Python: SPG Stateful Node#

An SPG node that reads back its own previous output, so a moving object drags a fading comet tail behind it. The whole node is one line, history[n] = current[n] + decay * history[n-1]: fade what you published last time, add what you were handed this time.

An output marked stateful keeps its resource across frames instead of being handed a fresh one, and is zero-initialised on first use, so the node needs no special case for the first frame. That resource is never published, so it needs no RenderVar.

SPG stateful node example output, a ball dragging a comet tail

The run writes two images from the same render step, _output/live.png and _output/trail.png, through the same node and the same tone map. Everything that differs between them came from the previous frame.

Prerequisites#

  • Python 3.10-3.13

  • uv

  • An NVIDIA RTX-capable GPU and a supported driver

Running#

uv run main.py                                # CUDA
uv run main.py --scene trail_scene_slang.usda # Slang

The node ships in both languages, so the two backends can be compared side by side. See Keep State Across Frames for what stateful means and how an output is marked.

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