opacity for different mesh/actor

[dansand]

Thanks to all the developers/contributors for this terrific package.

I want to plot a scalar field (from a 2d FEM simulation) with some opacity (e.g. 0.5), and then plot contours with no opacity. What seems to happen is that the contour opacity inherits the value I set in the scalar field.

Here is the example code ( I can provide input files and/or screenshots if that's needed):

vtk_data = pv.read(vtk_file)

contours = vtk_data.contour(isosurfaces=[295, 873,973], 
                             scalars='T')

p = pv.Plotter( window_size = (800, 250), border=True, border_color='black', border_width=2.0)

p.add_mesh(vtk_data, scalars = 'stress_xx', cmap=stress_cm, opacity = 0.5,
           clim  = [-50e6, 50e6], stitle='$\sigma_{xx}$ (MPa)', show_scalar_bar=True)

p.add_mesh(contours, color="black", line_width=2, opacity = 1.0)

p.view_xy()

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[banesullivan]

What seems to happen is that the contour opacity inherits the value I set in the scalar field.

This definitely shouldn't be happening. Can you share the mesh and a screenshot?

---

Here is an example similar to what I think you are trying to do:

import pyvista as pv
from pyvista import examples

vtk_data = examples.download_st_helens()
contours = vtk_data.contour()

p = pv.Plotter()
p.add_mesh(vtk_data, opacity=0.5)    
p.add_mesh(contours, color="black", line_width=2, opacity=1.0)
p.view_xy()
p.show()

The contours have no opacity, however, they are shown intersecting the original mesh which might add some distortion in your data (perhaps the "haze" you see over the contours from the other mesh being transparent).

There are a few mitigation techniques if this is the case

1. Move the points in the z-direction so they are on top of the transparent mesh: contours.points[:, -1] += 0.001
2. special VTK intersection mapping parameters
3. depth peeling: p.enable_depth_peeling()

[dansand]

Thanks for getting back to me. I think you're right that this is a "haze" issue. When you further decrease the opacity on the other mesh, the contours stand out much more clearly:

[dansand]

import pyvista as pv
pv.set_plot_theme("document")

vtk_file =  'temperature_0.vtu'
vtk_data = pv.read(vtk_file)

contours = vtk_data.contour(isosurfaces=[295, 873,973], 
                             scalars='T')

###################################

p = pv.Plotter( window_size = (1600, 500), border=True, border_color='black', border_width=2.0)

p.add_mesh(vtk_data, scalars = 'T', stitle = 'Temperature',  cmap='viridis', 
          clim = [273, 1500],
           scalar_bar_args = dict(title_font_size=48),
           opacity=0.75)

p.add_mesh(contours, color="black", line_width=6,
           opacity=1.0)

p.camera_position = [(416.0689376056465, 92366.4072261825, -37740.71276357017),
 (416.0689376056465, 92366.4072261825, 0.0),
 (0.0, 1.0, 0.0)]

p.add_text("temperature \n opacity 75%", 
           position = 'lower_left', font_size=22, shadow=True)

p.screenshot('./detachment_paper/opacity_test/opacity_75%.png') 

p.render()
p.show()

[dansand]

temperature_0.zip

[dansand]

Moving the points in the z-direction seems to be the best workaround from the suggestions provided.

Thanks!