The primary advantages of ray tracing over rasterization are:
Accurate shadows, without explicit sizing of shadow buffer resolutions or massive stencil volume overdraw. With reasonable area light source bundles for softening, this is the most useful and attainable near-term goal.
Accurate reflections without environment maps or subview rendering. This benefit is tempered by the fact that it is only practical at real time speeds for mirror-like surfaces. Slightly glossy surfaces require a bare minimum of 16 secondary rays to look decent, and even mirror surfaces alias badly in larger scenes with bump mapping. Rasterization approximations are inaccurate, but mip map based filtering greatly reduces aliasing, which is usually more important. I was very disappointed when this sunk in for me during my research – I had thought that there might be a place for a high end “ray traced reflections” option in upcoming games, but it requires a huge number of rays for it to actually be a positive feature.
Some other “advantages” that are often touted for ray tracing are not really benefits:
Accurate refraction. This won’t make a difference to anyone building an application.
Global illumination. This requires BILLIONS of rays per second to approach usability. Trying to do it with a handful of tests per pixel just results in a noisy mess.
Because ray tracing involves a log2 scale of the number of primitives, while rasterization is linear, it appears that highly complex scenes will render faster with ray tracing, but it turns out that the constant factors are so different that no dataset that fits in memory actually crosses the time order threshold.
Classic Whitted ray tracing is significantly inferior to modern rasterization engines for the vast majority of scenes that people care about. Only when two orders of magnitude more rays are cast to provide soft shadows, glossy reflections, and global illumination does the quality commonly associated with “ray tracing” become apparent. For example, all surfaces that are shaded with interpolated normal will have an unnatural shadow discontinuity at the silhouette edges with single shadow ray traces. This is most noticeable on animating characters, but also visible on things like pipes. A typical solution if the shadows can’t be filtered better is to make the characters “no self shadow” with additional flags in the datasets. There are lots of things like this that require little tweaks in places that won’t be very accessible with the proposed architecture.
The huge disadvantage is the requirement to maintain acceleration structures, which are costly to create and more than double the memory footprint. The tradeoffs that get made for faster build time can have significant costs in the delivered ray tracing time versus fully optimized acceleration structures. For any game that is not grossly GPU bound, a ray tracing chip will be a decelerator, due to the additional cost of maintaining dynamic accelerator structures.
Rasterization is a tiny part of the work that a GPU does. The texture sampling, shader program invocation, blending, etc, would all have to be duplicated on a ray tracing part as well. Primary ray tracing can give an overdraw factor of 1.0, but hierarchical depth buffers in rasterization based systems already deliver very good overdraw rejection in modern game engines. Contrary to some popular beliefs, most of the rendering work is not done to be “realistic”, but to be artistic or stylish.
I am 90% sure that the eventual path to integration of ray tracing hardware into consumer devices will be as minor tweaks to the existing GPU microarchitectures.
John Carmack - 2013
