Document that Isaac RTX enable_shadows cannot disable shadows

[mataylor-nvidia]

Description

IsaacRtxRendererGlobalSettingsCfg.enable_shadows reads as a working shadow switch. It is not one,
and has not been since #5449. This documents the limitation and its cause.

Setting it to False leaves every camera output byte-identical.

Root cause

enable_shadows writes /rtx/shadows/enabled — the shadow switch of the RaytracedLighting
pipeline. Isaac Lab used to pin that render mode (see #310, a workaround for its casing), so the
flag worked when it was added. #5449 switched the default to RealTimePathTracing for determinism,
and the flag became inert that day, silently.

RaytracedLighting is no longer selectable at all on Isaac Sim 6.0 — requesting it is rejected the
same way a nonsense token is:

requested /rtx/rendermode	readback
RaytracedLighting	RealTimePathTracing ← rejected
BogusMode123	RealTimePathTracing ← rejected
PathTracing / Minimal / MinimalRendering / RealTimePathTracing	kept

None of the three surviving pipelines reads the setting. RealTimePathTracing and PathTracing
always cast shadows by design; Minimal uses omni:rtx:minimal:castShadows, which Isaac Sim 6.0's
OmniRtxSettingsMinimalAPI_1 does not define (it has minimal:mode, minimal:constantColor and
the two sceneDb:ambientLight* properties). The ovrtx runtime carries a newer RTX that does
define it, which is why OVRTXRendererCfg.enable_shadows works.

Measurements

Slab, 1.2 m pillar, sphere, distant light at 45 degrees, 640x480, unmodified Camera /
IsaacRtxRendererCfg, reading the color buffer.

attempt	applied	result
enable_shadows=False	renderer init	identical
settings.set("/rtx/shadows/enabled", False)	runtime	identical
--/rtx/shadows/enabled=false	Kit startup	identical
rtx.shadows.enabled = false in the .kit file	app config	identical
per-light UsdLux ShadowAPI inputs:shadow:enable=False	before and after Fabric population	identical
omni:rtx:minimal:castShadows=False, incl. with OmniRtxSettingsMinimalAPI_1 applied to the prim	4 variants, in real Minimal mode	identical

Every one reports success at the layer it touches — the carb setting reads back False, and the USD
attributes report defined=True authored=True value=False. Nothing above the renderer misbehaves;
only the consumer is missing.

Shadow-free options that do work today

• Request the albedo data type. It is unlit, so there is nothing to shadow.
• Light the scene with ambient only (ambient_light_intensity), so no directional source casts.
• Use the OVRTX renderer with OVRTXRendererCfg(enable_shadows=False) in Minimal mode (Turn off OVRTX shadows by default #7454).

There is no way to keep RTPT or Minimal lighting and drop the cast shadows on this backend. That
needs an Isaac Sim RTX uplift to the build that defines castShadows.

Note on test coverage

test_isaac_rtx_global_settings does cover this field, but asserts
settings.get("/rtx/shadows/enabled") is True against a fake settings object. It verifies that the
config field writes the carb key, never that the key does anything, so it passed unchanged through
the regression. Catching this class of bug needs a pixel comparison; the rendering suite's
make_xfail_rendering_params would be the natural home, as a non-strict xfail that reports
XPASS (REVIEW XFAIL) when a future Isaac Sim honours the setting. Happy to add that here or as a
follow-up.

Type of change

• Documentation update

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works — docstring-only change; see the test-coverage note above
• I have added a changelog fragment under source/<pkg>/changelog.d/ for every touched package
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[isaaclab-review-bot]

Isaac Lab Review Bot

The change documents that IsaacRtxRendererGlobalSettingsCfg.enable_shadows is ineffective with the RTX version shipped in Isaac Sim 6.0 and adds the corresponding isaaclab_physx changelog fragment.

• Design and architecture: The limitation is documented on the backend-specific global settings field where users encounter it. The OVRTX alternative is referenced without introducing runtime coupling between the renderer packages.
• API: The public API remains unchanged: the field retains its bool | None type, None default, name, and configuration semantics. The added documentation clarifies current backend behavior and identifies the working OVRTX counterpart.
• Implementation: The warning and changelog consistently describe the affected setting and its /rtx/shadows/enabled path. The main non-blocking maintenance consideration is that the limitation is explicitly version-specific and should be revisited when Isaac Sim's RTX runtime is upgraded.

No blocking issues. No inline issue met the actionable-evidence threshold; the assessment above records the review feedback.

Automated review; human maintainers own approval decisions.

[greptile-apps]

Greptile Summary

This PR clarifies that IsaacRtxRendererGlobalSettingsCfg.enable_shadows does not affect rendered output with the RTX version shipped in Isaac Sim 6.0.

• Expands the field docstring with the backend limitation and points users to the effective OVRTX alternative.
• Adds a corresponding isaaclab_physx changelog fragment.

Confidence Score: 5/5

The PR appears safe to merge because it only clarifies existing renderer behavior and adds a matching changelog entry.

The changes are documentation-only and introduce no runtime, build, data, or security behavior changes requiring correction.

Important Files Changed

Filename	Overview
source/isaaclab_physx/isaaclab_physx/renderers/isaac_rtx_renderer_cfg.py	Documents the existing no-op shadow setting without changing runtime behavior.
source/isaaclab_physx/changelog.d/mataylor-isaac-rtx-enable-shadows-no-op.rst	Records the documentation correction and identifies the working OVRTX alternative.

_{Reviews (1): Last reviewed commit: "Document that Isaac RTX enable_shadows c..." | Re-trigger Greptile}