Volume Rendering

Volume render uniform mesh types like pyvista.UniformGrid or 3D NumPy arrays.

This also explores how to extract a volume of interest (VOI) from a pyvista.UniformGrid using the pyvista.UniformGridFilters.extract_subset() filter.

import pyvista as pv
from pyvista import examples

# Download a volumetric dataset
vol = examples.download_knee_full()
vol

Header	Data Arrays
UniformGrid Information N Cells 10225800 N Points 10368384 X Bounds 0.000e+00, 1.497e+02 Y Bounds 0.000e+00, 1.786e+02 Z Bounds 0.000e+00, 2.000e+02 Dimensions 208, 248, 201 Spacing 7.230e-01, 7.230e-01, 1.000e+00 N Arrays 1	Name Field Type N Comp Min Max SLCImage Points uint8 1 0.000e+00 1.740e+02

Simple Volume Render

# A nice camera position
cpos = [(-381.74, -46.02, 216.54), (74.8305, 89.2905, 100.0), (0.23, 0.072, 0.97)]

vol.plot(volume=True, cmap="bone", cpos=cpos)

Opacity Mappings

Or use the pyvista.Plotter.add_volume() method like below. Note that here we use a non-default opacity mapping to a sigmoid:

pl = pv.Plotter()
pl.add_volume(vol, cmap="bone", opacity="sigmoid")
pl.camera_position = cpos
pl.show()

You can also use a custom opacity mapping

opacity = [0, 0, 0, 0.1, 0.3, 0.6, 1]

pl = pv.Plotter()
pl.add_volume(vol, cmap="viridis", opacity=opacity)
pl.camera_position = cpos
pl.show()

We can also use a shading technique when volume rendering with the shade option

pl = pv.Plotter(shape=(1, 2))
pl.add_volume(vol, cmap="viridis", opacity=opacity, shade=False)
pl.add_text("No shading")
pl.camera_position = cpos
pl.subplot(0, 1)
pl.add_volume(vol, cmap="viridis", opacity=opacity, shade=True)
pl.add_text("Shading")
pl.link_views()
pl.show()

Cool Volume Examples

Here are a few more cool volume rendering examples.

Head Dataset

head = examples.download_head()

pl = pv.Plotter()
pl.add_volume(head, cmap="cool", opacity="sigmoid_6", show_scalar_bar=False)
pl.camera_position = [(-228.0, -418.0, -158.0), (94.0, 122.0, 82.0), (-0.2, -0.3, 0.9)]
pl.camera.zoom(1.5)
pl.show()

Bolt-Nut MultiBlock Dataset

Note

See how we set interpolation to 'linear' here to smooth out scalars of each individual cell to make a more appealing plot. Two actor are returned by add_volume because bolt_nut is a pyvista.MultiBlock dataset.

bolt_nut = examples.download_bolt_nut()

pl = pv.Plotter()
actors = pl.add_volume(bolt_nut, cmap="coolwarm", opacity="sigmoid_5", show_scalar_bar=False)
actors[0].prop.interpolation_type = 'linear'
actors[1].prop.interpolation_type = 'linear'
pl.camera_position = [(127.4, -68.3, 88.2), (30.3, 54.3, 26.0), (-0.25, 0.28, 0.93)]
cpos = pl.show(return_cpos=True)

Frog Dataset

frog = examples.download_frog()

pl = pv.Plotter()
pl.add_volume(frog, cmap="viridis", opacity="sigmoid_6", show_scalar_bar=False)
pl.camera_position = [(929.0, 1067.0, -278.9), (249.5, 234.5, 101.25), (-0.2048, -0.2632, -0.9427)]
pl.camera.zoom(1.5)
pl.show()

Extracting a VOI

Use the pyvista.UniformGridFilters.extract_subset() filter to extract a volume of interest/subset volume to volume render. This is ideal when dealing with particularly large volumes and you want to volume render only a specific region.

# Load a particularly large volume
large_vol = examples.download_damavand_volcano()
large_vol

Header	Data Arrays
UniformGrid Information N Cells 11003760 N Points 11156040 X Bounds 4.130e+05, 6.920e+05 Y Bounds 3.864e+06, 4.096e+06 Z Bounds -5.479e+04, 5.302e+03 Dimensions 280, 233, 171 Spacing 1.000e+03, 1.000e+03, 3.535e+02 N Arrays 1	Name Field Type N Comp Min Max data Points float32 1 9.782e-15 1.000e+02

opacity = [0, 0.75, 0, 0.75, 1.0]
clim = [0, 100]

pl = pv.Plotter()
pl.add_volume(
    large_vol,
    cmap="magma",
    clim=clim,
    opacity=opacity,
    opacity_unit_distance=6000,
)
pl.show()

Woah, that’s a big volume! We probably don’t want to volume render the whole thing. So let’s extract a region of interest under the volcano.

The region we will extract will be between nodes 175 and 200 on the x-axis, between nodes 105 and 132 on the y-axis, and between nodes 98 and 170 on the z-axis.

voi = large_vol.extract_subset([175, 200, 105, 132, 98, 170])

pl = pv.Plotter()
pl.add_mesh(large_vol.outline(), color="k")
pl.add_mesh(voi, cmap="magma")
pl.show()

Ah, much better. Let’s now volume render that region of interest!

pl = pv.Plotter()
pl.add_volume(voi, cmap="magma", clim=clim, opacity=opacity, opacity_unit_distance=2000)
pl.camera_position = [
    (531554.5542909054, 3944331.800171338, 26563.04809259223),
    (599088.1433822059, 3982089.287834022, -11965.14728669936),
    (0.3738545892415734, 0.244312810377319, 0.8947312427698892),
]
pl.show()