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 feedback is one line, history[n] = current[n] + decay * history[n-1]: fade what you kept last time, add what you were handed this time.

The frame it reads back is memory of the node, stated in the launch script as self.memory["History"]: SPG keeps it across frames instead of handing out a fresh resource, and zeroes it when it is created, so the node needs no special case for the first frame.

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

See Keep State Across Frames for what a node’s memory is and how it is stated.