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Comment by namuol | Hacker News Reader

> I really don't understand why are they chasing RT.

1. The highly-parallelizable nature of RT is ideal for many-core architecture we're heading toward. Note that the PowerVR hardware used in the demo has 4 cores, needs no fans, and uses 10x less power than a traditional rasterization-optimized GPU.

2. The simplicity of implementing physically-based effects leads to a much easier time for artists.

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I would argue that physically-based raytracers actually make things more difficult for artists, because they tend to be less flexible. Being physically correct is great if what you get out of the renderer is exactly what you want, but as soon as you need to cheat anything to achieve a particular look, it's much easier to not be bound by physical correctness.

You've described a struggle that's very real in production with the off-the-shelf renderers today. People seldom realise that.

The Disney BSDF [1, 2] is extremely flexible and offers a lot of artistic freedom. The only real limitation is that the BSDF is required to be energy conserving, but that's hardly a limitation. A physically-based renderer is still able to produce non-photorealistic results.

[1] http://blog.selfshadow.com/publications/s2012-shading-course...

[2] http://blog.selfshadow.com/publications/s2015-shading-course...

Maybe, depends on whether you're doing CG animation (Pixar, Dreamworks style) or more realistic stuff - VFX, photoreal stuff.

If the former, then maybe a bit.

But the beauty of physically-based is that in many cases, you can take a model that's been textured and lookdeved and it will work nicely in many different lighting setups. Before Physically-based, you practically had to re-do stuff completely when you changed the lighting to get the look you wanted.

However, Physically-based still isn't a complete win - you can't looked something close up and expect it to look good in the distance - it just won't. You need to be aware of the LODs and create different asset variations appropriately for different distances.

It would be really nice to be able to do things like this in RT:

http://www.zeitguised.com/geistxyz#geistxyz01 https://vimeo.com/150824660

Pixar switched to a path tracer for Monsters University, and it made it easier for the artists (once they unlearned the tricks and hacks they would have needed for less sophisticated renderers).

PathTracing (Though not regular Ray tracing) is actually pretty tricky to parallelize. I'd be interested in hearing what their hardware actually does? I'm guessing some hardware implementation of ray intersections with a BVH or kd-Tree?

The problem that occurs when trying to do path tracing on current hardware is that secondary (reflected) rays are incoherent so the gpu will stall one ray while waiting for another that hit something, leading to poor utilization of the gpu. It's still normally faster than on a cpu, but compared to "simpler" problems like some linear algebra (Machine Learning or similar) the perf of gpu vs cpu is low.

I thought GPU techniques do something like accumulate the secondary rays and use really fast GPU sorting algorithms on them before continuing, to bring back coherence.

Rasterization also is of highly-parallelizable nature and this parallelism has been exploited to great effect in the last decade(s). Given that Ray Tracing's only advantage over Rasterization is specular reflection it seems unlikely it is really superior in terms of efficiency. Especially since specular reflections and many other effects can be achieved via cheating in Rasterization.

For real GI, Path Tracing, Photon Mapping and so on are the methods of choice and not Ray Tracing.

Global Illumination, Path Tracing, Photon Mapping and Ray Tracing all belong to a category of algorithms that are better suited for multi-core general processors -- not the SIMD hardware of today's GPUs.

GPU's haven't stuck to SIMD parallelism for quite a few years now. NVIDIA is not there yet SMT/DPT isn't true parallelism but it's good enough even for path tracing. AMD GPU's are pretty much truly parallel with every execution unit capable of executing any instruction asynchronously.

Ray tracing has other advantages besides shiny reflections: it can handle refraction, shadows, non-triangle-based geometric primitives (including CSG), and has less sensitivity to the total amount of geometry in a scene.

Global illumination is where we want to go to surpass the realism of current games, but path tracing and photon mapping are both fundamentally based on ray-tracing. Any hardware that makes ray-tracing go faster (especially for incoherent rays) should also help to speed up global illumination.

Their hardware actually combines hardware rasterization and raytracing.

Depending on the implementation, raytracing can have many benefits over rasterization, such as: - ability to produce pixel-perfect shadows - sub-linear complexity on the amount of primitives (vs. linear for rasterization) - rays need not be coherent, i.e one can render non-linear projections or lots of small views

Path Tracing also is just another form of Raytracing. They demonstrated that their hardware can be used for it (just read the link).

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