Time series in vtkjs

Web Support
1

Hi all,

We use the vtk (python) to generate .vtkjs file that we load in a viewer using the vtkjs library.

I am interested in understanding how I can use time series in the above-mentioned workflow. I see in this example that a series of .vtp files are necessary for a time series animation. Is this the only way to have a time series animation?

Our use case
We use scalars of celldata to mount simulation data. In one particular case, we have 8760 arrays of simulation data. In this case, the geometry data (celldata) remains the same and only the scalar gets updated at each time step. If we produce 8760 .vtp files, the geometry data will be duplicated and it will unnecessarily increase storage needs. Is there a way to avoid this?

In my search for an answer, I do see objects such as vtkMultiBlockDataSet and vtkTemporalDataset. Can I use any of those objects to achieve what I have mentioned above?

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2

The way things get solved to prevent duplication with the vtkjs format is that we have all the native arrays in a same directory which are named with a hash. So even if we create a lot of polydata they actually point to the same memory space and therefore no duplication really happen.

Another approach could be to handle manually the swap of the scalar field on the JS side while keep a single polydata.

Also the most relevant example could be that one.

3

This is good and I can create a workflow to write time steps into a .vtkjs file. I spent some time trying to understand the structure of the .vtkjs file by looking into the timesteps.vtkjs used in the example pointed by you.

These are the questions I have;
1

  1. Why there’s a “1” folder in the .vtkjs? Is it necessary to name time steps in decimals such as 0.1, 0.2, etc?

  2. I see that a new object named “animation” is added to the index.json shown in the picture above. Is there a script some where that I can use?

  3. Does following folder structure make sense for my use case?

gridbased.vtkjs
    0
        0 (This is time step zero. All the sub-folders are for each datasets in the model.
        The index.jsons points to the data folder.)
            Aperture
                index.json
            Floor
                index.json
            Grid
                index.json
            RoofCeiling
                index.json
            Shade
                index.json
            Wall
                index.json
        1 (This is time step one. All the subfolders are for each datasets in the model.
        The index.jsons poin to the data folder.)
            Aperture
                index.json
            Floor
                index.json
            Grid
                index.json
            RoofCeiling
                index.json
            Shade
                index.json
            Wall
                index.json
            index.json
            This file maps folders 0 and 1 to time steps 0 and 1
    data
        all the unique binary files for all the time steps   
    index.json

In my case, only the values in a scalar in the dataset name “Grid” changes.

I am trying to learn the folder structure of this vtkjs file with time series data so that I can write a script to automate this.

4

The folder structure is really up to you. What matter is how you describe your various index.json to point to the right data.
To some extent I think you should be able to structure your data like so (did not validate my assumption)

/index.json
/data/*all binary *
/datasets/Aperture_000.json
/datasets/Aperture_001.json
...
1 Like
5

The current format is:

  • index.json : should contains an array named scene containing all of your objects in your scene. These objects are defined by their name, mapper, … and their vtkHttpDataSetSeriesReader::url, that is the name of the subfolder containing all of their data for all timesteps
  • in these subfolders you’ll find :
    • another index.json that maps an url (ie the name of a folder on the same level) to a timestep value,
    • a list of folder, one per timestep. In these folders you’ll see yet another index.json telling which data to use to construct the dataset (points cells, data arrays, etc)

So basically you can name your folders by whatever the name you want, however you cannot change the global form of the hierarchy (ROOT/DataSetName/TimeStepName/index.json).

As for a script generating the animation object in the root index.json, check Web/Python/vtkmodules/web/vtkjs_helper.py and Remoting/Animation/vtkSMAnimationSceneWebWriter.cxx, this should lead you to the right direction.

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6

The zipAllTimeSteps function takes which directory_path as an input? I believe this function does some of the automation I planned to do.

I am not sure if I can use vtkSMAnimationSceneWebWriter object. I am only using vtk python not paraview python.

8

Hi @timothee.chabat,
Can you please help?

9

Indeed you cannot use vtkSMAnimationSceneWebWriter if you want to only depend on VTK.

This is vtkSMAnimationSceneWebWriter algorithm :

  • create a place-holder index.json at the root of a folder with all necessary keys,
  • for each timestep:
    • use the vtkJSONSceneExporter class to export each frame in its own folder, at the same level of index.json
    • add the current timestep time to the root index.json (see index.json below)
  • at this point we have a folder, let’s say /home/user/somewhere/, containing :
    • index.json
    • scene_timestep1/
    • scene_timestep2/
    • […]
  • call zipAllTimeSteps('/home/user/somewhere/myFinalExport.vtkjs'). After that /home/user/somewhere/ will only contain myFinalExport.vtkjs.

If you wish to reimplement this algorithm you can pretty much copy paste the zipAllTimeSteps function (keep in mind the license though :slight_smile: ) and reimplement vtkSMAnimationSceneWebWriter algorithm.