The best open source alternative to Rhinoceros is Blender. If that doesn't suit you, our users have ranked more than 50 alternatives to Rhinoceros and nine of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Rhinoceros are FreeCAD, Bforartists, CAD Sketcher and Sverchok.
A comprehensive open-source 3D software suite for modeling, UV unwrapping, texturing, rigging, and animating. It supports renderings, simulations including fluid and cloth dynamics, complex compositing, video editing, and Python scripting, running across all major operating systems.
A powerful open-source parametric 3D modeler ideal for mechanical engineering, supporting Python scripting, running on Windows, Linux, and Mac, with modular architecture allowing extensive adaptability and use with scientific computing libraries.
Bforartists is a complete 3D suite to create 3D content. It offers you the full 3D art pipeline to create game graphics, prerendered movies and stills.
CAD Sketcher is a free and open-source project looking to enhance precision workflows in Blender by bringing CAD like tools, features and usability to Blender.
Sverchok is a parametric tool for architects and designers. You can use it to program objects' shapes without knowing any programming languages.
K-3D is free-as-in-freedom 3D modeling and animation software. It features a plugin-oriented procedural engine for all of its content, making K-3D a very versatile and powerful package.
Gmsh is a 3D finite element grid generator with a build-in CAD engine and post-processor. Its design goal is to provide a fast, light and user-friendly meshing tool with parametric input and advanced visualization capabilities.
Ayam is a free 3D modelling environment for the RenderMan interface, distributed under the BSD licence. This means that neither the author nor any contributors make money out of this software.
A game engine written in Python, extending support across multiple GPU devices with 3D rendering, PBR Renderer and much more.
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Can't stop laughing. As NURBS was the main concern ("don't use for real life" posted anypoop) and is now fully integrated, what else but snobbery could keep easy-payers wasting money on a proprietary software?