Keywords: Volume Visualization, Volumetric Ray Tracing, Virtual Endoscopy, Virtual Colonoscopy.
Ray tracing based volume visualization is a direct volume rendering
approach to visualizing various datasets. Common applications include virtual
colonoscopy, a medical visualization problem aiming to reconstruct internal
views of the human colon from CT (Computer Tomography) scans. Applying
volumetric ray tracing in interactive visualization has always been limited
by low rendering speeds, and attaining interactive rendering performance
on mainstream PC hardware has been a challenge for years.
This paper presents methods for accelerating first-hit ray tracing based virtual endoscopy with negligible impact on image quality, which aim at improving empty-space traversal (tracing all rays to the colon wall and storing the results in a depth buffer) and shading (surface normal approximation at hit locations and simple Phong shading applied to obtain pixel color). The method proposed to accelerate empty-space traversal exploits inter-ray coherence based on the fact that the colon wall is a (C2) continuous natural surface, which does not exhibit erratic behavior, such as sharp jumps, steps or edges. To improve shading performance inter-pixel coherence is exploited and an algorithm using conditional interpolation is investigated. Applying these methods, rendering times were reduced by 60-70% boosting lower resolution rendering beyond 25, and normal resolutions to around 10 frames per second. Resulting benchmarks from PC implementations are presented for various levels of acceleration.
With images of acceptable resolutions providing enough detail on significant features rendered at over 10 frames per second, it is concluded that volumetric ray tracing based virtual endoscopy is technically feasible and application-ready on mainstream PC hardware.