Tree Rendering with Billboard Clouds

This paper presents a method to render complex trees on high frame rates while providing accurate occlusion and parallax effects. Based on the recognition that a planar billboard is accurate if the represented polygon is in its plane, we find a billboard for each of those groups of tree leaves that lie approximately in the same plane. The tree is thus represented by a set of billboards, called billboard cloud. The billboards are built automatically by a clustering algorithm. Unlike classical billboards, the billboards of a billboard cloud are not rotated when the camera moves, thus the expected occlusion and parallax effects are provided. On the other hand, this approach allows the replacement of a large number of leaves by a single semi-transparent quadrilateral, which considerably improves the rendering performance. A billboard cloud well represents the tree from any direction and provides accurate depth values, thus the method is also good for shadow, obscurances, and indirect illumination computation. In order to provide high quality results even if the observer gets close to the tree, we also propose a novel approach to encode textures representing the billboards. This approach is called indirect texturing and generates very high resolution textures on the fly while requiring just moderate amount of texture memory.