Computer Graphics World

November/December 2014

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32 cgw n o v e m b e r . d e c e m b e r 2 0 1 4 ably," Hendrickson says. "The next step would be to go for it. I sent a memo out to the compa- ny early last year saying we were at a dividing point, a fork in the road where we had to decide whether to use Hyperion or not. The experiments were so favor- able; it looked like it could be the new way to work. So I invoked executive privilege. I said, 'We're doing this.' We never looked back. Because we couldn't." B R E A K T H R O U G H In June 2013, Burley, along with Christian Eisenacher, Gregory Nichols, and Andrew Selle, had pushed the idea far enough along to publish the paper "Sorted Deferred Shading for Production Path Tracing." It received the Best Paper distinc- tion at the 2013 Eurographics Symposium on Rendering. They wrote: "Raytraced global illumination is becom- ing widespread. However, incoherent ray traversal and shading has traditionally limited raytracing to scenes that fi t in memory. To combat these issues, we introduce a sorting strategy for large, potentially out-of-core ray batches, and we sort and defer shading of ray hits. As a result, we achieve perfectly coherent shading and texture access, removing the need for a shading cache." Sean Jenkins, the technical supervisor who, along with Chuck Tappan, helped move the renderer into production, simpli- fi es the description: "Hyperion's power is that it takes all the rays it casts out, sorts them, and then works on all the rays aim- ing toward one place, like the ceiling. Then, it works on aiming all the rays toward the table. It's a diff erent way of looking at the problem. Brent identi- fi ed coherence as the main bottleneck, as what would limit us as far as scale. By capitalizing on coherence, everything else got easier. The so ware doesn't have to swap back and forth between the ceiling and the table and the chairs. Sorting the rays is pretty straightforward. The algorithm we published in the paper hasn't changed a whole lot since then." Most raytracing systems send rays out from the camera and calculate the shading for each spot each ray hits. As the rays bounce, the renderer calculates the shading for each hit, and continues doing so through a complex spiderweb of rays bouncing and hitting objects in a scene. Thus, studios o en limit raytracers to as few bounc- es as possible. By contrast, Hyperion shades a fi xed set, a batch, of 33 million rays going in roughly the same direction. Burley provides more detail: "We start by sending rays from the camera, generate a few hundred million rays, and organize them into batches of 33 million going to a similar part of the image in a similar direction, " he explains. "The renderer records the ray hits and switches to shading mode. So, all the rays that hit a chair, for example, get processed all at once. The process of shading generates new rays and, unless the thing being shaded is a light, the rays bounce more. Those rays are sent to a queue where they are bucketed by directions. There's an open batch for each of six directions. Once the batch is full, we have a ready batch of rays going in the same direction. We trace, shade, trace, shade until there are no more rays. It happens over the course of a few seconds. Because we're sorting rays based on hit-point coherence, they are shading the same object. They have the same shader, the same texture, even a portion of a texture on an object. They do everything they need and are done with it." And that's why Hyperion could calculate 10,500,000,000 rays per frame. The eff ect was dramatic. "Because we could capitalize on coherence," Jenkins says, "we could trace more rays and trace rays through multiple bounces. We started seeing signifi cant results from six bounces. We still had energy fi lling in the overall scene. And it gave an overall illu- mination within the characters. Baymax is hollow and bright. We could bounce light through him many times." Hendrickson adds, "It's the fi rst time we could do caustics, SAN FRANSOKYO HAS 260,000 TREES, 250,000 STREETLIGHTS, 100,000 VEHICLES, AND 83,000 BUILDINGS. " WE NEVER LOOKED BACK. BECAUSE WE COULDN'T." VIDEO: GO TO EXTRAS IN THE NOVEMBER.DECEMBER 2014 ISSUE BOX C G W. C O M

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