how does the cow.vtp is generated about vtk-js

[jourdain]

Volume rendering is not yet supported by the vtk.js library.
VTK.js is still in development and hasn't been released yet.
The cow was generated using ParaView with the Python script from the repository.

If you have ParaView on your system and node.js, you can run the following:

$ npm install -g kitware/vtk-js
$ vtkDataConverter 

  Usage: vtkDataConverter [options]

  Options:

    -h, --help                output usage information
    -V, --version             output the version number
    -i, --input [file.vtk]    Input data file to convert
    -o, --output [directory]  Destination directory

    -m, --merge               Merge dataset
    -e, --extract-surface     Extract surface

    --paraview [path]         Provide the ParaView root path to use
    --sample-data [path]      Convert sample data from ParaViewData