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PhysX Research: Mass-Conserving Liquids and cloth with Long Range Attachments

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More interesting papers from PhysX research team and Dr. Matthias Müller-Fischer, PhysX SDK Research Lead in NVIDIA.

First paper, called “Mass-Conserving Eulerian Liquid Simulation“, present latest advancements in hybrid fluid solver development – a topic of active research for past several years [previous work].

We present a GPU friendly, Eulerian, free surface fluid simulation method that conserves mass locally and globally without the use of Lagrangian components. Local mass conservation prevents small scale details of the free surface from disappearing, a problem that plagues many previous approaches, while global mass conservation ensures that the total volume of the liquid does not decrease over time. Our method handles moving solid boundaries as well as cells that are partially filled with solids. Due to its stability, it allows the use of large time steps which makes it suitable for both off-line and real-time applications.

We achieve this by using density based surface tracking with a novel, unconditionally stable, conservative advection scheme and a novel interface sharpening method. While our approach conserves mass, volume loss is still possible but only temporarily. With constant mass, local volume loss causes a local increase of the density used for surface tracking which we detect and correct over time. We also propose a density post-processing method to reveal sub-grid details of the liquid surface.We show the effectiveness of the proposed method in several practical examples all running either at interactive rates or in real-time.

At some point this research may be made into new APEX module, according to our information.

Another new paper “Long Range Attachments – A Method to Simulate Inextensible Clothing in Computer Games” – presents a new method to effectively simulate high-resolution inextensible clothing on characters withing a typical resource budget of a computer game.

Inextensibility is one of the most fundamental properties of cloth. Existing approaches to handle inextensibility often require solving global non-linear systems and remain computationally expensive for computer game uses. Real time performance can be achieved by allowing damping or stretching at reduced solver costs, but these compromise visual realism – the cloth either looks stretchy or fine wrinkles get lost.

Our long range attachment (LRA) method exploits that typical game character clothing tends to be attached to some kinematic parts of the character. LRA method applies unilateral distance constraint between free particles of the cloth to distant attachment point on the character, preventing them from stretching away from the kinematically driven attachments (e.g. shoulder for a cape). This simple step provides an efficient shortcut for enforcing global inextensibility that can be readily implemented into existing game physics methods such as PBD.

Likewise, we are expecting this improvement to be integrated into APEX Clothing module in the future.

Update: Long Range Attachments were implemented in the latest version of PhysX 3.x cloth solver, which is now available as part of APEX Clothing 1.2 module.

Written by Zogrim

July 20th, 2012 at 6:20 pm

One Response to 'PhysX Research: Mass-Conserving Liquids and cloth with Long Range Attachments'

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  1. The Hybrid Fluid Solver is pretty amazing :)




    21 Jul 12 at 2:12 am

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