Getting Started with VTK for Python

The visualization toolkit (VTK) is a open source library displaying scientific data. VTK is maintained by Kitware, the same company which gave us CMake. VTK is written in C/C++ but it comes with Python bindings and can be installed from https://pypi.org/project/vtk/. In this post, I am going to show how to start using VTK from Python using PyQt5.

Qt has two package for using with Python: PySide2 and PyQt5. PySide2 is the official module for Python but for a long time there was no official module and only PyQt5 was available. You can refer to https://www.learnpyqt.com/blog/pyqt5-vs-pyside2/ to understand the differences (they are mostly same) between two modules. I am going to use PyQt5 but the VTK module itself supports both Qt modules.

VTK provides a QVTKRenderWindowInteractor class which inherits from QWidget, QGLWidget, or any other custom class inherited from QWidget. We will add QVTKRenderWindowInteractor to a QMainWindow and use vtkRenderer to render a Hello, World sphere. To decouple user interface (Qt) and rendering (VTK) I will create a VtkWindow class and use it from a MainWindow which is purely for VTK.

Lets first create the MainWindow:

from PyQt5 import QtCore, QtWidgets
import sys
import VtkWindow

class MainWindow(QtWidgets.QMainWindow):
	def __init__(self, parent=None):
		super(MainWindow, self).__init__(parent)
		self.setWindowState(QtCore.Qt.WindowMaximized)
		
		self.mVtkWindow = VtkWindow.VtkWindow()
		self.setCentralWidget(self.mVtkWindow)
		
if __name__ == '__main__':
	app = QtWidgets.QApplication(sys.argv)
	window = MainWindow()
	window.show()
	app.exec_()

If we comment lines 10 and 11 and run the MainWindow.py, it will display a blank Qt Window. Now lets see how to add VTK support to it by adding a VtkWindow class:

from PyQt5 import QtWidgets
import vtk
import vtkmodules.qt
vtkmodules.qt.QVTKRWIBase = "QGLWidget"
from vtk.qt.QVTKRenderWindowInteractor import QVTKRenderWindowInteractor

# VtkWindow must be derived from QFrame: https://vtk.org/Wiki/VTK/Examples/Python/Widgets/EmbedPyQt
class VtkWindow(QtWidgets.QFrame):
	def __init__(self, parent=None):
		super(QtWidgets.QWidget, self).__init__(parent)
		
		# Create a VTK widget and add it to the QFrame.
		self.setLayout(QtWidgets.QVBoxLayout())
		self.mVtkWidget = QVTKRenderWindowInteractor(self)
		self.layout().addWidget(self.mVtkWidget)
		self.layout().setContentsMargins(0, 0, 0, 0)
		
		# Get the render window and set an interactor.
		self.mRenderWindow = self.mVtkWidget.GetRenderWindow()
		self.mInteractor   = self.mRenderWindow.GetInteractor()
		self.mInteractor.SetInteractorStyle(vtk.vtkInteractorStyleTrackballCamera())
		self.mInteractor.Initialize()
		
		# Create a new renderer and set the background color.
		self.mRenderer = vtk.vtkRenderer()
		self.setBackgroundColor([0.5, 0.5, 0.5])
		self.mRenderWindow.AddRenderer(self.mRenderer)
		
		# Set the Vtk Window title.
		self.mTitleActor = None
		self.setTitle("pyVtkLib Demo")
		
	# Called when QFrame is resized.
	def resizeEvent(self, newSize):
		textSize = [0, 0]
		self.mTitleActor.GetSize(self.mRenderer, textSize)
		
		width  = int( (self.width() - textSize[0]) / 2.0)
		height = self.height() - textSize[1]
		self.mTitleActor.SetPosition(width, height - 10)
		
	def setBackgroundColor(self, color):
		self.mRenderer.SetBackground(color)
		
	def setTitle(self, title):
		if not self.mTitleActor:
			self.mTitleActor = vtk.vtkTextActor()
			self.mTitleActor.GetTextProperty().SetFontFamilyAsString("Georgia")
			self.mTitleActor.GetTextProperty().SetFontSize(30)
			self.mTitleActor.GetTextProperty().SetColor([1, 0, 0])
			self.mTitleActor.SetInput(title)
			self.mTitleActor.SetPosition(0, 0)
			self.mRenderer.AddActor(self.mTitleActor)
		else:
			self.mTitleActor.SetInput(title)
	

VTK module for Python comes with a QVTKRenderWindowInteractor class which by default inherits from QWidget for PyQt5. In lines 4-5, we first change it to to use QGLWidget so that rendering will be done using OpenGL instead of software renderer. Next, we create a class called VtkWindow which inherits from QWidget so that it can be use from Qt UI. Note, that it is recommended to inherit from QFrame and not QWidget as QVTKRenderWindowInteractor cannot be reparented. More discussion on this topic can be found at EmbedPyQt example on VTK website. Next, we create an instance of QVTKRenderWindowInteractor and add it to VtkWindow class through a QVBoxLayout.

After that it is usual VTK stuff of creating a vtkRenderingWindow, vtkRenderWindowInteractor, and vtkRenderer. I prefer to use vtkInteractorStyleTrackballCamera which I find far more intuitive than the default vtkInteractorStyleJoystickCamera.

I render scene title at the top-middle of the screen and in order to place it here I listen to QFrame::resizeEvent to determine te current width and height of the QFrame.

Run the MainWindow.py from a terminal and it will display a windows with text pyVtkLib Demo printed in the middle-center of the window. In the next tutorial I will show how to measure and show frames per second to the VtkWindow.

The code from this tutorial and any other future enhancements I will do will be available from saurabhg17/pyVtkLib repository at GitHub.