Backwards Memory Allocation and Improved OIT

Abstract

Order independent transparency (OIT) is a graphics technique which sorts surfaces per-pixel for correct alpha blending. The sorting stage requires relatively large amounts of temporary memory in shaders that is usually conservatively allocated at a maximum, which impacts occupancy and performance. To address this issue we introduce backwards memory allocation (BMA), a strategy which creates a set of shaders with varying static allocation size in lieu of dynamic allocation. Batches of threads are then executed directly with the appropriate shader. This also allows optimizations for each generated shader such as choosing the sorting algorithm based on allocation size with no additional overhead. BMA gives both a more flexible OIT (BMA-OIT) for dynamic scenes of varying depth complexity and up to a 3x speedup.