TLAS Hit Testing: closest_hit vs any_hit

This document tests and visualizes the difference between closest_hit and any_hit functions in the TLAS implementation using the trace_rays API.

Test Setup

Test 1: Single Ray Through Center

Test a ray through the center that passes through all three spheres.

Test 2: Multiple Rays from Different Positions

Test multiple rays to ensure both functions work correctly.

Test 3: Complex Scene

Demonstrate ray tracing through a complex scene with many overlapping objects.

Key Findings:

• closest_hit continues searching and updates ray's t_max to find the nearest intersection

• any_hit exits on the first intersection during TLAS traversal (useful for shadow rays)

• Both always agree on whether a hit occurred (hit vs miss)

• any_hit is typically faster than closest_hit due to early termination

Performance Comparison

Compare the performance of closest_hit vs any_hit.

Summary

This document demonstrated:

1. trace_rays - A convenient function for tracing rays against a TLAS and collecting results for visualization

   • Returns a RayIntersectionResult bundling rays, hit data, and the TLAS

   • Automatically builds a StaticTLAS for traversal

2. Makie visualization recipe - Automatic visualization via plot(result)

   • Automatically renders TLAS geometry, rays, and hit points

   • Customizable colors, transparency, markers, and labels

   • Works with any Makie backend (GLMakie, WGLMakie, CairoMakie)

3. closest_hit correctly identifies the nearest intersection among multiple overlapping primitives

   • Returns: (hit_found::Bool, triangle::Triangle, distance::Float32, bary_coords::SVector{3,Float32}, instance_id::UInt32)

   • Use sum(bary_coords .* triangle.vertices) to convert to world-space hit point

4. any_hit efficiently determines if any intersection exists, exiting early

   • Returns: Same format as closest_hit

   • Can exit early on first hit found, making it faster for occlusion testing

5. Both functions handle miss cases correctly (returning hit_found=false)

6. any_hit is typically faster than closest_hit due to early termination

All tests passed! ✓