Polypostors: 2D Polygonal Impostors for 3D Crowds

Ladislav Kavan
Trinity College Dublin
Simon Dobbyn
Trinity College Dublin
Steven Collins
Trinity College Dublin
Jiří Žára
Czech Technical University, Prague

Carol O'Sullivan
Trinity College Dublin

Our algorithm converts a 3D polygonal character (a) to 2D textured polygons, animated efficiently by displacing their vertices with associated texture coordinates (b). This view-dependent representation (which we call Polypostors) achieves dramatic simplification with low texture memory overhead. The intended application is real time rendering of large crowds (c).


Various methods have been proposed to animate and render large crowds of humans in real time for applications such as games and interactive walkthroughs. Recent methods have been developed to render large numbers of pre-computed image-based human representations (Impostors) by exploiting commodity graphics hardware, thus achieving very high frame-rates while maintaining visual fidelity. Unfortunately, these images consume a lot of texture memory, no in-betweening is possible, and the variety of animations that can be shown is severely restricted. This paper proposes an alternative method that significantly improves upon pre-computed impostors: automatically generated 2D polygonal characters (or Polypostors). When compared with image-based crowd rendering systems, Polypostors exhibit a similarly high level of rendering efficiency and visual fidelity, with considerably lower memory requirements (up to a factor of 30 in our test cases). Furthermore, Polypostors enable simple in-betweening and can thus deliver a greater variety of animations at any required level of smoothness with almost no overhead.

accompanying video


Ladislav Kavan, Simon Dobbyn, Steven Collins, Jiří Žára, Carol O'Sullivan. Polypostors: 2D Polygonal Impostors for 3D Crowds. Symposium on Interactive 3D Graphics and Games, 2008.  

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We would like to thank the anonymous reviewers for their helpful comments and suggestions. This work was supported by Science Foundation Ireland (project Metropolis).