UV Preservation

Existing UVs can be preserved after processing with RapidPBR, as long as the materials do not require full baking.

RapidPBR converts materials differently depending on their complexity, and will always attempt to preserve the existing UVs if possible. However some material setups require the creation of a new "atlas" UV layout, with no overlaps, and with all the UVs arranged within the 0-1 UV space.

Atlas UVs are generated if a material uses geometry-dependent inputs, and if the existing UV has overlaps or is not within the 0-1 UV space. 

Examples of geometry-dependent inputs: VRayDirt, VRayCurvature, Vertex Color, procedural textures (e.g. Noise, Checker, Gradient Ramp, etc.). 

Atlas UVs are also generated for materials relying on specific UV layouts, for example UDIMs or textures using Real-World Scale.

The metal material on the left has no texture inputs, so the UVs are simply ignored. The brick and fabric materials in the middle use textures and color adjustments, so they can re-use their existing UV layouts. The chipped-plastic example on the right uses procedural textures and edge wear, which are both geometry-dependent, so this material must be baked into an "atlas" UV layout.