Sheared AABB Bounds for Tighter Ray Tracing Hierarchies
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Solution Overview
Problem
Existing ray tracing technologies face inefficiencies due to the use of axis-aligned bounding boxes (AABBs) that poorly fit non-axis-aligned geometry, leading to increased false positives and computational overhead, especially in real-time graphics processing.
Innovation Solution
Implementing sheared axis-aligned bounding boxes (SAABBs) through localized shear transforms to approximate oriented bounding boxes, applying inverse shear transforms to rays before intersection tests, thereby reducing false positives and improving computational efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If axis-aligned bounding boxes (AABBs) are used for bounding geometry, then the bounding volumes are simple and easy to test, but they poorly fit non-axis-aligned geometry leading to increased false positives
Solution Approach 1:
The patent transforms the coordinate system by applying a shear transform to the geometry, converting non-axis-aligned geometry into axis-aligned geometry in the transformed space. This allows standard AABBs to effectively bound the transformed geometry, resolving the contradiction between simplicity and accuracy.
Solution Approach 2:
The patent introduces a shear transform as an intermediary coordinate system transformation. By transforming both the geometry and rays into this intermediate shear space, standard AABB structures can be used effectively, achieving both simplicity and accuracy without requiring complex oriented bounding boxes.
2Measurement precision
If oriented bounding boxes (OBBs) are used to tightly bound non-axis-aligned geometry, then false positives are reduced, but the computational complexity and storage requirements increase
Solution Approach 1:
Instead of storing complex OBB data structures with rotation matrices and multiple parameters, the patent copies the simplicity of AABB structures into shear space. The bounding volumes remain as simple AABBs in the transformed coordinate system, avoiding the complexity of OBB representations while achieving similar bounding accuracy.
Solution Approach 2:
The patent replaces the mechanical rotation operation required for OBBs with a shear transform. The shear transform can be represented with simpler parameters and applied more efficiently, substituting the complex rotational mechanics of OBBs with a computationally lighter transformation that achieves the same bounding effectiveness.
3Measurement precision
If shear transforms are applied to create tighter fitting bounding volumes, then false positives are reduced, but additional transform computations are required
Solution Approach 1:
The patent applies the shear transform during the scene construction and acceleration structure building phase, transforming the geometry into shear space beforehand. This preliminary transformation allows standard AABB tests to be used during ray tracing without requiring additional transform computations at query time, amortizing the transform cost over the entire scene processing.
Solution Approach 2:
Instead of transforming the bounding volumes to fit the geometry, the patent transforms the rays and geometry into a space where standard AABBs naturally fit. By inverting the problem—transforming into shear space rather than transforming the bounding volumes—we achieve tight fitting with simpler computations.
Data Source
AI summary
To improve the efficiency of bounding volumes in a hardware based ray tracer, we employ a sheared axis-aligned bounding box to approximate an oriented bounding box typically defined by rotations. To achieve this, the bounding volume hierarchy builder shears an axis-aligned box to fit tightly around its enclosed oriented geometry in top level or bottom level space, then computes the inverse shear transform. The bounds are still stored as axis-aligned boxes in memory, now defined in the new sheared coordinate system, along with the derived parameters to transform a ray into the sheared coordinate system before testing intersection with the boxes. The ray-bounding volume intersection test is performed as usual, just in the new sheared coordinate system. Additional efficiencies are gained by constraining the number of shear dimensions, constraining the shear transform coefficients to a quantized list, sharing a shear transform across a collection of bounds, performing a shear transform only for ray-bounds testing and not for ray-geometry intersection testing, and adding a specialized shear transform calculator/accelerator to the hardware.