Camera panning issues with asyncio loop and vtklocal.LocalView for animations

[ng-daniel]

Hello, I am having trouble debugging an issue with animating Vtk objects using an asyncio loop. In my application, I'd like to have a simple animation playing while the user can pan around the scene.

However, there is currently a bug where, when the updates are pushed from the server to the WASM client, the camera jolts towards the right and to the top. I've noticed that this jolt is often proportional to the distance from the mouse to the bottom left corner, so whatever code calculating the panning may be unable to retrieve the panning start position, and defaults to the origin as a starting point. I've attached a short screen recording to demonstrate the issue (the motorcycle model rotation is intended).

Recording.2026-04-07.160812.mp4

This example demonstration is made with code copied from the motorcycle example on the trame docs, with the only differences being:

1. Additional imports of import asyncio and import random
2. Adding the following function to the bottom of the TrameApp class:

async def animation_coroutine(self, **kwargs):
    """
    Coroutine I added to create a simple animation that plays every second.
    A simple rotation animation that randomly rotates the bike and updates the view.
    """
    
    while True:
        print("Updating animation frame")
        self.bike_actor.RotateX(random.uniform(-180, 180))
        self.ctrl.view_update(push_camera=False)
        await asyncio.sleep(1)

3. And adding the following line to the end of the init function:

"""
Code I added to start the animation coroutine when the server is ready.
"""
self.ctrl.on_server_ready.add_task(self.animation_coroutine)

The reasoning behind this animation methodology is that I was referencing this previous discussion post about animations, where an asyncio loop is suggested. However, that example did not use vtklocal.LocalView, so I guess there are some issues here that aren't present in that discussion post.

Is there a way to fix the camera panning inconsistencies, with this animation implementation approach? Or is there a better method of implementing animations for Trame? All help is appreciated.

[jourdain]

I guess we would need to investigate, but I suspect, that when the scene state update on the client side, some event handling internal state reset.

[jourdain]

This is going to be investigated here: Kitware/trame-vtklocal#64