Volumetric Billboards

The Volumetric Billboards representation extends the classic billboards representation used to render complex 3D objects in real-time. It uses volumetric images of an object stored into 3D textures, instead of 2D images as for classic billboards. Combined with a dedicated real-time rendering algorithm based on GPU geometry shader, volumetric billboards offer full parallax effect of the objects from any viewing direction without popping artifact, along with improved anti-aliasing of distant objects. The objects can have transparency and can be arbitrarily distributed in a 3D scene. The algorithm correctly handles transparency between the multiple and possibly overlapping objects. Furthermore, volumetric billboards can be easily integrated into common rasterization-based renderers, which allows for their concurrent use with polygonal models and standard rendering techniques such as shadow-mapping.

They may be of interest in many cases including levels-of-detail representation for multiple intersecting complex objects, volumetric textures, animated complex objects and construction of high-resolution objects by assembling instances of low-resolution volumetric billboards.

More details and comparison with other techniques can be found in this paper.

Representation

Our representation is composed of a set of triangular prism-shaped 3D cells arbitrarily positioned in a 3D scene with 3D texture coordinates at their vertices, and a set of volume data stored into 3D textures which are mapped into the cells. In typical uses, the cells are likely to overlap.

Here, the eiffel volume resolution is 256x128x128 (4.8 MB of compressed texture memory) and each tree is a 128³ volume (2.4 MB of compressed texture memory for colors).

Volume data generation

Volume data representing an object is created offline. It is stored into a MIP-mapped 3D texture. The building process (figure below) ensures that view-dependant information is preserved as long as possible.

Voxelization of an object is performed by rendering 6 stacks of axis-aligned slices of the object. Each stack of slices is obtained by fitting an orthographic camera on the object along one of the 6 directions (+X, -X, +Y, -Y, +Z, -Z) and then, for each slice image, by setting the near and far clip planes of the camera to match the slice position and thickness.

From these 6 stacks of slices we get 6 3D volumes. Each volume is MIP-mapped independently. For each level of MIP-map, the 6 corresponding volumes are then combined into one volume. These volumes finally form the 3D MIP-mapped texture of the object.

Rendering algorithm

At runtime, volumetric billboards are rendered with a dedicated slice-based volume rendering algorithm that renders all the volumetric billboards at once and correctly handles transparency without requiring any sorting.

To that purpose, the algorithm generates slice planes parallel to the screen from back to front with respect to the camera. For each slice it computes all the polygons that correspond to the intersection of the given slice with all the cells. It draws these polygons and then moves to the next slice. Thus, the polygons are drawn in a back-to-front order with respect to the camera and do not need any additionnal sorting for correct transparency compositing.
Slicing: Intersection of all cells with slice 1 is drawn first, then with slice 2, and so on.

Slab partitioning : The algorithm avoids slicing the regions with no cells and avoids considering all the cells for intersection by partitioning the space with planes orthogonal to the viewing direction axis (slabs) before drawing the cells.

Adaptive slicing : The distance between two successive slices is adapted to render distant volumetric billboards more efficiently while ensuring accurate anti-aliasing and MIP-map. This also avoids over or under-sampling artifacts during the slice-based volume reconstruction.

Prism/plane intersection

The performance of the slicing algorithm relies on the computation of the prism/plane intersections. This step is performed on GPU through a geometry shader.

The result of the intersection of a prism with a plane may be empty or may be a polygon having 3, 4 or 5 edges depending on the plane/prism configurations. The shader algorithm is depicted by the figure and pseudo-code below.

Some results

- Ph. Decaudin, F. Neyret. Volumetric Billboards. Computer Graphics Forum, Volume 28 (8), p 2079-–2089, 2009.

- My other publications.