McUsd Model

This model is meant as a simple test scene with a variety of UsdPreviewSurface 2.4 materials and other USD elements, such as a camera and two light sources (not visible by default). There are two versions: 1 meter blocks and 10 cm blocks for viewing on phones, etc.

McUsd: JG-RTX textures, rendered in Omniverse

Clockwise from “12 o’clock”: diamond block, iron block, gold block, fern, prismarine, sunflower, purple stained glass, rails, chiseled quartz block atop quartz pillar, piston, and in the middle is lava.

Note: here and further on, you can click on the image to see the full resolution version, which may be scaled down to fit on this README page.

Model Sources

This model was generated by laying down a few blocks in Classic (Java) Minecraft, then using Mineways to export the model to USDA format. The JG-RTX resource pack, released under the Creative Commons Attribution-ShareAlike 4.0 International Public License, is used at a resolution of 256x256 textures for the surfaces. Textures are of five possible types:

Albedo/Opacity - sRGB color RGBA file with no suffix to the root file name, e.g., “dirt.png”
Normal - linear RGB vector directions in tangent space, mapped from (-1,-1,-1) to (1,1,1), suffix “_n.png”
Roughness - grayscale roughness mapped to 0.0 (smooth) to 1.0 (rough), suffix “_r.png”
Metallic - metalness, typically binary, mapped to 0.0 (non-metallic) to 1.0 (metallic), suffix “_m.png”
Emissive - sRGB emissive color, suffix “_e.png”

Technical Details

The McUsd.usda file is provided so that it is simple to examine the lights, camera, and model itself. As USD uses a film-industry specification, it is not obvious wha the camera’s field of view is. The camera in McUsd has a field of view of 60 degrees horizontally, almost 36 degrees vertically. It is also specified in

Near the top of the file is the line:

double3 xformOp:scale = (100, 100, 100)

By default USD uses centimeters as its units. Minecraft blocks are 1 meter “in real life”, so this scale setting makes it so that the meshes are properly meters. Doing so saves having a lot of zeroes in the USDA file.

Note that the USD “metersPerUnit” could have been set from the default of 0.01 to 1, but doing so can lead to a camera specification issue.

For now, the distant Sun light and environment DomeLight are set in this file to be invisible at the start. They have been given intensity values that work well for NVIDIA’s USD Composer. For viewing in UsdView, change the Sun intensity from 1560 to about 30000 and the DomeLight intensity from 32 to 0.2.

Currently the UsdLux and related light specifications use a film-related relative pair of values, “exponent and intensity”, not tied to any physical units. See, for example, this light comparison page showing how different renderers respond differently to lights. Lighting in USD looks to evolve to something providing a more physically based specification, which will help improve model interchange among applications.

Another element not fully specified for the UsdPreviewSurface material is the full meaning of the “emissiveColor”. As of Version 2.4 it is minimally specified as the “Emissive component.” There are two questions that affect rendering: 1) how does an object with an emissive color appear when directly viewed? and 2) how does this emission color work with other lights?

For the first question, one idea is to say that the emissiveColor should be treated as a fixed color for the surface, the color that is always shown. However, USD has an elaborate camera model, including an exposure attribute, which implies that the appearance of the light should change as the exposure changes.

For both questions, in McUsd the emissive lava_still and lava_flow textures are scaled by a factor of 20:

def Shader “emissive_texture” { float4 inputs:scale = (20, 20, 20, 1.0) }

Doing so gives off a reasonable amount of light to surrounding objects when viewed in NVIDIA’s USD Composer. This scale factor may need to be rescaled for other applications, and may need modification in the future depending on how the specification evolves. Also, note this scaling factor can be applied only to textures, not colors. USD Composer is used because it provides access to various global illumination renderers so can show the effect of emissive surfaces on the surround environment.

USD Composer Adjustments

In the McUsd.usda file are some render settings at the top that are specific to NVIDIA’s USD Composer:

bool “rtx:indirectDiffuse:enabled” = 1 bool “rtx:raytracing:fractionalCutoutOpacity” = 1 token “rtx:rendermode” = “PathTracing” float3 “rtx:sceneDb:ambientLightColor” = (0, 0, 0)

The indirectDiffuse hint gives better lighting; in turn, the last line turns off USD Composer’s ambient lighting. Turning on the fractionalCutoffOpacity favors quality over speed for the USD Composer real-time renderer, providing better rendering for glass and other semitransparent objects. The rendermode hint asks for path tracing to be used on load.

In addition, the Camera uses:

custom vector3d omni:kit:centerOfInterest = (0, 0, -850)

to provide a hint to the user interface for where the camera should focus when rotating around the scene. The Euler angle order is ZXY, allowing roll along the Z (view) axis, pitch along the X (left/right) axis, and yaw along the Y (up) axis.

USDView

It is possible to build usdview from scratch, but NVIDIA’s Omniverse Launcher provides a pre-built version of the USDView application. Images shown were generated with USDView 0.23.2.

Load procedure to match camera: File -> Open and select McUsd.usda. Press F11 to toggle on the hierarchy view (if not already visible). Open the “root” by double-clicking on it. Select the “Camera” and right-click, then pick (at the bottom) “Set As Active Camera”.

Here is the rendering of McUsd in USDView, using NVIDIA’s prebuilt version 0.23.2:

USDView version 0.23.2

As expected from a basic rasterizer, shadows, reflections, and emitted light from surfaces are not rendered. By default, USDView adds a light “at the eye”, which is shown in the rendering above. This additional light can be turned off via the viewport’s Lights -> Enable Default Camera Light. The DomeLight in McUsd appears to have no effect in USDView, neither to direct illumination nor as a background environment map.

Variant Model

Two test models are provided:

McUsd.usda - the main test file.
McUsd_10cm.usda - instead of making the blocks 1 meter wide, make each 10 cm wide, by changing the scale factor.

To see exact differences, perform a “diff” of the USDA files. USDZ versions of each model are also provided. The 10 cm version of the file is useful for XR applications, as this version of the model then easily fits inside a room by default.

License

CC-NC-BY-SA

Textures from the JG-RTX resource pack, which has the same Creative Commons license.

Contact

Email the author, Eric Haines, at erich@acm.org.

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