Rendering to image files using Python

Omniverse Developer
1

Hi,

I have set up a scene with physics enabled. I would like to start the timeline programmatically via a Python script running headlessly and render frames to image files.

I know how to do this using Isaac Sim. Is there also a way to achieve this using Kit only, without any Isaac Sim dependencies/extensions?

Best,
Bruno

2

What I have achieved so far is this:

I run a simple python script like this:

kit --exec "test.py" --no-window --enable [some extensions]

In the script I open an existing USD scene with physics driven objects:

ctx = get_context()
ctx.open_stage("omniverse://nucleus.dsdlab.db.de/Users/brunovetter/kit_test/nucleus_test_0ad68bf8.usd")
stage = ctx.get_stage()

I was also able to do some physics stepping like this, and retrieve an object’s position after each step:

...
for i in range(20):
    physx_interface.update_simulation(0.1, 0.0)
    physx_interface.update_transformations(
        True, True, True, True)
    train_world_xform = train_xform.ComputeLocalToWorldTransform(Usd.TimeCode.Default())
    train_xlat = train_world_xform.ExtractTranslation()

So here is another question: In this setup, is it possible to just start the timeline instead and use some callbacks to react on physics steps?

Thanks for any help
Bruno

3

Hi Bruno,
You should be able to start a timeline like this:
omni.timeline.get_timeline_interface().play()
(if async, add await omni.kit.app.get_app().next_update_async())

You can also add callbacks to physics events like
physx_interface.subscribe_physics_step_events(callback_fn)

Generally, I suggest you to look into World, SimulationContext and PhysicsContext classes of Isaac SIM to have an idea on how to do these. In IS we wrap these concepts around a simpler to use API but they use PhysX and Kit APIs under the hood. If you take a look into the classes you will find most of these functionalities and calls.