GPU Hardware Ray Tracing is the dedicated fixed-function hardware (NVIDIA RT Cores, AMD Ray Accelerators, Intel Ray Tracing Units) that accelerates the computationally intensive ray-scene intersection tests required for photorealistic rendering — traversing bounding volume hierarchies (BVH) and computing ray-triangle intersections at hundreds of billions of tests per second, enabling real-time ray tracing for reflections, shadows, ambient occlusion, and global illumination in games, film production, and scientific visualization.

Why Hardware Acceleration

Ray tracing requires testing each ray against potentially millions of triangles. Software BVH traversal on shader cores achieves ~1-5 billion ray-box tests/second. RT Cores achieve 50-300 billion tests/second — a 10-100× speedup. This hardware acceleration transforms ray tracing from offline rendering (hours per frame) to real-time (16-33 ms per frame at 30-60 FPS).

BVH (Bounding Volume Hierarchy)

The acceleration structure that makes ray tracing tractable:

• Construction: Scene triangles are recursively partitioned into groups, each enclosed by an axis-aligned bounding box (AABB). The root AABB contains the entire scene; leaf nodes contain 1-8 triangles.
• Traversal: A ray tests against the root AABB. If it intersects, test both children. Recursively descend into intersected nodes, skip non-intersected subtrees. Average complexity: O(log N) triangle tests instead of O(N).
• Quality vs. Build Time: SAH (Surface Area Heuristic) BVH construction produces optimal traversal trees but is expensive to build. LBVH (Linear BVH) uses Morton codes for fast construction (suitable for dynamic scenes). TLAS/BLAS split: Bottom-Level AS (BLAS) per object (rebuilt rarely), Top-Level AS (TLAS) for scene arrangement (rebuilt every frame).

RT Core Architecture (NVIDIA)

• BVH Traversal Unit: Dedicated hardware that traverses the BVH tree, testing ray-AABB intersections at each node. One traversal step per clock — 2-3 AABB tests per cycle. Operates concurrently with shader execution on SM cores.
• Ray-Triangle Intersection Unit: Computes Möller-Trumbore ray-triangle intersection for leaf nodes. Reports hit distance, barycentric coordinates, and triangle ID.
• Opacity Micro-Map (Hopper+): Hardware-accelerated alpha-test evaluation. Encodes per-micro-triangle opacity, allowing RT Cores to skip fully transparent triangles and classify semi-transparent regions — 2× speedup for foliage and particle effects.

Ray Tracing Pipeline (DXR/Vulkan RT/OptiX)

1. Ray Generation Shader: Launches rays (one per pixel for primary rays, additional for reflections/shadows). 2. BVH Traversal (hardware): RT Core traverses TLAS → BLAS hierarchy. 3. Intersection Shader (optional): Custom intersection test for non-triangle primitives (spheres, curves, SDF). 4. Any-Hit Shader: Called for each potential hit — used for alpha-test transparency. Can accept or reject the hit. 5. Closest-Hit Shader: Called for the nearest intersection. Computes shading (material, lighting, launches secondary rays). 6. Miss Shader: Called when no intersection found — returns environment/sky color.

Performance Metrics

NVIDIA RTX 4090: 191 billion RT Core TFLOPS equivalent, ~30-60 FPS in fully ray-traced scenes at 4K with DLSS. AMD RDNA 3 (RX 7900 XTX): significant improvement over RDNA 2 but still trails NVIDIA in pure RT throughput. Intel Arc provides competitive RT performance in its class.

GPU Hardware Ray Tracing is the fixed-function acceleration that transformed photorealistic rendering from an offline computation to a real-time capability — dedicated silicon that makes the physically-based lighting, reflections, and shadows of ray tracing achievable within the millisecond-per-frame budgets of interactive applications.