MIT Laboratory for Computer Science, March 2003
417
Billboard Clouds for Extreme Model Simplification
Xavier D´
ecoret, Fr´
edo Durand, Franc¸ois X. Sillion, Julie Dorsey
Introduction: We introduce billboard clouds – a new approach for extreme simplification in the context of real-
time rendering. 3D models are simplified onto a set of planes with texture and transparency maps. We present an
optimization approach to build a billboard cloud for a textured polygonal model and a given geometric error threshold.
We demonstrate our technique on a large class of models, including smooth manifolds and composite objects.
(a) (b) (c) (d)
Figure 1: Example of a billboard cloud: (a) Original model (5,138 polygons) (b) false-color rendering using one color per billboard
to show the faces that were grouped on it (c) View of the (automatically generated) 32 textured billboards (d) the billboards side by
side.
Approach:We iteratively replace large sets of faces by a textured plane. The texture is generated by projecting the
associated set of faces on the plane. The faces and the plane are chosen so that the distance between vertices and their
projection is bounded (error threshold), and that the projected area of faces is maximized. To approximate the whole
model with a minimum number of planes, we uses a greedy approach that iteratively chooses planes that approximate
great quantities of faces, which we call dense planes. Density is evaluated in plane space, using a discretization and
adaptative refinement for efficiency. A good surface approximation is ensured by favoring planes that are “nearly
tangent” to the model. This method does not require connectivity information, but still avoids cracks by projecting
primitives onto multiple planes when needed. To avoid large textures with a lot of empty space, which can occur when
distant parts of the model are simplified on the same plane, our algorithm takes into account the compacity of the
generated textures when selecting set of faces to be simplified. For extreme simplification, our approach combines the
strengths of mesh decimation and image-based impostors. Billboard clouds is a new representation that can be used
not only for rendering but also quick shadow display or fast ray-object intersection as shown on Figure 2.
(a) (b) (c) (d)
Figure 2: Billboard clouds encode an object shape and appearance through a few quads and transparent textures.
This representation can be used for fast rendering of complex objects (a), for shadow computations (b) or ray-object
intersection (c). Storing additional information such as normal map in the textures also allow real-time relighting for
improved realism (d).
Progress:The simplification of complex object with billboard clouds gives good results. The shape and details can be
captured with a surprisingly low number of billboards. But this low number of billboards can not be straightforwardly
compared to the number of faces in a mesh simplification. The rendering of a billboard cloud is penalized mainly
by the fill rate : when 100 interleaved billboards are rendered, lots of pixels are drawn many times (the worst case
being parallel billboards viewed from front). Therefore, a billboard clouds is useful when its projected screen size is
