Blender Geometry Nodes → Cycles Render Engine: Does Execution Order Affect Render Output?
29.5K reputation · 13 Feb 2021, 20:52 UTC
Integration Boundary: Geometry Nodes and Cycles
The goal is to determine whether the execution order of Geometry Nodes that modify the same mesh and feed into a single downstream node is explicitly defined in Blender’s documentation. Current evidence suggests the order is undocumented, potentially leading to unpredictable vertex positions and shading artifacts that differ between Blender releases and hardware configurations.
Key constraints include version variability (Blender 3.4 vs 3.5+), GPU acceleration introducing nondeterminism, and the risk that complex node trees mask the issue. A minimal test scene is required to isolate the problem and verify consistency across CPU and GPU.
Unresolved questions:
1. Is the node execution order deterministic across Blender releases when multiple Geometry Nodes output to the same downstream node?
2. Does GPU acceleration modify the implicit execution order, causing divergent render outputs?
3. Can a minimal test scene reliably expose and document the order ambiguity?
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2,070 reputation · 14 Feb 2021, 03:37 UTC
To clarify the boundary between the modifier stack and the node graph: while the previous response correctly identifies that the modifier order (top-to-bottom) is sequential, it is important to distinguish this from the internal flow of a single Geometry Nodes tree. Within a single node group, execution is determined by the connection lines (the data flow), not the physical position of nodes on the canvas.
For those testing for non-determinism across GPU/CPU, focus on Attribute persistence. If multiple branches merge into a single Join Geometry node, the final state of a named attribute depends on which geometry stream is processed last. To verify this in Blender 3.4+:
- Use the Spreadsheet Editor to inspect the final attribute values before rendering.
- If the Spreadsheet values are identical across CPU and GPU but the Cycles render differs, the issue lies in the render engine's interpolation or GPU kernel, not the Geometry Nodes execution order.