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IDL: Volume Visualization

1. 3-D Transformations
2. Visualizing an Iso-Surface
3. The IDL Slicer
4. Displaying a Surface with the Slicer

3-D Transformations

a=read_binary('/usr/local/rsi/idl_5.4/idl_5.4/examples/data/head.dat', $
data_dims=[80,100,57],data_type=1)

There are a number of ways to set up a transformation matrix in IDL.  One way is that a transformation matrix can be entered explicitly into the system variable !P.T. This method is rather difficult, because you have to figure out the transformation yourself.

Another method, is the SURFACE and SHADE_SURF commands, which automatically create a 3-D transformation based on the datasets being visualized.

1. For example, specify a slice of the data:

slice=a[*,*,25]

2. Now surface the slice specified:

surface, slice

A number of different IDL procedures that simplify the creation of 3-D transformations can be used. Keyword arguments to some of these procedures allow you to set viewing angles and data ranges. The procedures then create the appropriate transformation matrix for you and store it in !P.T. These procedures include T3D, SCALE3, SCALE3D, and SURFR.

Visualizing an Iso-Surface

Like many other IDL commands, POLYSHADE accepts the T3D keyword that makes POLYSHADE use a user-defined 3D transformation. Before you can use POLYSHADE to render the final image, you need to set up an appropriate three-dimensional transformation. The XRANGE, YRANGE, and ZRANGE keywords accept 2-element vectors, representing the minimum and maximum axis values, as arguments.The POLYSHADE function returns an image based upon the list of vertices, v, and list of polygons, p. The T3D keyword tells POLYSHADE to use the previously-defined 3D transformation. The TV procedure displays the shaded-surface image.  Enter the following lines:

1. Create the polygons and vertices that define the iso-surface with a value of 70.  Return the vertices in v and the polygons in p:

shade_volume,a,70,v,p,/low

2. Set appropriate limits for the X, Y, and Z axes with the SCALE3 procedure:

scale3,xrange=[0,80],yrange=[0,100],zrange=[0,57]

3. Display a shaded-surface representation of the previously generated arrays of vertices and polygons:

tv,polyshade(v,p,/t3d)

The IDL Slicer

The IDL Slicer provides many other volume visualization techniques. As the name implies, the slicer allows you to look at slices through a volume dataset.

1. To use the slicer with dataset a, it is first required to pass in a pointer to the data set by entering the following at the IDL Command Line:

adata=ptr_new(a)

2. Then enter:

slicer3,adata

3. The IDL Slicer appears. The Slicer window will come up empty by default though the data is loaded. Be sure that the Mode pull-down menu is set to Slice which is the default. Position the pointer within the cube. Hold down the left mouse button and move the mouse.  An outline of the cutting plane appears.  This plane moves only in the direction indicated by the orientation display.

Note: These slices look smoothed because the slicer uses bilinear interpolation by default. Smoothing can be turned on and off by clicking on the Interpolation buttons.

4. To make slices in different orientations, move the cursor into the large drawing window and press the right mouse button.

5. Click the right button a second time to see the third possible orientation.

6. Make slices in these orientations by clicking on the mouse button and dragging the cutting plane outline to the desired location.

Displaying a Surface with the Slicer

1. To create a surface in Slicer similar to the one you created previously at the IDL command line, click on the Surface option on the Mode pull-down menu on the Slicer. A Surface Threshold window, a slider, and a number of new buttons should appear.

2. Click in the Surface Threshold and slide the determiner line to choose the Display button. A status window reports on the number of vertices and polygons generated and then the iso-surface appears.

When you are done experimenting with the Slicer, before continuing with other
sections in this tutorial, you should dismiss the Slicer window.

3. To exit the volume window, enter the following at the Command Line:

wdelete

4. To exit the IDL Slicer, choose the path File -> Quit.