IR restarts with IK and Constraint tags

[JonStoll]

I have do not have a fix yet and am shocked this is not being addressed. I have a very standard setup . latest graphics driver and everything. I have worked hard with chat gpt with this and found out a few things. I had some python script that was old and causing issues but is fixed now then I ran into this which is not fixable for 30 scenes I have .
TITLE
Corona for Cinema 4D IR continuously restarts when IK and Constraint tags are enabled

ENVIRONMENT

• Cinema 4D 2026.3.2
• Corona Renderer 15 Hotfix 1
• Windows 11
• Intel i9-13900K
• 128 GB RAM
• NVIDIA RTX 4090
• Scene: TW_Scene_07_Created_v04.c4d

SUMMARY
Corona Interactive Rendering repeatedly restarts or remains in a preparing/parsing loop while standard Cinema 4D IK and Constraint tags are enabled on the character rig.

If I disable the rig’s IK and Constraint tags, Corona IR works normally and accumulates passes. This is not a practical workaround because the rig must remain active during animation and look development.

A separate Python PointRig tag was previously found to force spline updates every evaluation cycle. That script was repaired. The current scene contains no remaining matching PointRig Python tags, but the Corona IR problem continues when IK and Constraints are enabled.

CONFIRMED BEHAVIOR

1. With IK and Constraint tags enabled, Corona IR repeatedly restarts or fails to settle.
2. Disabling those tags allows IR to render normally.
3. Right-clicking anywhere in the Cinema 4D Object Manager temporarily allows IR to begin clearing. When the context menu is closed, the instability returns.
4. The issue occurs at a stationary timeline frame.
5. No measurable transform changes were detected during repeated evaluations of the same frame.

DIAGNOSTIC RESULTS
A whole-scene diagnostic repeatedly evaluated the document at the same frame:

• Objects scanned: 230
• Expression-type tags found: 7
• Warmup passes: 2
• Measured passes: 10

The strongest repeated updates were:

LEFT WRIST
Object:
Left Arm / _L_Ruka / _L_Plecho / _L_Locot / _L_Kist

• No measurable transform change
• MATRIX dirty counter changed on 10 of 10 passes
• Enabled Cinema 4D Constraint tag
• Target:
  Left Arm / _L_Kist MOVE / _L_Kist_Rotate
• Priority mode: 2
• Priority value: 0
• Camera Dependent: False
• The Constraint tag’s own dirty counters did not change

RIGHT WRIST
Object:
Right Arm / _R_Ruka / _R_Plecho / _R_Locot / _R_Kist

• No measurable transform change
• MATRIX dirty counter changed on 10 of 10 passes
• Enabled Cinema 4D Constraint tag
• Target:
  Right Arm / _R_Kist MOVE / _R_Kist_Rotate
• Priority mode: 2
• Priority value: 0
• Camera Dependent: False
• The Constraint tag’s own dirty counters did not change

DOWNSTREAM GEOMETRY
The arm meshes and Subdivision Surface objects were also marked dirty on every pass:

• Left arm polygon: DATA=10, CACHE=10
• Right arm polygon: DATA=10, CACHE=10
• Left Subdivision Surface: CACHE=10
• Right Subdivision Surface: CACHE=10

No measurable transforms changed during these evaluations.

EXPECTED RESULT
Corona IR should continue accumulating while a valid Cinema 4D rig with standard IK and Constraint tags is enabled, as long as the final evaluated scene state has not meaningfully changed.

ACTUAL RESULT
Corona IR repeatedly prepares, parses, or restarts while the rig expressions are enabled. Disabling the expressions stabilizes IR but breaks the live rig.

REPRODUCTION STEPS

1. Open TW_Scene_07_Created_v04.c4d.
2. Use Corona 15 Hotfix 1 as the renderer.
3. Keep the character IK and Constraint tags enabled.
4. Start Corona Interactive Rendering.
5. Observe repeated preparing/parsing/restarting.
6. Right-click an item in the Object Manager and leave the context menu open.
7. Observe that IR temporarily begins to settle.
8. Close the context menu.
9. Observe that the instability returns.
10. Disable the rig’s IK and Constraint tags.
11. Restart IR.
12. Observe that IR now accumulates normally.
13. Re-enable the tags.
14. Observe that the problem returns.

WORKAROUNDS TESTED

• Repaired continuously updating PointRig Python tags: helped other files, but not this remaining issue.
• Disabled IK and Constraint tags: resolves IR, but disables the animated rig.
• Camera Dependent priority: disabled on the main detected wrist Constraint tags.
• Baking the entire rig and disabling expressions is not a reasonable general workflow for look development and animation.

LIKELY INTERPRETATION
Cinema 4D may legitimately reevaluate IK and Constraints and mark matrices or deformation caches dirty even when the resulting transforms do not numerically change.

Corona IR appears to react to these repeated dirty/evaluation notifications as if meaningful geometry changed, causing repeated rebuilds or restarts.

QUESTIONS

1. Is Corona IR known to restart from standard Cinema 4D IK or Constraint evaluation when final matrices remain unchanged?
2. Can Corona distinguish a redundant dirty/evaluation notification from an actual transform or geometry change?
3. Is there a debug log that identifies which Cinema 4D notification triggers each IR restart?
4. Can Corona ignore unchanged matrix or deformation-cache evaluations?
5. Is this related to the known Corona for Cinema 4D expression issue?
6. Can this be fixed so live rigs remain enabled during IR?

FILES TO ATTACH

• TW_Scene_07_Created_v04.c4d, collected with assets
• Short screen recording showing:
  1. IR restarting with expressions enabled
  2. IR settling while the Object Manager context menu is open
  3. IR working after IK and Constraint tags are disabled
• Cinema 4D Console diagnostic output
• Corona log files

I am not attaching these as my rigs are pretty basic. But if someone can help I can send some files.

[JonStoll]

I used chat to make a scirpt that selects all the ik and constraints in your project and in the Attribute Basic menu you and uncheck Enable then you can use the IPR for your lighting and materials , then use the script to select them all and turn them back on .

run this in Python

import c4d


def iter_objects(obj):
    """Walk through every object in the document hierarchy."""
    while obj:
        yield obj

        child = obj.GetDown()
        if child:
            yield from iter_objects(child)

        obj = obj.GetNext()


def main():
    doc = c4d.documents.GetActiveDocument()

    if not doc or not doc.GetFirstObject():
        c4d.gui.MessageDialog("The current document contains no objects.")
        return

    # Standard C4D rigging tag types to select.
    tag_types = {
        c4d.Tcaconstraint,  # Constraint
        c4d.Tcaik,          # IK
        c4d.Tcaikspline     # IK-Spline
    }

    found_tags = []

    for obj in iter_objects(doc.GetFirstObject()):
        tag = obj.GetFirstTag()

        while tag:
            if tag.GetType() in tag_types:
                found_tags.append(tag)

            tag = tag.GetNext()

    if not found_tags:
        c4d.gui.MessageDialog(
            "No Constraint, IK, or IK-Spline tags were found."
        )
        return

    # Select the first tag and begin a new tag selection.
    doc.SetActiveTag(
        found_tags[0],
        c4d.SELECTION_NEW
    )

    # Add all remaining tags to the selection.
    for tag in found_tags[1:]:
        doc.SetActiveTag(
            tag,
            c4d.SELECTION_ADD
        )

    # Refresh Cinema 4D's internal tag selection state.
    doc.GetActiveTag()
    c4d.EventAdd()

    c4d.gui.MessageDialog(
        "Selected {} Constraint/IK tags.\n\n"
        "Use the Basic tab → Enabled to turn them all off or on.".format(
            len(found_tags)
        )
    )


if __name__ == "__main__":
    main()

[maru]

Hi, thanks for your hard work! Please contact us here with a sample scene: https://support.chaos.com/hc/en-us/requests/new

[JonStoll]

Sure I submitted my files to your link. It would be great to know how to fix this.

[maru]

Thank you. This will be continued in support ticket #499409.