Volume example fails: Shader failed to compile

[mvkma]

Hello,

I'm trying to run a slightly modified version of the volume example but it fails with the following error message:

 JS Error => 2025-09-16 15:21:08.233 (   0.258s) [          4BFA94]   vtkShaderProgram.cxx:1145   ERR| vtkShaderProgram (0x1523ef0): ERROR: 0:273: '!=' : wrong operand types - no operation '!=' exists that takes a left-hand operand of type 'highp float' and a right operand of type 'const int' (or there is no acceptable conversion)
 JS Error => 2025-09-16 15:21:08.233 (   0.258s) [          4BFA94]vtkOpenGLGPUVolumeRayCa:2855   ERR| vtkOpenGLGPUVolumeRayCastMapper (0x93dcf0): Shader failed to compile

The only change I made was to remove the VtkRemoteView in lines 108 to 111.

I traced back the origin of the shader code and the problematic line seems to come from here: vtkVolumeShaderComposer.h#L792. The variable range is a float and the 0 is interpreted as an int so the comparison fails.

However, this code has not been touched in a long time, so I first wanted to confirm that this is not due to a problem with my setup.

I use Firefox 142.0.1 (64-bit) on Linux and ran the example as follows:

> git clone 'https://github.com/Kitware/trame-vtklocal'
Cloning into 'trame-vtklocal'...
remote: Enumerating objects: 1841, done.
remote: Counting objects: 100% (264/264), done.
remote: Compressing objects: 100% (148/148), done.
remote: Total 1841 (delta 137), reused 181 (delta 111), pack-reused 1577 (from 2)
Receiving objects: 100% (1841/1841), 20.91 MiB | 32.20 MiB/s, done.
Resolving deltas: 100% (1036/1036), done.

> cd trame-vtklocal

> uv venv
Using CPython 3.12.10
Creating virtual environment at: .venv
Activate with: source .venv/bin/activate

> uv pip install trame "trame-vtklocal[vtk]" trame-vuetify trame-vtk
Resolved 33 packages in 189ms
Installed 33 packages in 25ms
 + aiohappyeyeballs==2.6.1
 + aiohttp==3.12.15
 + aiosignal==1.4.0
 + attrs==25.3.0
 + contourpy==1.3.3
 + cycler==0.12.1
 + fonttools==4.59.2
 + frozenlist==1.7.0
 + idna==3.10
 + kiwisolver==1.4.9
 + matplotlib==3.10.6
 + more-itertools==10.8.0
 + msgpack==1.1.1
 + multidict==6.6.4
 + numpy==2.3.3
 + packaging==25.0
 + pillow==11.3.0
 + propcache==0.3.2
 + pyparsing==3.2.4
 + python-dateutil==2.9.0.post0
 + pyyaml==6.0.2
 + six==1.17.0
 + trame==3.12.0
 + trame-client==3.10.2
 + trame-common==1.0.1
 + trame-server==3.6.0
 + trame-vtk==2.9.1
 + trame-vtklocal==0.15.2
 + trame-vuetify==3.0.3
 + typing-extensions==4.15.0
 + vtk==9.5.1
 + wslink==2.4.0
 + yarl==1.20.1

> uv run python ./examples/vtk/volume.py

[mvkma]

Looking a bit more at what the relevant shader code does I found that I can work around this by disabling gradient opacity on the volume property, e.g. with volumeProperty.SetDisableGradientOpacity(True).

I still think there is a problem in the shader code I linked to, but maybe this helps to explain why I'm running into this now.

[jourdain]

Volume rendering is not yet fully supported in VTK.wasm. @jspanchu will know more.