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# Loading scalar data: the `heart.vtk` example

This section describes a simple example with the `heart.vtk` file. This is a simple volume of 3D data (32 x 32 x 12 points) with scalars at each point (the points are equally spaced). The data is a structured dataset (an ImageData in fact), we’ll read more about these later but you can think of it as a cube of points regularly spaced with some scalar data associated with each point. The data apparently represents a CT scan of a heart. I have no idea whose heart! The file is a readable text file, look at it in a text editor if you’d like to.

> 1. With `mayavi2` started, we start by opening the data file. Go to the File->Load data->Open file menu item and then in the file dialog, navigate to the directory that contains the sample data. There select the `heart.vtk` file.

>    Once you choose the data, you will see a new node on the Mayavi tree view on the left that says VTK file (heart.vtk). Note that you will not see anything visualized on the TVTK scene yet.

> 2. To see an outline (a box) of the data, navigate to the Visualize->Modules menu item and select the Outline module. You will immediately see a white box on the TVTK scene. You should also see two new nodes on the tree view, one called Colors and legends and one underneath that called Outline.

> 3. You can change properties of the outline displayed by clicking on the Outline node on the left. This will create an object editor window on left bottom of the window (the object editor tab) below the tree view. Play with the settings here and look at the results. If you double-click a node on the tree view it will pop up an editor dialog rather than show it in the embedded object editor.

>    Note that in general, the editor window for a Module will have a section for the Actor, one for the Mapper and one for Property. These refer to TVTK/VTK terminology. You may think of Properties as those related to the color, representation (surface, wireframe, etc.), line size etc. Things grouped under Actor are related to the object that is rendered on screen and typically the editor will let you toggle its visibility. In VTK parlance, the word Mapper refers to an object that converts the data to graphics primitives. Properties related to it will be grouped under the Mapper head.

> 4. To interact with the TVTK scene window, look at the section on Interaction with the scene for more details. Experiment with these options till you are comfortable.

> 5. Now, with the Outline node selected, create an iso-surface by selecting the Visualize->Modules->IsoSurface menu item. You will see a new IsoSurface node on the left and an iso-contour of the scalar data on the scene. The iso-surface is colored as per the particular iso-value chosen. Experiment with the settings of this module.

> 6. To produce meaningful visualizations you need to know what each color represents. To display this legend on the scene, click on the Colors and legends node on the tree view and on the object editor activate the Show scalar bar check-box. This will show you a legend on the TVTK scene. The legend can be moved around on the scene by clicking on it and dragging on it. It can also be resized by clicking and dragging on its edges. You can change the nature of the color-mapping by choosing various options on the object editor.

> 7. Create a simple “grid plane” to obtain an idea of the actual points on the grid. This can be done using the GridPlane module, and created via the Visualize->Modules->GridPlane menu item.

> 8. You can delete a particular module by right clicking on it and choosing delete. Try this on the GridPlane module. Try the other right click menu options as well.

> 9. Experiment with the ContourGridPlane module and also the ScalarCutPlane module a little.

>    The ScalarCutPlane module features a very powerful feature called 3D widgets. On the TVTK scene window you will see a cut plane that slices through your data showing you colors representing your data. This cut plane will have a red outline and an arrow sticking out of it. You can click directly on the cut plane and move it by dragging it. Click on the arrow head to rotate the plane. You can also reset its position by using the editor window for the scalar cut plane.

> 10. You can save the visualization to an image produced by clicking on the little save icon on the TVTK scene or via any of the options on the File->Save Scene As menu.

You should have a visualization that looks something like the one shown below.

The nice thing about mayavi is that although in this case all of the above was done using the user interface, all of it can be done using pure Python scripts as well. More details on this are available in the mlab: scripting for 3D plotting section (see also Advanced Scripting with Mayavi for a more in-depth coverage).

Opening data files and starting up modules can also be done from the command line. For example we could simply have done:

    $ mayavi2 -d /path/to/heart.vtk -m Outline -m IsoSurface \
    > -m GridPlane -m ScalarCutPlane

More details are available in the Command line arguments section.

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