Optimal depth buffer for low-cost graphics hardware

Abstract

3D applications using hardware depth buffers for visibility testing are confronted with multiple choices of buffer types, sizes and formats. Some of the options are not exposed through 3D API or may be used by the driver without application’s knowledge. As a result, it becomes increasingly diffkult to select depth buffer optimal for desired balance between performance and precision. In this paper we provide comparative evaluation of depth precision for main depth buffer types with different size and format combinations. Results indicate that integer storage is preferred for some buffer types, while others achieve maximal depth resolution with floating-point format optimized for known scene parameters. We propose to give 3D applications full control of the depth buffer optimization by supporting multiple storage formats with the same buffer size and exposing them in 3D API. In the search for a unified depth buffer solution, we describe new type of the depth buffer and compare it with other options. Complementary floating-point Z buffer is a combination of a reversed-direction Z buffer and an optimal floating-point storage format. Non-linear mapping and storage format compensate each other’s effect on the depth precision; as a result, depth errors become significantly less dependent on the eye-space distance, improving depth resolution by the orders of magnitude in comparison with standard Z buffer. Results show that complementary Z buffer is also superior to inverse W buffer at any storage size. At 16 and 24 bits/pixel, average depth errors of complementary Z buffer remain 2 times larger than for true W buffer utilizing expensive high-precision per-pixel division. However, it provides absolutely best precision at 32 bits/pixel, when errors are limited by floating-point per-vertex input. Results suggest that complementary floating-point Z buffer can be considered as a candidate for replacement of both screen Z and inverse W buffers, at the same time making hardware investment in the true W buffer support less attractive. CR