Improved Accuracy for Prism-Based Motion Blur

Journal article, 2022

For motion blur of dynamic triangulated objects, it is common to construct a prism-like shape for each triangle, from the linear trajectories of its three edges and the triangle’s start and end position during the delta time step. Such a prism can be intersected with a primary ray to find the time points where the triangle starts and stops covering the pixel center. These intersections are paired into time intervals for the triangle and pixel. Then, all time intervals, potentially from many prisms, are used to aggregate a motion-blurred color contribution to the pixel.

For real-time rendering purposes, it is common to linearly interpolate the ray-triangle intersection and uv coordinates over the time interval. This approximation often works well, but the true path in 3D and uv space for the ray-triangle intersection, as a function of time, is in general nonlinear.

In this article, we start by noting that the path of the intersection point can even partially reside outside of the prism volume itself: i.e., the prism volume is not always identical to the volume swept by the triangle. Hence, we must first show that the prisms still work as bounding volumes when finding the time intervals with primary rays, as that may be less obvious when the volumes differ. Second, we show a simple and potentially common class of cases where this happens, such as when a triangle undergoes a wobbling- or swinging-like motion during a time step. Third, when the volumes differ, linear interpolation between two points on the prism surfaces for triangle properties works particularly poorly, which leads to visual artifacts. Therefore, we finally modify a prism-based real-time motion-blur algorithm to use adaptive sampling along the correct paths regarding the triangle location and uv coordinates over which we want to compute a filtered color. Due to being adaptive, the algorithm has a negligible performance penalty on pixels where linear interpolation is sufficient, while being able to significantly improve the visual quality where needed, for a very small additional cost.