When Will Ray Tracing Replace Rasterization?

Ray Tracing Myths

Ray tracing is not the ultimate solution though, and it's time to tear down the myths that surround it.

First of all, many gamers think that ray tracing is intrinsically a better algorithm than rasterization because "it's what the movies use." That's false. The majority of films using synthesized images (and all of Pixar's) use an algorithm called REYES that's based on rasterization. Pixar added ray tracing to RenderMan, its rendering engine, only later for the production of Cars. But even for Cars, ray tracing was used sparingly to avoid excessive computing overhead. Until then, Pixar used an external module for its limited use of ray tracing, such as for ambient occlusion (AO) effects.

The second widespread myth among defenders of ray tracing has to do with the complexity of the scenes that can be rendered with ray tracing and with rasterization. To get a handle on this, we need to take a closer look at each algorithm.

Here's how rasterization works for each triangle of the scene:

  • the set of pixels that the triangle covers is determined
  • for each pixel covered, its depth is compared with that of the nearest pixel

The main rasterization loop is based on the number of triangles. The algorithm has a complexity of O(n), where n is the number of triangles. The algorithm thus operates in linear fashion as a function of the number of triangles, since for each frame, the list of triangles has to be processed, one after the other.

Conversely, ray tracing operates as follows:

For each pixel of the frame:

  • a ray is cast to determine which triangles are the closest
  • for each triangle, the distance is calculated from the triangle to the image plane

As you can see, the loop is inverted, in a sense. In the first case, we take each polygon and look for which pixels it covers, and in the other, we take each pixel and then see which polygon it corresponds to. So, you might think that ray tracing is less dependent on the number of triangles than rasterization, since the number of triangles doesn't affect the main loop. But in practice that isn't so. In fact, to determine which triangle will be intersected by our ray, we also have to process all the triangles in the scene. Here again, partisans of ray tracing say that, in practice, it's not necessary to process all the triangles in the scene with each ray. Using the appropriate type of data structure, they say it's very easy to organize the triangles in such a way that only a small percentage of them need to be tested for each ray, concluding that ray tracing therefore has a complexity of O(log n) where n is the number of polygons.

And in fact, they're quite right. But these pundits are being a little dishonest in that that's also true of rasterization. Game engines have been using binary space partitioning (BSP) trees and other techniques for years to limit the number of polygons that need to be processed for each frame calculated. Another debatable point about this line of reasoning is that the data structure is particularly effective for static data. All that's needed is to calculate data once, and then simply access the data, which produces very good results. But what about dynamic data? There, the data structure has to be recalculated for each image, and to do that, there's no miracle formula. Each polygon has to be examined.

  • IzzyCraft
    Greed? You give an inch they take a mile? Very pessimistic conclusion although it helps drive the industry so hard to really complain. ;)
  • Ramar
    I'm definitely the kind of person that would prefer to lose some performance in exchange for elegance and perfection. The eye can tell when something is done cheaply in a render. I've made this argument that quite often we find computationally cheap methods of doing something in a game, and after time it seems to me that we've got a 400 horsepower muscle car that, on close inspection, is held together with duct tape and dreams. I'd much rather have a V6 sedan that's spotless and responds properly.

    Okay, well in real life, the Half Life 2 buggy would be a lot cooler to drive around than a Jetta, but you get the analogy.
  • stray_gator
    Great article!
  • zodiacfml
    i still like the simplicity of ray tracing and how close it is to physics/science. it is just how it works, bounce light to everything.

    there are a lot of diminishing returns i can see in the future, some are, how complex can rasterization can get? what is the diminishing returns for image resolution especially on the desktop/living room?
    ray tracing has a lot of room for optimization.

    for years to come, indeed, raster is good for what is possible in hardware. look further ahead,more than 5 years, we'll have hardware fast enough and efficient algorithm for ray tracing. not to mention the big cpu companies, amd & intel, who will push this and earn everyones money.
  • stray_gator
    aargh. start typing, then sign in to find your first words posted.
    Anyway, what I liked about this article is its being under the hood, but not related to a new product, announcement or such.
    "deep tech" articles accompanying product launches tend inevitably to follow the lines of press kits, PR slides, etc.
    Articles like this, while take longer to research, are exactly that - they are researched rather than detailing "company X implemented techniques Y and Z in their new product, which works this way, benefits performance that way and is really cool.". it gives an independent, comprehensive view of the subject, and gives the reader real understanding in the field.
  • enewmen
    The ray-tracing code on the business card was way cool. I was hoping (real-time)ray-tracing and photo-realistic rendering will come with DX11 and GPGPU offloading - this seems completely unrealistic.
    I still never read of any dedicated ray-tracing hardware, at any price. It seems the better we understand ray-tracing and it's limitations, the more cloudy the future becomes.
  • shurcooL
    Nice article. Seems to be fairly accurate.
    Ray tracing will inevtiably replace rasterization. It will just flat out look better to the human perception, when in motion, than pure rasterization, and that is all that is required.

    Heh... this article brought to you by Nvidia.
  • annymmo
    Hopefully GPGPU (OpenCL)
    will make raytracing possible.
    (Together with a huge number of processing cores per graphic card and an advanced raytracing algorithm.)
  • Inneandar
    nice article.
    I wouldn't mind having just a little bit more technical depth, but I'd be glad to seem more like this on Tom's.