Realtime Ray Tracing and Interactive Global Illumination - Scientific ...

22 Chapter 3: A Brief Survey of Ray Tracing Acceleration Methods
have to be traced, and can achieve interactive update rates even on a single
desktop PC. The number of rays shot in vertex tracing also depends to a large
degree on how often triangles are subdivided. Obviously, coarser subdivision
is much faster, but is also likely to miss certain features, and thus to generate
disturbing artifacts. In order to increase interactivity, vertex tracing allows
for interactively changing the quality parameters: During user interaction,
reduced quality parameters are used for fast user feedback, while the image
is progressively improved as soon as interaction stops.
However, using reduced quality settings can result in rather poor rendering
quality due to excessive interpolation and under-sampling of features.
This becomes especially pronounced if the image to be rendered contains
many detailed features.
3.1.3 The Render Cache
Another combination of interpolation and sparse sampling of the image plane
is Walter et al.’s “Render Cache” approach [Walter99, Walter02]: The render
cache keeps a cache of “old” image samples from previous frames, and reprojects
those to each new frame. These reprojected samples provide a sparse
sampling of the image plane, our of which the full image is then reconstructed
using certain heuristics to resolve occlusion, disocclusion, and interpolation
inbetween samples.
Asynchronously to this reprojection process, new samples are generated
by continuously tracing new rays. To improve performance, the render cache
uses heuristics to decide which regions of the image plane need to be sampled
most, and preferably generates new samples in those regions. New samples
are inserted into the render cache data structure, thereby evicting older samples.
The render cache allows for decoupling frame rate from sample generation
speed, and therefore allows interactive frame rates even for very slow
renderers. However, the reprojection and image reconstruction steps are
quite costly, and only really pay off for extremely costly renderers. As such,
the render cache is most beneficial for extremely complex computations, such
as global illumination. For simple ray tracing, however, it is often faster to
simply ray trace the whole image.
However, the worst problem of the render cache is its limited rendering
quality: Even though the render cache uses several heuristics to resolve the
worst artifacts, not all artifacts can be avoided: While the render cache
quickly converges to a high-quality image when no user interaction occurs,
under-sampling of certain objects or image regions, use of outdated samples,
and excessive blurring (especially across under-sampled discontinuities) are