Screen-Space Ambient Occlusion Using A-Buffer Techniques

Abstract

Computing ambient occlusion in screen-space (SSAO) is a common technique in real-time rendering applications which use rasterization to process 3D triangle data. However, one of the most critical problems emerging in screenspace is the lack of information regarding occluded geometry which does not pass the depth test and is therefore not resident in the G-buffer. These occluded fragments may have an impact on the proximity-based shadowing outcome of the ambient occlusion pass. This not only decreases image quality but also prevents the application of SSAO on multiple layers of transparent surfaces where the shadow contribution depends on opacity.We propose a novel approach to the SSAO concept by taking advantage of per-pixel fragment lists to store multiple geometric layers of the scene in the G-buffer, thus allowing order independent transparency (OIT) in combination with high quality, opacity-based ambient occlusion (OITAO). This Abuffer concept is also used to enhance overall ambient occlusion quality by providing stable results for low-frequency details in dynamic scenes. Furthermore, a flexible compression-based optimization strategy is introduced to improve performance while maintaining high quality results.

Z‐Thickness Blending: Effective Fragment Merging for Multi‐Fragment Rendering

“…It is important to represent and restore abundant geometric information with a limited number of fragments in a multi‐fragment rendering. For example, many screen‐space rendering algorithms [BCL∗07, BKKB13, MMNL14, FHSS18] use a limited number of fragments to calculate the geometric interactions for dynamic rendering effects in real‐time.…”

“…Specifically, this technique performs two rendering processes in a common graphics processing units (GPU) pipeline (i.e., raster‐based graphics), including the storage of three‐dimensional (3D) geometric information into framebuffers and the generation of final images by exploiting the stored geometric information (i.e., screen‐space information). The memory access capabilities of modern graphics hardware allow multi‐fragment rendering to utilize screen‐space geometric information, which helps increase the demand for screen‐space or deferred rendering algorithms [KG13, MY18] in real‐time such as order‐independent‐transparency (OIT) [VPF15, MKKP18], dynamic photorealistic rendering techniques [BKKB13, MMNL14, FHSS18], and interactive scientific visualizations (e.g., multivariate surface data representations [RASS16] or hybrid data representations for the fusion of multiple volumetric and potentially transparent surface data [LFS∗15]).…”

A Survey of Multifragment Rendering

“…This way, the AO estimator is improved, at the cost of slightly overestimating occlusion. Moreover, Bauer et al [BKKB13] stored additional properties for translucent objects, such as opacity, on an linked-list A-buffer and computed AO using a modified estimator, considering both the interior and exterior occlusion of objects. For each sample covered by translucent objects, AO is computed for each fragment in the list separately.…”

Order-Independent Transparency for Programmable Deferred Shading Pipelines

“…These capabilities can be used to implement an A‐Buffer [Car84], which stores the fragments generated during rasterization, to enable order‐independent transparency and a large number of other multi‐fragment effects. In particular, it has already been used for screen‐space ambient occlusion [BKB13], depth of field [YWY10], screen‐space collision detection [JH08], illustrative visualization for computer aided design (CAD) applications [CFM*12], constructive solid geometry (CSG) operations [RFV13] or nearest‐neighbor search algorithms [RBA08,BGO09].…”