Visual Artifacts Issues in Official vMaterials MDL Shaders

[wiwiwisper]

Description

Many official vMaterials in Isaac Sim exhibit visual artifacts when rendered. Some materials appear to intersect geometry or produce unexpected visual effects, making surfaces look incorrect or inconsistent. This affects a range of MDL shaders including composites, fabrics, glass, liquids, plastics, and stones. The issue is purely visual and does not affect geometry or physics.

material_issue.py

Isaac Sim version

5.1.0

Operating System (OS)

22.04

GPU Name

4090D

GPU Driver and CUDA versions

Driver Version: 570.195.03

Logs

No response

Additional information

No response

[wiwiwisper]

vMaterial's Water_Murky_Perlinwaves.mdl Carbon_Fiber.mdl Fiberglass.mdl Glass_Glazing_Spandrel.mdl Paper_Plain.mdl Polypropylene_Cloudy.mdl...

This visual error can produce an effect similar to that of a penetration model

[wiwiwisper]

After enabling /rtx/pathtracing/fractionalCutoutOpacity, the issue was resolved. This parameter appears to control how light interacts with materials that have alpha values < 1 in ray tracing:
When disabled: regions where alpha < threshold are treated as fully transparent, so light passes through directly.
When enabled: regions with alpha < 1 partially block light, showing the material's actual color and producing a semi-transparent effect.
However, it's unclear why this parameter has such a significant impact on rendering behavior. Could this be unintended or a bug? Please advise.

[vick-yu]

Great find, @wiwiwisper! You're absolutely correct that enabling /rtx/pathtracing/fractionalCutoutOpacity resolves these vMaterial rendering artifacts.

Why This Works

The fractionalCutoutOpacity setting controls how the RTX renderer handles materials with semi-transparent or cutout opacity features. By default, it's disabled for performance reasons, which causes the visual artifacts you're seeing with vMaterials (especially composites, fabrics, glass, liquids, plastics, and stones that rely on advanced opacity/alpha features).

Solution for Both Rendering Modes

This setting exists for both rendering modes:

PathTracing Mode:

import carb
carb.settings.get_settings().set("/rtx/pathtracing/fractionalCutoutOpacity", True)

RayTracing/Real-Time Mode:

import carb
carb.settings.get_settings().set("/rtx/raytracing/fractionalCutoutOpacity", True)

Documentation Gap

There's an important documentation ambiguity here: The Isaac Sim Performance Optimization Handbook mentions this setting only under "Using RTX – Realtime Mode", which could mislead PathTracing users into thinking it doesn't apply to them.

In reality, both rendering modes require this setting for proper vMaterial rendering.

Note from the handbook: "This affects opacity, and user may need to set partial opacity checkbox"

Internal Usage

This setting is already used in Isaac Sim's own Actor SDG workflows (source/tools/actor_sdg/sdg_scheduler.py:142) where it's explicitly enabled for Digital Human characters that require accurate opacity rendering.

Performance Tradeoff

Enabling this does add rendering overhead, but it's essential for accurate material visualization. For workflows requiring proper vMaterial rendering, the quality improvement is worth the performance cost.

Thanks for sharing this solution - it will help others encountering the same issue!