Python: SPG Ray Generation#

An SPG node that traces the scene itself instead of post-processing an AOV. One primary ray per pixel through the render product’s camera, a shadow ray toward the light, and the exact geometric normal of every surface recovered from two extra probe rays. The whole image is produced by the node; nothing in it is a renderer AOV.

A ray-traced Cornell box rendered by a single SPG ray-generation node

Surface colour is the recovered normal, encoded and modulated by the lighting, so each face’s hue states its orientation: the floor’s +Y reads green-dominant, the left wall’s +X red-dominant, the back wall’s +Z blue-dominant. The two blocks are rotated about Y, so their faces are not axis aligned and each still resolves to its own correct normal.

A shader under a RenderProduct receives the scene acceleration structure and its camera’s transform with no authored input and no connection, which is what lets it author rays in ordinary camera space. No vertex data is exposed, so the normal is derived from the trace itself: three points on a flat triangle define its plane.

Prerequisites#

Python 3.10-3.13
uv
An NVIDIA RTX-capable GPU and a supported driver
The renderer running on Vulkan, which is the default

Running#

uv run main.py                                          # inline RayQuery
uv run main.py --scene cornell_box_pipeline_scene.usda  # ray-tracing pipeline

The same room, traced two ways. Both runs print the same checks, and the two images agree on 99.85% of pixels. Refer to Trace the Scene for what the two forms differ in.

main.py checks the render with no golden image: that no ray escaped the room, that each wall’s dominant colour channel matches its normal, that a real population of off-axis normals is present, and that both shadowed and lit pixels exist. See Trace the Scene for the ray-generation dispatch and the implicit scene binding.