Deferred Any-Hit Shaders for Reduced SIMD Ray-Tracing Divergence
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
In ray tracing on SIMD architectures, the immediate execution of any hit shaders for candidate hits leads to significant control flow divergence due to varying operations required by different rays, resulting in inefficiencies.
Innovation Solution
Deferring the execution of any hit shaders until multiple contexts are accumulated allows for grouping these executions together, reducing divergence by overlapping BVH traversal with shader execution and culling unnecessary contexts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If any hit shaders are executed immediately upon detecting a candidate hit, then the ray tracing process can proceed without delay for individual rays, but significant control flow divergence occurs because different rays require varying operations
Solution Approach 1:
The patent applies preliminary action by deferring the execution of any hit shaders until multiple candidate hits are accumulated. Instead of executing shaders immediately when a single ray finds a candidate hit, the system waits and accumulates multiple contexts before executing the shaders in a grouped manner. This preliminary accumulation phase allows the system to prepare multiple execution contexts in advance, reducing control flow divergence when the actual shader execution occurs.
2Loss of time
If any hit shaders are executed immediately for each candidate hit, then processing can be done without waiting for other rays, but the overall processing time increases due to repeated shader invocations
Solution Approach 1:
The patent applies merging by combining multiple any hit shader executions into a single grouped execution. Instead of invoking shaders separately for each ray that finds a candidate hit, the system accumulates multiple candidate hits from different rays and then executes the corresponding any hit shaders together in a unified operation. This merging of execution operations reduces the total number of shader invocations and improves overall processing throughput.
Solution Approach 2:
The patent maintains continuity of useful action by overlapping BVH traversal with shader execution. While any hit shaders are being executed for accumulated candidate hits, the system continues traversing the BVH for other rays without idle waiting. This overlapping ensures that processing resources remain continuously utilized, eliminating gaps where rays would otherwise wait for shader execution to complete, thereby improving overall efficiency.
Data Source
AI summary
Techniques for reducing SIMD divergence for ray tracing are provided. In ray tracing on a SIMD architecture, rays are cast into a scene. Part of such operations includes evaluating a ray cast for intersection with a triangle, which is performed using an acceleration structure. SIMD execution is performed for multiple work-items (e.g., rays) in parallel, but control flow can become divergent if the work-items need to perform different operations. During traversal, it is possible that rays require execution of an any hit shader to evaluate a candidate hit as accepted or rejected. However, if such execution is performed immediately upon detection of a candidate hit, a high degree of control flow divergence can occur, since it is likely that such execution occurs only for a single ray. By deferring this execution, it is possible to group the execution of an any hit shader for multiple work-items together, thereby reducing divergence.