MWC 16: Imagination Technologies Ray Tracing Accelerator
Subject: Graphics Cards, Mobile, Shows and Expos | February 23, 2016 - 08:46 PM | Scott Michaud
Tagged: raytracing, ray tracing, PowerVR, mwc 16, MWC, Imagination Technologies
For the last couple of years, Imagination Technologies has been pushing hardware-accelerated ray tracing. One of the major problems in computer graphics is knowing what geometry and material corresponds to a specific pixel on the screen. Several methods exists, although typical GPUs crush a 3D scene into the virtual camera's 2D space and do a point-in-triangle test on it. Once they know where in the triangle the pixel is, if it is in the triangle, it can be colored by a pixel shader.
Another method is casting light rays into the scene, and assigning a color based on the material that it lands on. This is ray tracing, and it has a few advantages. First, it is much easier to handle reflections, transparency, shadows, and other effects where information is required beyond what the affected geometry and its material provides. There are usually ways around this, without resorting to ray tracing, but they each have their own trade-offs. Second, it can be more efficient for certain data sets. Rasterization, since it's based around a “where in a triangle is this point” algorithm, needs geometry to be made up of polygons.
It also has the appeal of being what the real world sort-of does (assuming we don't need to model Gaussian beams). That doesn't necessarily mean anything, though.
At Mobile World Congress, Imagination Technologies once again showed off their ray tracing hardware, embodied in the PowerVR GR6500 GPU. This graphics processor has dedicated circuitry to calculate rays, and they use it in a couple of different ways. They presented several demos that modified Unity 5 to take advantage of their ray tracing hardware. One particularly interesting one was their quick, seven second video that added ray traced reflections atop an otherwise rasterized scene.
It was a little too smooth, creating reflections that were too glossy, but that could probably be downplayed in the material ((Update: Feb 24th @ 5pm Car paint is actually that glossy. It's a different issue). Back when I was working on a GPU-accelerated software renderer, before Mantle, Vulkan, and DirectX 12, I was hoping to use OpenCL-based ray traced highlights on idle GPUs, if I didn't have any other purposes for it. Now though, those can be exposed to graphics APIs directly, so they might not be so idle.
The downside of dedicated ray tracing hardware is that, well, the die area could have been used for something else. Extra shaders, for compute, vertex, and material effects, might be more useful in the real world... or maybe not. Add in the fact that fixed-function circuitry already exists for rasterization, and it makes you balance gain for cost.
It could be cool, but it has its trade-offs, like anything else.