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Visualization: VTK and Blender

VTK and Blender

Notes:

  • 2008-07-03: Jon Crall from Kitware requested access to UV coordinates so that he could use the models from Big Buck Bunny in VTK. The function BlenderToPolyData() now will export UV coords if they exist, and has an optional argument to specify the UV layer (the active layer will be used if no argument is supplied). The code, examples, and documentation have also been updated to reflect the new Blender API syntax for linking objects to scenes: scene.link(object) is now supposed to be scene.objects.link(object).
  • 2006-08-23: Some bugfixes and speedups -- thanks to Fritz Mielert from the University of Stuttgart for his valuable feedback!
  • 2006-07-11: The VTKBlender.py module and the demos have been updated for VTK 5.x and for Blender 2.41 and later. These new versions no longer uses the Blender 'NMesh' module to manupulate meshes, and instead uses the new 'Mesh' module for faster access.

    The recent increase in popularity of open source software has created some interesting possibilities in the realms of computer graphics and scientific visualization. Two great software packages that exploit this development model are the scientific visualization library VTK and the 3D modeling, animation, and rendering package Blender.

    Can these packages be used together in an almost seamless fashion? Yes, and we will discuss how to do this using the open source language Python as the glue that binds these two powerful pieces of software together. We will exploit the fact that the VTK libraries have python wrappers, and Blender has a built-in python interpreter. It should be noted that a complete tutorial for beginners of VTK and Blender would be a huge undertaking, so it is recommended that the reader has some familiarity already -- check the websites of these projects for additional documentation.


    Step 1: Get the VTKBlender Module

    You can download the python module that makes this all work here: VTKBlender.py

    Make a note of what directory you save this file in, as we will need this information in the next step.


    Step 2: Getting Blender to find the VTK libraries and the VTKBlender module

    Blender's python interpreter needs to find the VTK modules and the VTKBlender module in order to work successfully. There are two ways to help python to find these modules: and you can either a) set your PYTHONPATH in your environment, or you can b) hard code the locations in your script using the sys.path variable, e.g.,

    a)

    We are assuming a UNIX environment, so Windows users might want to consult some additional documentation on how to set python related environment variables.

    For Bourne compatible shells, at the prompt, before starting blender, type:

       PYTHONPATH=$VTK_ROOT/Wrapping/Python:${LIBRARY_OUTPUT_PATH}
       PYTHONPATH=$PYTHONPATH:${PATH_TO_VTKBlender_MODULE}
       export PYTHONPATH
       

    For csh compatible shells, at the prompt, before starting blender, type:

       setenv PYTHONPATH $VTK_ROOT/Wrapping/Python:${LIBRARY_OUTPUT_PATH}
       setenv PYTHONPATH ${PYTHONPATH}:${PATH_TO_VTKBlender_MODULE}
       

    You may want to add these commands to your shell's startup scripts.

    b)

    add the following to your script near the beginning, before importing VTK or VTKBlender:

       import sys
       sys.path.append($VTK_ROOT/Wrapping/Python)
       sys.path.append(${LIBRARY_OUTPUT_PATH})
       sys.path.append(${PATH_TO_VTKBlender_MODULE})
       

    Be sure to replace $VTK_ROOT and ${LIBRARY_OUTPUT_PATH} with values that are relevant to your system. These values can be found by starting vtkpython with no arguments and typing:

       import sys
       print sys.path
       

    Usually the first two items reported are the ones you want.

    Also replace ${PATH_TO_VTKBlender_MODULE} with wherever you have put the VTKBlender module.


    What does the VTKBlender python module do?

    The VTKBlender module converts vtkPolyDataMapper's to blender meshes and converts blender meshes to vtkPolyData. There are two python functions that do these conversions:

    • VTKBlender.PolyDataMapperToBlender(pmapper, me=None)

      If the function is run with only one argument, this function takes a vtkPolyDataMapper pmapper and returns a new blender mesh with the converted polydata. A second optional argument, which takes a pre-existing blender mesh, may be provided, causing the existing mesh to be overwritten with the polydata. Please note that the new mesh is not added to the scene, and can be added afterwards via:

      sc = Blender.Scene.GetCurrent()
      ob = Blender.Object.New('Mesh')
      ob.link(me)
      sc.objects.link(ob)
      

      The reason why the function takes a vtkPolyDataMapper object as an argument (instead of a vtkPolyData object) is because the vtkPolyDataMapper can also contain a look up table to color the data, in which case the blender mesh will have vertex colors set accordingly.

    • VTKBlender.BlenderToPolyData(me, uvlayer=None)

      This function take a blender mesh and returns a vtkPolyData object that contains the geometry contained in the mesh. If UV coordinates exist they will be exported. The name of a UV layer can be used as an optional argument to export a particular UV layer. The active layer will be exported if no layer is specified, so for example if the mesh has only one UV layer, that layer will be exported without using the optional argument.


    Examples

    Example 1

    Let's test that the VTKBlender actually works by running an example file:

    VTKBlender_demo.zip

    Load this file into blender. In the main 3D window, you will see a cube, a ring, and a monkey head. Below this you will see some buttons. On the right part of the screen you will see two text editors with python scripts, the top one called vtk_to_blender.py and the other called blender_to_vtk.py.



    Example 1a

    Lets try running the top script, vtk_to_blender.py. The main purpose of this script is to demonstrate how the VTKBlender.PolyDataMapperToBlender() function can be used to get different kinds of geometry created with vtk into blender. With your mouse cursor in the top script sub-window, press Alt-P to run the script, or select A"Run Python Script" from that window's file menu.

    The script will create some points, some lines, a tube, a cube, a cylinder, and the familiar quadric isosurface from the VTK examples. Notice how the cube actually turns into the quadric isosurfaces: this is an example of VTKBlender.PolyDataMapperToBlender() being passed the cube mesh as an argument, thus overwriting the cube's mesh.

    The other thing that is of interest with this script is that it has some code to detect whether it is running inside of blender or not, and if it determines that it is not running inside blender, it will call the VTK library to render the objects created. In the script window's File menu, select save to write the script to an external text file and try it for yourself!

    Example 1b

    The next example shows interaction between VTK and Blender in the opposite direction: geometry created in blender (the monkey head) will be passed to VTK, run through a probe filter to color it (using the quadric scalar field) and returned to the blender scene as a new object. At the same time, the ring object will be passed to VTK, run through a tube filter, and returned to blender. Place your mouse cursor in the bottom text window and press Alt-P to run the script.

    Where are the colors? Lets switch to shaded mode to see them (press Shift-Z with your mouse cursor in the 3D window to switch display modes).

    We can then render this scene with blender's scanline renderer to produce the following image:

    Hmmm, that doesn't look like a particularly impressive render! Blender has a built in scanline render and a build in raytracer, and has support to use the external YAFRay raytracer. In the right hands, Blender can make some very impressive images, of which the above image is not. We'll do something a bit more fancy in the next example, but in the meantime, check out the Blender website gallery archive to see why Blender is an excellent choice for 3D Graphics:

    Blender Gallery

    Example 2

    In this example we will use Blender to animate and render a visualization created in VTK.

    Download the example file here:

    VTKBlender_demo2.zip

    The magic that runs this example is something called scriptlink-ing. This is the ability to have Blender run a python script when an event occurs.

    Scriptlinks are set up in the scriptlink window. Here we have created a FrameChanged scriptlink: we are telling Blender that we would like the script anim.py to be executed every time the frame changes. To see the effect of anim.py, change the frame using the left or right arrow keys or watch the scene being animated by pressing Alt-A when the mouse pointer is in the 3D window.

    The script anim.py creates the VTK pipeline the first time it is run, creating some isosurfaces in a field generated from a vtkQuadric object. On subsequent invocations the parameters of the quadric filter are modified based on the frame number, and new isosurfaces are created. A special module called Blender.Registry is used to preserve data between script invocation so that we don't have to constantly recreate our pipeline.

    We can use the scanline renderer to render individual frames, or we can create a 30 frame animation by pressing the Anim button.

    OK, do you remember in the last example when I said we would be doing something more fancy? Well, here it is: press the 2 key with your mouse pointer in the 3D Window to reveal the hidden layer number 2, enable the raytracer with the Ray button, and press Anim.

    We are going to be raytracing so I hope you have a lot of spare CPU time! If you wait long enough, the final result will look something like this (click to download an mpeg of the animation, let the animation loop in your viewer):

    That should be enough to get people started using VTK and Blender together. To get full use of VTK and Blender, refer to the corresponding websites to download or purchase additional documentation:

    Have fun!

    Questions or comments? Please contact: research.support@ualberta.ca


    Revised: March 15, 2005

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