Guide for beginners: What is OpenSubdiv?

OpenSubdiv (often used when programming 3D configurators) refers to a set of open source libraries that implement a powerful subdivision surface evaluation on parallel CPU and GPU architectures. This codepath is optimized for drawing deforming surfaces with static topology at interactive frame rates.

OpenSubdiv is an API that is ready to be integrated with third-party digital content creation tools. It is neither an application nor a tool that can be used directly to create digital assets.

Why Fast Subdivision?

Subdivision Surfaces are often used for the final rendering of character shapes for smooth and controllable interfaces. However, Subdivision Surfaces are typically drawn in interactive applications, as is the case with the polygonal control hull. The polygonal control hull is an approximation offset from the true interface. When looking at an approximation in the interactive app, it is difficult to see the exact contact, such as fingers touching a water bottle or hands touching a cheek. It also makes it difficult to see breakthroughs in fabric simulation when skin and fabric are both approximations. This problem is particularly severe when one sign is much larger than another and uneven subdivision area sizes cause approximation errors to be magnified.

Maya and Pixar’s proprietary Presto animation system can take 100 ms to divide a character from 30,000 polygons into the second subdivision (500,000 polygons). The ability to perform the same operation in less than 3 ms allows the user to interact with the smooth, accurate boundary at any time.

Research.

The new GPU technology behind OpenSubdiv is the result of joint research by Pixar and Microsoft.

• “Adaptive GPU rendering of Catmull-Clark-Subdivision Surfaces with adaptive graphics card” by Niessner, Loop, Meyer and DeRose.
• “Efficient evaluation of semi-smooth wrinkles in Catmull-Clark-Subdivision Surfaces” by Niessner, Loop and Greiner.
• “Analytical Displacement Mapping with Hardware Tesselation” by Niessner and Loop.

History.

This is the fifth generation subdivision library used by Pixar’s priority animation system in a line that began in 1996 with code by Tony DeRose and Tien Truong for Geri’s game. Each generation was a completely rewritten version, building on Pixar’s experience with Subdivision Surfaces for developing animated films. This code is online, so that Pixar’s changes to OpenSubdiv for current and future movies will be released as open source, while simultaneously introducing them to Pixar animation production.

• “Subdivision for modeling and animation” by Zorin and Schroder.
• “Subdivision Surfaces in character animations” by DeRose, Kass and Truong.
• “Recursively generated B-Pline surfaces on any topological mesh” by Catmull and Clark.

Licensing.

OpenSubdiv is licensed under Apache and may be used freely for commercial or non-commercial purposes. This is the same code that Pixar uses internally for animated film production.

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