1

I want to show a simple polygon using vtkPolyData / vtkPolygon , but seems the result is strange

Points

    points->InsertNextPoint(0, -1, 2);
    points->InsertNextPoint(1, -0.5, 2);
    points->InsertNextPoint(2, -3, 2);
    points->InsertNextPoint(2, 3, 2);
    points->InsertNextPoint(1, 0.5, 2);
    points->InsertNextPoint(0, 1, 2);

Results:

image

#include <vtkActor.h>
#include <vtkCellArray.h>
#include <vtkNamedColors.h>
#include <vtkNew.h>
#include <vtkPointData.h>
#include <vtkPoints.h>
#include <vtkPolyData.h>
#include <vtkPolyDataMapper.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkRenderer.h>
#include <vtkTriangle.h>
#include <vtkUnsignedCharArray.h>

// For compatibility with new VTK generic data arrays.
#ifdef vtkGenericDataArray_h
#define InsertNextTupleValue InsertNextTypedTuple
#endif
#include <vtkPolygon.h>

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

    // Setup points.
    vtkNew<vtkPoints> points;
    points->InsertNextPoint(0, -1, 2);
    points->InsertNextPoint(1, -0.5, 2);
    points->InsertNextPoint(2, -3, 2);
    points->InsertNextPoint(2, 3, 2);
    points->InsertNextPoint(1, 0.5, 2);
    points->InsertNextPoint(0, 1, 2);

    // Define some colors.
    unsigned char red[3] = {255, 0, 0};
    unsigned char green[3] = {0, 255, 0};
    unsigned char blue[3] = {0, 0, 255};

    // Setup the colors array.
    vtkNew<vtkUnsignedCharArray> colors;
    colors->SetNumberOfComponents(3);
    colors->SetName("Colors");

    // Add the three colors we have created to the array.
    colors->InsertNextTupleValue(red);
    colors->InsertNextTupleValue(green);
    colors->InsertNextTupleValue(blue);
    colors->InsertNextTupleValue(red);
    colors->InsertNextTupleValue(green);
    colors->InsertNextTupleValue(blue);

    // Create a triangle.
    vtkNew<vtkCellArray> triangles;
    vtkNew<vtkPolygon> triangle;
    triangle->GetPointIds()->SetNumberOfIds(6);
    triangle->GetPointIds()->SetId(0, 0);
    triangle->GetPointIds()->SetId(1, 1);
    triangle->GetPointIds()->SetId(2, 2);
    triangle->GetPointIds()->SetId(3, 3);
    triangle->GetPointIds()->SetId(4, 4);
    triangle->GetPointIds()->SetId(5, 5);
    triangles->InsertNextCell(triangle);

    // Create a polydata object and add everything to it.
    vtkNew<vtkPolyData> polydata;
    polydata->SetPoints(points);
    polydata->SetPolys(triangles);
    polydata->GetPointData()->SetScalars(colors);

    // Visualize
    vtkNew<vtkPolyDataMapper> mapper;
    mapper->SetInputData(polydata);

    vtkNew<vtkActor> actor;
    actor->SetMapper(mapper);

    vtkNew<vtkRenderer> renderer;
    vtkNew<vtkRenderWindow> renderWindow;
    renderWindow->AddRenderer(renderer);
    renderWindow->SetWindowName("TriangleColoredPoints");

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

    renderer->AddActor(actor);
    renderer->SetBackground(nc->GetColor3d("SlateGray").GetData());

    renderWindow->Render();
    renderWindowInteractor->Start();

    return EXIT_SUCCESS;
}