Add modsim_toolbox example to showcase fVDB for multi-modal image sim

modsim_toolbox/.gitignore

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modsim_toolbox/.python-version

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+3.12

modsim_toolbox/README.md

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+# ModSim Toolbox
+
+ModSim Toolbox is a minimal example of building reusable, accelerated, and
+differentiable infrastructure for multimodal simulation on top of
+[fVDB](https://github.com/AcademySoftwareFoundation/openvdb/tree/master/fvdb).
+It is not a simulator for any particular sensor. Instead, it factors out the
+geometric work that most sensor simulations share, so that the physics
+specific to a given modality can be written separately, on top of a common
+tensor-based interface.
+
+## Components
+
+The toolbox covers three areas:
+
+- **geometry** — rotations and the coordinate frames a sensor is mounted in.
+- **voxels** — converting group-jagged meshes into an `fvdb.GridBatch`, and
+  carrying per-corner attributes onto the resulting voxels.
+- **ray tracing** — casting a bundle of rays against a voxel grid batch using
+  fVDB's voxel ray tracing, finding the nearest surface each ray hits, and
+  computing visibility between a surface and a direction (for example,
+  toward a light source).
+
+There is also a viewer (`visualization.Visualizer`) for displaying the
+result. Because ray tracing runs against voxel grids rather than triangle
+meshes, and because fVDB's operations are differentiable, the same trace can
+be used both for fast forward simulation and for gradient-based tasks such as
+calibration or inverse rendering.
+
+The toolbox does not model physics: it has no notion of spectra, materials,
+or units of radiance, and it does not read files. Every function operates on
+tensors and returns tensors, and the modules are stateless, with the
+exception of `visualization.Visualizer`, which owns a live viser server. What
+a modality's outputs represent — radiance, a range value, a label — is
+decided by the caller. This is what allows the same underlying trace to
+support different modalities, such as a panchromatic camera or a
+spectrometer, without changes to the toolbox itself.
+
+## Example
+
+[`examples/car`](examples/car) shows one modality built on the toolbox: a
+DIRSIG vehicle-glint scene, voxelized per OBJ group, imaged by the
+panchromatic frame camera described in the scene's own platform file, and
+shaded using solar irradiance and Beard-Maxwell BRDFs. The physics specific
+to that example — spectral reflectance, shading, band integration — is
+implemented in the example itself, not in the toolbox.

modsim_toolbox/examples/car/.gitignore

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+/scene

modsim_toolbox/examples/car/README.md

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+# Car: vehicle-glint example
+
+This example uses the toolbox to simulate a panchromatic frame camera imaging
+a car, based on RIT's DIRSIG vehicle-glint demo scene. The toolbox handles
+voxelization and ray tracing; this example adds the optical physics —
+solar irradiance, Beard-Maxwell BRDFs, spectral reflectance, and band
+integration — in `radiometry.py` and `shade`.
+
+## Layout
+
+| File | Contents |
+|---|---|
+| `main.py` | `build_scene`, `build_sensor`, `simulate`, `serve` |
+| `radiometry.py` | Solar spectrum, Beard-Maxwell, Fresnel, spectral resampling |
+| `loader.py` | Wavefront OBJ and DIRSIG `.platform` / `.ppd` / `.mat` / `.ems` / `.fit` parsing |
+
+## Scene data
+
+The scene data is not checked in. Download the DIRSIG vehicle-glint demo
+scene from the [DIRSIG demo scenes page](https://dirsig.cis.rit.edu/) and
+unpack it into `scene/`, subject to RIT's license terms for that data. This
+example reads five files from it:
+
+```
+scene/geometry/infiniti_g35_vn.obj   car geometry, per group, with vertex normals
+scene/demo.platform                  320x240 at 16 um, f = 100 mm, 0.400-0.800 um "Pan"
+scene/demo.ppd                       platform pose: 150 m up, yawed 135 deg
+scene/materials/demo.mat             material table, referencing:
+scene/materials/*.ems, *.fit         emissivity curves and Beard-Maxwell fits
+```
+
+The 6 km ground plane declared in the scene is replaced with a 40 m patch,
+and capture time and location are constants in `main.py` rather than parsed
+from the scene files. Everything else the demo ships is unused.
+
+## Running it
+
+```bash
+uv run python main.py
+```
+
+This opens a viser page showing the voxelized scene alongside the simulated
+focal plane, with controls for solar azimuth, elevation, and shadows.