Interactive Order-Independent Transparency

Correctly rendering non-refractive transparent surfaces with core OpenGL functionality [9] has the vexing requirements of depth-sorted traversal and nonintersecting polygons. This is frustrating for most application developers using OpenGL because the natural order of scene traversal (usually one object at a time) rarely satisfies these requirements. Objects can be complex, with their own transformation hierarchies. Even more troublesome, with advanced graphics hardware, the vertices and fragments of objects may be altered by user-defined per-vertex or per-fragment operations within the GPU. When these features are employed, it becomes intractable to guarantee that fragments will arrive in sorted order for each pixel. The technique presented here solves the problem of order dependence by using a technique we call depth peeling. Depth peeling is a fragment-level depth sorting technique described by Mammen using Virtual Pixel Maps [7] and by Diefenbach using a dual depth buffer [3]. Though no dual depth buffer hardware fitting Diefenbach’s description exists, Bastos observed that shadow mapping hardware in conjunction with alpha test can be used to achieve the same effect [2]. Using this variation of depth peeling, each unique depth in the scene is extracted into layers, and the layers are composited in depth-sorted order to produce the correctly blended final image. The peeling of a layer requires a single order-independent pass over the scene. Figure 1 contrasts correct and incorrect rendering of transparent surfaces. (a) (b)