MATLAB®

Exploring Volumes with Slice Planes

Example — Slicing Fluid Flow Data

A slice plane (which does not have to be planar) is a surface that takes on coloring based on the values of the volume data in the region where the slice is positioned. Slice planes are useful for probing volume data sets to discover where interesting regions exist, which you can then visualize with other types of graphs (see the slice example). Slice planes are also useful for adding a visual context to the bound of the volume when other graphing methods are also used (see coneplot and Example — Stream Line Plots of Vector Data for examples).

Use the slice function to create slice planes. This example slices through a volume generated by the flow M-file.

1. Investigate the Data

Generate the volume data with the command

[x,y,z,v] = flow;

Determine the range of the volume by finding the minimum and maximum of the coordinate data.

xmin = min(x(:)); 
ymin = min(y(:)); 
zmin = min(z(:));

xmax = max(x(:)); 
ymax = max(y(:)); 
zmax = max(z(:));

2. Create a Slice Plane at an Angle to the X-Axes

To create a slice plane that does not lie in an axes plane, first define a surface and rotate it to the desired orientation. This example uses a surface that has the same x- and y- coordinates as the volume.

hslice = surf(linspace(xmin,xmax,100),...
	linspace(ymin,ymax,100),...
	zeros(100));

Rotate the surface by -45 degrees about the x-axis and save the surface XData, YData, and ZData to define the slice plane; then delete the surface.

rotate(hslice,[-1,0,0],-45)
xd = get(hslice,'XData');
yd = get(hslice,'YData');
zd = get(hslice,'ZData');
delete(hslice)

3. Draw the Slice Planes

Draw the rotated slice plane, setting the FaceColor to interp so that it is colored by the figure colormap, and set the EdgeColor to none. Increase the DiffuseStrength to .8 to make this plane shine more brightly after adding a light source.

h = slice(x,y,z,v,xd,yd,zd);
set(h,'FaceColor','interp',...
	'EdgeColor','none',...
	'DiffuseStrength',.8)

Set hold to on and add three more orthogonal slice planes at xmax, ymax, and zmin to provide a context for the first plane, which slices through the volume at an angle.

hold on
hx = slice(x,y,z,v,xmax,[],[]);
set(hx,'FaceColor','interp','EdgeColor','none')

hy = slice(x,y,z,v,[],ymax,[]);
set(hy,'FaceColor','interp','EdgeColor','none')

hz = slice(x,y,z,v,[],[],zmin);
set(hz,'FaceColor','interp','EdgeColor','none')

4. Define the View

To display the volume in correct proportions, set the data aspect ratio to [1,1,1] (daspect). Adjust the axis to fit tightly around the volume (axis) and turn on the box to provide a sense of a 3-D object. The orientation of the axes can be selected initially using rotate3d to determine the best view.

Zooming in on the scene provides a larger view of the volume (camzoom). Selecting a projection type of perspective gives the rectangular solid more natural proportions than the default orthographic projection (camproj).

daspect([1,1,1])
axis tight
box on
view(-38.5,16)
camzoom(1.4)
camproj perspective

5. Add Lighting and Specify Colors

Adding a light to the scene makes the boundaries between the four slice planes more obvious because each plane forms a different angle with the light source (lightangle). Selecting a colormap with only 24 colors (the default is 64) creates visible gradations that help indicate the variation within the volume.

lightangle(-45,45)
colormap (jet(24))
set(gcf,'Renderer','zbuffer')

The Modifying the Color Mapping section shows how to modify how the data is mapped to color.

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Modifying the Color Mapping

The current colormap determines the coloring of the slice planes. This enables you to change the slice plane coloring by

• Changing the colormap

• Changing the mapping of data value to color

Suppose, for example, you are interested in data values only between -5 and 2.5 and would like to use a colormap that mapped lower values to reds and higher values to blues (that is, the opposite of the default jet colormap).

Customizing the Colormap

The first step is to flip the colormap (colormap, flipud).

colormap (flipud(jet(24)))

Adjusting the Color Limits

Adjusting the color limits enables you to emphasize any particular data range of interest. Adjust the color limits to range from -5 to 2.4832 so that any value lower than the value -5 (the original data ranged from -11.5417 to 2.4832) is mapped into the same color. (See caxis and Axis Color Limits - The CLim Property in Axes Properties in the MATLAB^® documentation for an explanation of color mapping.)

caxis([-5,2.4832])

Adding a color bar provides a key for the data-to-color mapping.

colorbar('horiz')

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Techniques for Visualizing Scalar Volume Data

Connecting Equal Values with Isosurfaces