Efficient depth peeling via bucket sort

In this paper we present an efficient algorithm for multi-layer depth peeling via bucket sort of fragments on GPU, which makes it possible to capture up to 32 layers simultaneously with correct depth ordering in a single geometry pass. We exploit multiple render targets (MRT) as storage and construct a bucket array of size 32 per pixel. Each bucket is capable of holding only one fragment, and can be concurrently updated using the MAX/MIN blending operation. During the rasterization, the depth range of each pixel location is divided into consecutive subintervals uniformly, and a linear bucket sort is performed so that fragments within each subintervals will be routed into the corresponding buckets. In a following fullscreen shader pass, the bucket array can be sequentially accessed to get the sorted fragments for further applications. Collisions will happen when more than one fragment is routed to the same bucket, which can be alleviated by multi-pass approach. We also develop a two-pass approach to further reduce the collisions, namely adaptive bucket depth peeling. In the first geometry pass, the depth range is redivided into non-uniform subintervals according to the depth distribution to make sure that there is only one fragment within each subinterval. In the following bucket sorting pass, there will be only one fragment routed into each bucket and collisions will be substantially reduced. Our algorithm shows up to 32 times speedup to the classical depth peeling especially for large scenes with high depth complexity, and the experimental results are visually faithful to the ground truth. Also it has no requirement of pre-sorting geometries or post-sorting fragments, and is free of read-modify-write (RMW) hazards.