def compute(self):
        r = self.get_input("renderer").vtkInstance
        window = vtk.vtkRenderWindow()
        w = self.force_get_input("width", 512)
        h = self.force_get_input("height", 512)
        window.OffScreenRenderingOn()
        window.SetSize(w, h)
        # r.ResetCamera()

        widget = None
        if system.systemType=='Darwin':
            from PyQt4 import QtCore, QtGui
            widget = QtGui.QWidget(None, QtCore.Qt.FramelessWindowHint)
            widget.resize(w, h)
            widget.show()
            window.SetWindowInfo(str(int(widget.winId())))    
       
        window.AddRenderer(r)
#        window.Start()
        window.Render()
        win2image = vtk.vtkWindowToImageFilter()
        win2image.SetInput(window)
        win2image.Update()
        writer = vtk.vtkPNGWriter()
        if hasattr(writer, 'SetInput'):
            writer.SetInput(win2image.GetOutput())
        else:
            # VTK 6 uses SetInputData
            writer.SetInputData(win2image.GetOutput())
        output = self.interpreter.filePool.create_file(suffix='.png')
        writer.SetFileName(output.name)
        writer.Write()
        window.Finalize()
        if widget == None:
            widget.close()
        self.set_output("image", output)
dataset/ETHPy150Open VisTrails/VisTrails/vistrails/packages/vtk/offscreen.py/VTKRenderOffscreen.compute/operator `!=` -> `==`