How to access and modify the default gravity value of the scene using Python script in Cinema 4D?

lakpo

Hi everyone,

I'm working on a Python script in Cinema 4D and I'm trying to toggle the default gravity of the scene between 0 and -981. I've tried several approaches, but I can't seem to find the correct method to access and modify this value.

Here's what I've attempted so far:

from typing import Optional
import c4d

doc: c4d.documents.BaseDocument  # The active document
op: Optional[c4d.BaseObject]  # The active object, None if unselected

def main() -> None:
    def object() -> Optional[c4d.BaseObject]:
        return doc.GetActiveObject()

    active_object = object()
    if not active_object:
        return

    current_gravity = active_object[c4d.PBDSCENE_DEFAULTGRAVITY]
    if current_gravity == 0:
        active_object[c4d.PBDSCENE_DEFAULTGRAVITY] = -981
    else:
        active_object[c4d.PBDSCENE_DEFAULTGRAVITY] = 0

if __name__ == '__main__':
    main()
    c4d.EventAdd()

Unfortunately, this doesn't work. If anyone can point me to the correct way to access and modify this value, that would be great!

Thanks in advance for your help!

ferdinand

Hey @lakpo,

Welcome to the Maxon developers forum and its community, it is great to have you with us!

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About your First Question

The answer to your question depends on what you would consider the 'default gravity value of the scene'. There are basically two interpretations:

You want to change the value for the default simulation scene as accessible via the Attribute Manager in document mode. You would here have to dig out the default simulation scene from its scene hook. That scene object will never be part of the normal object tree and you can therefor not access it with BaseDocument.GetActiveObject. We recently talked about this very subject here including some example code.
You want to reset the gravity value of a simulation scene, no matter if the default or a custom one, to its default value. You would have to access the description of the node, get the parameter you are interested in, and then read DESC_DEFAULT.

Cheers,
Ferdinand

lakpo

Hi Ferdinand,

I just wanted to thank you for your invaluable help! Thanks to your guidance, I was able to solve my issue with toggling the default gravity in Cinema 4D. Honestly, I didn't think it would be this complicated!

For anyone who might encounter a similar problem, here's the script I used, based on your code. It allows access to the global default simulation scene in the simulation hook and toggles the gravity value between 0 and -981:

import c4d

doc: c4d.documents.BaseDocument  # The currently active document.
op: c4d.BaseObject | None  # The primary selected object in the document. Can be None.

c4d.SHsimulation: int = 1057220  # The ID of the simulation hook.

def main() -> None:
    """Called by Cinema 4D when the script is executed."""
    
    # Get the simulation hook of the active document
    simulationHook: c4d.BaseList2D | None = doc.FindSceneHook(c4d.SHsimulation)
    if not simulationHook:
        raise RuntimeError("Cannot find the simulation hook.")
    
    # Retrieve the object branch from the simulation hook
    branches: list[dict] = simulationHook.GetBranchInfo()
    data: dict | None = next(n for n in branches if n["id"] == c4d.Obase)  # We're looking for the object branch.
    if not data:
        raise RuntimeError("Malformed simulation hook with missing object branch.")
    
    # Get the simulation scene
    simulationScene: c4d.BaseObject | None = data["head"].GetFirst()
    if not simulationScene:
        raise RuntimeError("Malformed simulation hook with missing object branch children.")
    
    # Toggle gravity between 0 and -981
    if simulationScene[c4d.PBDSCENE_DEFAULTGRAVITY] == 0:
        simulationScene[c4d.PBDSCENE_DEFAULTGRAVITY] = -981
    else:
        simulationScene[c4d.PBDSCENE_DEFAULTGRAVITY] = 0

    c4d.EventAdd()

if __name__ == '__main__':
    main()

This script toggles the default gravity of the simulation scene between 0 and -981 depending on its current value.

Thanks again for your help! I hope this can be useful to others who might face the same issue.