Transactions on Graphics Paper- The Design and Evolution of Disney's Hyperion Renderer

The August 2018 issue of ACM Transactions on Graphics (Volume 37 Issue 3) is partially a special issue on production rendering, featuring five systems papers describing notable, major production renderers in use today. I got to contribute to one of these papers as part of the Hyperion team at Walt Disney Animation Studios! Our paper, titled “The Design and Evolution of Disney’s Hyperion Renderer”, discusses exactly what the title suggests. We present a detailed look inside how Hyperion is designed today, discuss the decisions that went into its current design, and examine how Hyperion has evolved since the original EGSR 2013 “Sorted Deferred Shading for Production Path Tracing” paper that was the start of Hyperion. A number of Hyperion developers contributed to this paper as co-authors, along with Hank Driskill, who was the technical supervisor on Big Hero 6 and Moana and was one of the key supporters of Hyperion’s early development and deployment.

Image from paper Figure 1: Production frames from Big Hero 6 (upper left), Zootopia (upper right), Moana (bottom left), and Olaf’s Frozen Adventure (bottom right), all rendered using Disney’s Hyperion Renderer.

Here is the paper abstract:

Walt Disney Animation Studios has transitioned to path-traced global illumination as part of a progression of brute-force physically based rendering in the name of artist efficiency. To achieve this without compromising our geometric or shading complexity, we built our Hyperion renderer based on a novel architecture that extracts traversal and shading coherence from large, sorted ray batches. In this article, we describe our architecture and discuss our design decisions. We also explain how we are able to provide artistic control in a physically based renderer, and we demonstrate through case studies how we have benefited from having a proprietary renderer that can evolve with production needs.

The paper and related materials can be found at:

We owe a huge thanks to Matt Pharr, who came up with the idea for a TOG special issue on production rendering and coordinated the writing of all of the papers, and Kavita Bala, who as editor-in-chief of TOG supported all of the special issue papers. This issue has actually been in the works for some time; Matt Pharr contacted us over a year ago about putting together a special issue, and we began work on our paper in May 2017. Matt and Kavita generously gave all of the contributors to the special issue a significant amount of time to write, and Matt provided a lot of valuable feedback and suggestions to all five of the final papers. The end result is, in my opinion, something special indeed. The five rendering teams that contributed papers in the end were Solid Angle’s Arnold, Sony Imageworks’ Arnold, Weta Digital’s Manuka, Pixar’s Renderman, and ourselves. All five of the papers in the special issue are fascinating, well-written, highly technical rendering systems papers (as opposed to just marketing fluff), and absolutely worth a read!

Something important that I want to emphasize here is that the author lists for all five papers are somewhat deceptive. One might think that the author lists represent all of the people responsible for each renderers’ success; this idea is, of course, inaccurate. For Hyperion, the authors on this paper represent just a small fraction of all of the people responsible for Hyperion’s success. Numerous engineers not on the author list have made significant contributions to Hyperion in the past, and the project relies enormously on all of the QA engineers, managers/leaders, TDs, artists, and production partners that test, lead, deploy, and use Hyperion every day. We also owe an enormous amount to all of the researchers that we have collaborated directly with, or who we haven’t collaborated directly with but have used their work. The success of every production renderer comes not just from the core development team, but instead from the entire community of folks that surround a production renderer; this is just as true for Hyperion as it is for Renderman, Arnold, Manuka, etc. The following is often said in our field but nonetheless true: building an advanced production renderer in a reasonable timeframe really is only possible through a massive team effort.

This summer, in addition to publishing this paper, members of the Hyperion team also presented the following at SIGGRAPH 2018:

  • Peter Kutz was on the “Design and Implementation of Modern Production Renderers” panel put together by Matt Pharr to discuss the five TOG production rendering papers. Originally Brent Burley was supposed to represent the Hyperion team, but due to some outside circumstances, Brent wasn’t able to make it to SIGGRAPH this year, so Peter went in Brent’s place.
  • Matt Jen-Yuan Chiang presented a talk on rendering eyes, titled “Plausible Iris Caustics and Limbal Arc Rendering”, in the “It’s a Material World” talks session.

Disney Animation Data Sets

Today at EGSR 2018, Walt Disney Animation Studios announced the release of two large, production quality/scale data sets for rendering research purposes. The data sets are available on a new data sets page on the official Disney Animation website. The first data set is the Cloud Data Set, which contains a large and highly detailed volumetric cloud data set that we used for our “Spectral and Decomposition Tracking for Rendering Heterogeneous Volumes” SIGGRAPH 2017 paper, and the second data set is the Moana Island Scene, which is a full production scene from Moana.

Figure 1: The Moana Island Data Set, rendered using Disney's Hyperion Renderer.

Figure 2: The Cloud Data Set, rendered using Disney's Hyperion Renderer.

In this post, I’ll share some personal thoughts, observations, and notes. The release of these data sets was announced by my teammate, Ralf Habel, at EGSR today, but this release has been in the works for a very long time now, and is the product of the collective effort of an enormous number of people across the studio. A number of people deserve to be highlighted: Rasmus Tamstorf spearheaded the entire effort and was instrumental in getting the resources and legal approval needed for the Moana Island Scene. Heather Pritchett is the TD that did the actual difficult work of extracting the Moana Island Scene out of Disney Animation’s production pipeline and converting it from proprietary data formats into usable, industry-standard data formats. Sean Palmer and Jonathan Garcia also helped in resurrecting the data from Moana. Hyperion developers Ralf Habel and Peter Kutz led the effort to get the Cloud Data Set approved and released; the cloud itself was made by artists Henrik Falt and Alex Nijmeh. On the management side of things, technology manager Rajesh Sharma and Disney Animation CTO, Nick Cannon, provided crucial support and encouragement. Matt Pharr has been crucial in collaborating with us to get these data sets released. Matt was highly accommodating in helping us get the Moana Island Scene into a PBRT scene; I’ll talk a bit more about this later. Intel’s Embree team also gave significant feedback. My role was actually quite small; along with other members of the Hyperion development team, I just provided some consultation throughout the whole process.

Please note the licenses that the data sets come with. The Cloud Data Set is licensed under a Creative Commons Attribution ShareAlike 3.0 Unported License; the actual cloud is based on a photograph by Kevin Udy on his Colorado Clouds Blog, which is also licensed under the same Creative Commons license. The Moana Island Scene is licensed under a more restrictive, custom Disney Enterprises research license. This is because the Moana Island Scene is a true production scene; it was actually used to produce actual frames in the final film. As such, the data set is being released only for pure research and development purposes; it’s not meant for use in artistic projects. Please stick to and follow the licenses these data sets are released under; if people end up misusing these data sets, then it makes releasing more data sets into the community in the future much harder for us.

This entire effort was sparked two years ago at SIGGRAPH 2016, when Matt Pharr made an appeal to the industry to provide representative production-scale data sets to the research community. I don’t know how many times I’ve had conversations about how well new techniques or papers or technologies will scale to production cases, only to have further discussion stymied by the lack of any true production data sets that the research community can test against. We decided as a studio to answer Matt’s appeal, and last year at SIGGRAPH 2017, Brent Burley and Rasmus Tamstorf announced our intention to release both the Cloud and Moana Island data sets. It’s taken nearly a year from announcement to release because the process has been complex, and it was very important to the studio to make sure the release was done properly.

One of the biggest challenges was getting all of the data out of the production pipeline and our various proprietary data formats into something that the research community can actually parse and make use of. Matt Pharr was extremely helpful here; over the past year, Matt has added support for Ptex textures and implemented the Disney Bsdf in PBRT v3. Having Ptex and the Disney Bsdf available in PBRT v3 made PBRT v3 the natural target for an initial port to a renderer other than Hyperion, since internally all of Hyperion’s shading uses the Disney Bsdf, and all of our texturing is done through Ptex. Our texturing also relies heavily on procedural SeExpr expressions; all of the expression-drive texturing had to be baked down into Ptex for the final release.

Both the Cloud and Moana Island data sets are, quite frankly, enormous. The Cloud data set contains a single OpenVDB cloud that weighs in at 2.93 GB; the data set also provides versions of the VDB file scaled down to half, quarter, eighth, and sixteenth scale resolutions. The Moana Island data set comes in three parts: a base package containing raw geometry and texture data, an animation package containing animated stuff, and a PBRT package containing a PBRT scene generated from the base package. These three packages combined, uncompressed, weigh in at well over 200 GB of disk space; the uncompressed PBRT package along weighs in at around 38 GB.

For the Moana Island Scene, the provided PBRT scene requires a minimum of around 90 GB if RAM to render. This many seem enormous for consumer machines, because it is. However, this is also what we mean by “production scale”; for Disney Animation, 90 GB is actually a fairly mid-range memory footprint for a production render. On a 24-core, dual-socket Intel Xeon Gold 6136 system, the PBRT scene took me a little over an hour and 15 minutes to render from the ‘shotCam’ camera. Hyperion renders the scene faster, but I would caution against using this data set to do performance shootouts between different renders. I’m certain that within a short period of time, enthusiastic members of the rendering community will end up porting this scene to Renderman and Arnold and Vray and Cycles and every other production renderer out there, which will be very cool! But keep in mind, this data set was authored very specifically around Hyperion’s various capabilities and constraints, which naturally will be very different from how one might author a complex data set for other renderers. Every renderer works a bit differently, so the most optimal way to author a data set for every renderer will be a bit different; this data set is no exception. So if you want to compare renderers using this data set, make sure you understand the various ways how the way this data set is structured impacts the performance of whatever renderers you are comparing.

For example, Hyperion subdivides/tessellates/displaces everything to as close to sub-poly-per-pixel as it can get while still fitting within computational resources. This means our scenes are usually very heavily subdivided and tessellated. However, the PBRT version of the scene doesn’t come with any subdivision; as a result, silhouettes in the following comparison images don’t fully match in some areas. Similarly, PBRT’s lights and lighting model differ from Hyperion’s, and Hyperion has various artistic controls that are unique to Hyperion, meaning the renders produced by PBRT versus Hyperion differ in many ways:

Figure 3a: 'shotCam' camera angle, rendered using Disney's Hyperion Renderer.

Figure 3b: 'shotCam' camera angle, rendered using PBRT v3.

Figure 4a: 'beachCam' camera angle, rendered using Disney's Hyperion Renderer.

Figure 4b: 'beachCam' camera angle, rendered using PBRT v3.

Figure 5a: 'dunesACam' camera angle, rendered using Disney's Hyperion Renderer.

Figure 5b: 'dunesACam' camera angle, rendered using PBRT v3. Some of the plants are in slightly different locations than the Hyperion render; this was just a small change that happened in data conversion to the PBRT scene.

Figure 6a: 'flowersCam' camera angle, rendered using Disney's Hyperion Renderer.

Figure 6b: 'flowersCam' camera angle, rendered using PBRT v3. Note that the silhouette of the flowers is different compared to the Hyperion render because the Hyperion render subdivides the flowers, whereas the PBRT render displays the base cage.

Figure 7a: 'grassCam' camera angle, rendered using Disney's Hyperion Renderer.

Figure 7b: 'grassCam' camera angle, rendered using PBRT v3. The sand dune in the background looks particularly different from the Hyperion render due to subdivision and displacement.

Figure 8a: 'palmsCam' camera angle, rendered using Disney's Hyperion Renderer.

Figure 8b: 'palmsCam' camera angle, rendered using PBRT v3. The palm leaves look especially different due to differences in artistic lighting shaping and curve shading differences. Most notably, the look in Hyperion depends heavily on attributes that vary along the length of the curve, which is something PBRT doesn't support yet. Some more work is needed here to get the palm leaves to look more similar between the two renders.

Figure 9a: 'rootsCam' camera angle, rendered using Disney's Hyperion Renderer.

Figure 9b: 'rootsCam' camera angle, rendered using PBRT v3. Again, the significant difference in appearance in the rocks is probably just due to subdivision/tesselation/displacement.

Another example of a major difference between the Hyperion renders and the PBRT renders is in the water, which Hyperion renders using photon mapping to get the caustics. The provided PBRT scenes use unidirectional pathtracing for everything including the water, hence the very different caustics. Similarly, the palm trees in the ‘palmsCam’ camera angle look very different between PBRT and Hyperion because Hyperion’s lighting controls are very different from PBRT; Hyperion’s lights include various artistic controls for custom shaping and whatnot, which aren’t necessarily fully physical. Also, the palm leaves are modeled using curves, and the shading depends on varying colors and attributes along the length and width of the curve, which PBRT doesn’t support yet (getting the palm leaves is actually the top priority for if more resources are freed up to improve the data set release). These difference between renderers don’t necessarily mean that one renderer is better than the other; they simply mean that the renderers are different. This will be true for any pair of renderers that one wants to compare.

The Cloud Data Set includes an example render from Hyperion, which implements our Spectral and Decomposition Tracking paper in its volumetric rendering system to efficiently render the cloud with thousands of bounces. This render contains no post-processing; what you see in the provided image is exactly what Hyperion outputs. The VDB file expresses the cloud as a field of heterogeneous densities. Also provided is an example Mitsuba scene, renderable using the Mitsuba-VDB plugin that can be found on Github. Please consult the README file for some modifications in Mitsuba that are necessary to render the cloud. Also, please note that the Mitsuba example will take an extremely long time to render, since Mitsuba isn’t really meant to render high-albedo heterogeneous volumes. With proper acceleration structures and algorithms, rendering the cloud only takes us a few minutes using Hyperion, and should be similarly fast in any modern production renderer.

One might wonder just why production data sets in general are so large. This is an interesting question; the short answer across the industry basically boils down to “artist time is more expensive and valuable than computer hardware”. We could get these scenes to fit into much smaller footprints if we were willing to make our artists spend a lot of time aggressively optimizing assets and scenes and whatnot so that we could fit these scenes into smaller disk, memory, and compute footprints. However, this isn’t actually always a good use of artist time; computer hardware is cheap compared to wasting artist time, which often could be better spent elsewhere making the movie better. Throwing more memory and whatnot at huge data sets is also simply more scalable than using more artist resources, relatively speaking.

Both data sets come with detailed README documents; the Moana Island Scene’s documentation in particular is quite extensive and contains a significant amount of information about how assets are authored and structured at Disney Animation, and how renders are lit, art-directed, and assembled at Disney Animation. I highly recommend reading all of the documentation carefully if you plan on working with these data sets, or just if you are generally curious about how production scenes are built at Disney Animation.

Personally, I’m very much looking forward to seeing what the rendering community (and the wider computer graphics community at large) does with these data sets! I’m especially excited to see what the realtime world will be able to do with this data; seeing the Moana Island Scene in its full glory in Unreal Engine 4 or Unity would be something indeed, and I think these data sets should provide a fantastic challenge to research into light transport and ray tracing speed as well. If you do interesting things with these data sets, please write to us at the email addresses in the provided README files!

Also, Matt Pharr has written on his blog about how the Moana Island Scene has further driven the development of PBRT v3. I highly recommend giving Matt’s blog a read!

Scandinavian Room Scene

Almost three years ago, I rendered a small room interior scene to test an indoor, interior illumination scenario. Since then, a lot has changed in Takua, so I thought I’d revisit an interior illumination test with a much more complex, difficult scene. I don’t have much time to model stuff anymore these days, so instead I bought Evermotion’s Archinteriors Volume 48 collection, which is labeled as Scandinavian interior room scenes (I don’t know what’s particularly Scandinavian about these scenes, but that’s what the label said) and ported one of the scenes to Takua’s scene format. Instead of simply porting the scene as-is, I modified and added various things in the scene to make it feel a bit more customized. See if you can spot what they are:

Figure 1: A Scandinavian room interior, rendered in Takua a0.8 using VCM.

I had a lot of fun adding all of my customizations! I brought over some props from the old complex room scene, such as the purple flowers and vase, a few books, and Utah teapot tray, and also added a few new fun models, such as the MacBook Pro in the back and the copy of Physically Based Rendering 3rd Edition in the foreground. The black and white photos on the wall are crops of my Minecraft renders, and some of the books against the back wall have fun custom covers and titles. Even all of the elements that came with the original scene are re-shaded. The original scene came with Vray’s standard VrayMtl as the shader for everything; Takua’s base shader parameterization draws some influence from Vray, but also draws from the Disney Bsdf and Arnold’s AlShader and as a result has a parameterization that is sufficiently different that I wound up just re-shading everything instead of trying to write some conversion tool. For the most part I was able to re-use the textures that came with the scene to drive various shader parameters. The skydome is from the noncommercial version of VizPeople’s HDRi v1 collection.

Speaking of the skydome… the main source of illumination in this scene comes from the sun in the skydome, which presented a huge challenge for efficient light sampling. Takua has had domelight/environment map importance sampling using CDF inversion sampling for a long time now, which helps a lot, but the indoor nature of this scene still made sampling the sun difficult. Sampling the sun in an outdoor scene is fairly efficient since most rays will actually reach the sun, but in indoor scenes, importance sampling the sun becomes inefficient without taking occlusion into account since only rays that actually make it outdoors through windows can reach the sun. The best known method currently for handling domelight importance sampling through windows in an indoor scene is Portal Masked Environment Map Sampling (PMEMS) by Bitterli et al. I haven’t actually implemented PMEMS yet though, so the renders in this post all wound up requiring a huge number of samples per pixel to render; I intend on implementing PMEMS at some point in the near future.

Apart from the skydome, this scene also contains several other practical light sources, such as the lamp’s bulb, the MacBook Pro’s screen, and the MacBook Pro’s glowing Apple logo on the back of the screen (which isn’t even visible to camera, but is still enabled since it provides a tiny amount of light against the back wall!). In addition to choosing where on a single light to sample, choosing which light to sample is also an extremely important and difficult problem. Until this rendering this scene, I hadn’t really put any effort into efficiently selecting which light to sample. Most of my focus has been on the integration part of light transport, so Takua’s light selection has just been uniform random selection. Uniform random selection is terrible for scenes that contain multiple lights with highly varying emission between different lights, which is absolutely the case for this scene. Like any other importance sampling problem, the ideal solution is to send rays towards lights with a probability proportional to the amount of illumination we expect each light to contribute to each ray origin point.

I implemented a light selection strategy where the probability of selecting each light is weighted by the total emitted power of each light; essentially this boils down to estimating the total emitted power of each light according to the light’s surface texture and emission function, building a CDF across all of the lights using the total emission estimates, and then using standard CDF inversion sampling to pick lights. This strategy works significantly better than uniform random selection and made a huge difference in render speed for this scene, as seen in Figures 2 through 4. Figure 2 uses uniform random light selection with 128 spp; note how the area lit by the wall-mounted lamp is well sampled, but the image overall is really noisy. Figure 3 uses power-weighted light selection with the same spp as Figure 2; the lamp area is more noisy than in Figure 2, but the render is less noisy overall. Notably, Figure 3 also took a third of the time compared to Figure 2 for the same sample count; this is because in this scene, sending rays towards the lamp is significantly more expensive due to heavier geometry than sending rays towards the sun, even when rays towards the sun get occluded by the walls. Figure 4 uses power-weighted light selection again, but is equal-time to Figure 2 instead of equal-spp; note the significant noise reduction:

Figure 2: The same frame from Figure 1, 128 spp using uniform random light selection. Average pixel RMSE compared to Figure 1: 0.439952.

Figure 3: Power-weighted light selection, with equal spp to Figure 2. Average pixel RMSE compared to Figure 1: 0.371441.

Figure 4: Power-weighted light selection again, but this time with equal time instead of equal spp to Figure 2. Average pixel RMSE compared to Figure 1: 0.315465.

Figure 5: Zoomed crops of Figures 2 through 4. From left to right: uniform random sampling, equal sample power-weighted sampling, and equal time power-weighted sampling.

However, power-weighted light selection still is not even close to being the most optimal technique possible; this technique completely ignores occlusion and distance, which are extremely important. Unfortunately, because occlusion and distance to each light varies for each point in space, creating a light selection strategy that takes occlusion and distance into account is extremely difficult and is a subject of continued research in the field. In Hyperion, we use a cache point system, which we described on page 97 of our SIGGRAPH 2017 Production Volume Rendering course notes. Other published research on the topic includes Practical Path Guiding for Efficient Light-Transport Simulation by Muller et al, On-line Learning of Parametric Mixture Models for Light Transport Simulation by Vorba et al, Product Importance Sampling for Light Transport Path Guiding by Herholz et al, Learning Light Transport the Reinforced Way by Dahm et al, and more. At some point in the future I’ll revisit this topic.

For a long time now, Takua has also had a simple interactive mode where the camera can be moved around in a non-shaded/non-lit view; I used this mode to interactively scout out some interesting and fun camera angles for some more renders. Being able to interactively scout in the same renderer used to final rendering is an extremely powerful tool; instead of guessing at depth of field settings and such, I was able to directly set and preview depth of field with immediate feedback. Unfortunately some of the renders below are noisier than I would like, due to the previously mentioned light sampling difficulties. All of the following images are rendered using Takua a0.8 with VCM:

Figure 6: A MacBook Pro running Takua Render to produce Figure 1.

Figure 7: Physically Based Rendering Third Edition sitting on the coffee table.

Figure 8: Closeup of the same purple flowers from the old Complex Room scene.

Figure 9: Utah Teapot tea set on the coffee table.

Figure 10: A glass globe with mirror-polished metal continents, sitting in the sunlight from the window.

Figure 11: Close-up of two glass and metal mugs filled with tea.

Beyond difficult light sampling, generally complex and difficult light transport with lots of subtle caustics also wound up presenting major challenges in this scene. For example, note the subtle caustics on the wall in the upper right hand part of Figure 10; those caustics are actually visibly not fully converged, even though the sample count across Figure 10 was in the thousands of spp! I intentionally did not use adaptive sampling in any of these renders; instead, I wanted to experiment with a common technique used in a lot of modern production renderers for noise reduction: in-render firefly clamping. My adaptive sampler is already capable of detecting firefly pixels and driving more samples at fireflies in the hopes of accelerating variance reduction on firefly pixels, but firefly clamping is a much more crude, biased, but nonetheless effective technique. The idea is to detect on each pixel spp if a returned sample is an outlier relative to all of the previously accumulated samples, and discard or clamp the sample if it in fact is an outlier. Picking what threshold to use for outlier detection is a very manual process; even Arnold provides a tuning max-value parameter for firefly clamping.

I wanted to be able to directly compare the render with and without firefly clamping, so I implemented firefly clamping on top of Takua’s AOV system. When enabled, firefly clamping mode produces two images for a single render: one output with firefly clamping enabled, and one with clamping disabled. I tried re-rendering Figure 10 using unidirectional pathtracing and a relatively low spp count to produce as many fireflies as I could, for a clearer comparison. For this test, I set the firefly threshold to be samples that are at least 250 times brighter than the estimated pixel value up to that sample.

Figure 12: The same render as Figure 10, but rendered with a lower sample count and using unidirectional pathtracing instead of VCM to draw out more fireflies.

Figure 13: From the same run of Takua Render as Figure 12, but the firefly-clamped render output instead of the raw render.

Note how Figure 13 appears to be completely firefly-free compared to Figure 12, and how Figure 13 doesn’t have visible caustic noise on the walls compared to Figure 10. However, notice how Figure 13 is also missing significant illumination in some areas, such as in the corner of the walls near the floor behind the wooden step ladder, or in the deepest parts of the purple flower bunch. Finding a threshold that eliminates all fireflies without loosing significant illumination in other areas is very difficult or, in some cases, impossible since some of these types of light transport essentially manifest as firefly-like high energy samples that only smooth out over time. For the final renders in Figure 1 and Figures 6 through 11, I wound up not actually using any firefly clamping. While biased noise-reduction techniques are a necessary evil in actual production, I expect that I’ll try to avoid relying on firefly clamping in the vast majority of what I do with Takua, since Takua is meant to just be a brute-force, hobby kind of thing anyway.

Aventador Renders Revisited

A long time ago, I made some posts that featured a cool Lamborghini Aventador model. Recently, I revisited that model and made some new renders using the current version of Takua, mostly just for fun. To me, one of the most important parts of writing a renderer has always been being able to actually use the renderer to make fun images. The last time I rendered this model was something like four years ago, and back then Takua was still in a very basic state; the renders in those old posts don’t even have any shading beyond 50% grey lambertian surfaces! The renders in this post utilize a lot of advanced features that I’ve added since then, such as a proper complex layered Bsdf and texturing system, advanced bidirectional light transport techniques, huge speed improvements to ray traversal, advanced motion blur and generalized time capabilities, and more. I’m way behind in writing up many of these features and capabilities, but in the meantime, I thought I’d post some for-fun rendering projects I’ve done with Takua.

All of the renders in this post are directly from Takua, with a basic white balance and conversion from HDR EXR to LDR PNG being the only post-processing steps. Each render took about half a day to render (except for the wireframe render, which was much faster) on a 12-core workstation at 2560x1440 resolution.

Figure 1: An orange-red Lamborghini Aventador, rendered in Takua a0.7 using VCM.

Shading the Aventador model was a fun, interesting exercise. I went for a orange-red paint scheme since, well, Lamborghinis are supposed to look outrageous and orange-red is a fairly exotic paint scheme (I suppose I could have picked green or yellow or something instead, but I like orange-red). I ended up making a triple-lobe shader with a metallic base, a dielectric lobe, and a clear-coat lobe on top of that. The base lobe uses a GGX microfacet metallic Brdf. Takua’s shading system implements a proper metallic Fresnel model for conductors, where the Fresnel model includes both a Nd component representing refractive index and a k component representing the extinction coefficient for when an electromagnetic wave propagates through a material. For conductors, the final Fresnel index of refraction for each wavelength of light is defined by a complex combination of Nd and k. For the base metallic lobe, most of the color wound up coming from the k component. The dielectric lobe is meant to simulate paint on top of a car’s metal body; the dielectric lobe is where most of the orange-red color comes from. The dielectric lobe is again a GGX microfacet Brdf, but with a dielectric Fresnel model, which has a much simpler index of refraction calculation than the metallic Fresnel model does. I should note that Takua’s current standard material implementation actually only supports a single primary specular lobe and an additional single clear-coat lobe, so for shaders authored with both a metallic and dielectric component, Takua takes a blend weight between the two components and for each shading evaluation stochastically selects between the two lobes according to the blend weight. The clear-coat layer on top has just a slightly amount of extinction to provide just a bit more of the final orange look, but is mostly just clear.

All of the window glass in the render is tinted slightly dark through extinction instead of through a fixed refraction color. Using proper extinction to tint glass is more realistic than using a fixed refraction color. Similarly, the red and yellow glass used in the head lights and tail lights are colored through extinction. The brake disks use an extremely high resolution bump map to get the brushed metal look. The branding and markings on the tire walls are done through a combination of bump mapping and adjusting the roughness of the microfacet Brdf; the tire treads are made using a high resolution normal map. There’s no displacement mapping at all, although in retrospect the tire treads probably should be displacement mapped if I want to put the camera closer to them. Also, I actually didn’t really shade the interior of the car much, since I knew I was going for exterior shots only.

Eventually I’ll get around to implementing a proper car paint Bsdf in Takua, but until then, the approach I took here seems to hold up reasonable well as long as the camera doesn’t get super close up to the car.

I lit the scene using two lights: an HDR skydome from HDRI-Skies, and a single long, thin rectangular area light above the car. The skydome provides the overall soft-ish lighting that illuminates the entire scene, and the rectangular area light provides the long, interesting highlights on the car body that help with bringing out the car’s shape.

For all of the renders in this post, I used my VCM integrator, since the scene contains a lot of subtle caustics and since the inside of the car is lit entirely through glass. I also wound up modifying my adaptive sampler; it’s still the same adaptive sampler that I’ve had for a few years now, but with an important extension. Instead of simply reducing the total number of paths per iteration as areas reach convergence, the adaptive sampler now keeps the number of paths the same and instead reallocates paths from completed pixels to high-variance pixels. The end result is that the adaptive sampler is now much more effective at eliminating fireflies and targeting caustics and other noisy areas. In the above render, some pixels wound up with as few as 512 samples, while a few particularly difficult pixels finished with as many as 20000 samples. Here is the adaptive sampling heatmap for Figure 1 above; brighter areas indicate more samples. Note how the adaptive sampler found a number of areas that we’d expect to be challenging, such as the interior through the car’s glass windows, and parts of the body with specular inter-reflections.

Figure 2: Adaptive sampling heatmap for Figure 1. Brighter areas indicate more samples.

I recently implemented support for arbitrary camera shutter curves, so I thought doing a motion blurred render would be fun. After all, Lamborghinis are supposed to go fast! I animated the Lamborghini driving forward in Maya; the animation was very basic, with the main body just translating forward and the wheels both translating and rotating. Of course Takua has proper rotational motion blur. The motion blur here is effectively multi-segment motion blur; generating multi-segment motion blur from an animated sequence in Takua is very easy due to how Takua handles and understands time. I actually think that Takua’s concept of time is one of the most unique things in Takua; it’s very different from how every other renderer I’ve used and seen handles time. I intend to write more about this later. Instead of an instantaneous shutter, I used a custom cosine-based shutter curve that places many more time samples near the center of the shutter interval than towards the shutter open and close. Using a shutter shape like this wound up being important to getting the right look to the motion blur; even the car is moving extremely quickly, the overall form of the car is still clearly distinguishable and the front and back of the car appear more motion-blurred than the main body.

Figure 3: Motion blurred render, using multi-segment motion blur with a cosine-based shutter curve.

Since Takua has a procedural wireframe texture now, I also did a wireframe render. I mentioned my procedural wireframe texture in a previous post, but I didn’t write about how it actually works. For triangles and quads, the wireframe texture is simply based on the distance from the hitpoint to the nearest edge. If the distance to the nearest edge is smaller than some threshold, draw one color, otherwise, draw some other color. The nearest edge calculation can be done as follows (the variable names should be self-explanatory):

float calculateMinDistance(const Poly& p, const Intersection& hit) const {
    float md = std::numeric_limits<float>::infinity();
    const int verts = p.isQuad() ? 4 : 3;
    for (int i = 0; i < verts; i++) {
        const glm::vec3& cur = p[i].m_position;
        const glm::vec3& next = p[(i + 1) % verts].m_position;
        const glm::vec3 d1 = glm::normalize(next - cur);
        const glm::vec3 d2 = hit.m_point - cur;
        const float l = glm::length((cur + d1 * glm::dot(d1, d2) - hit.m_point));
        md = glm::min(md, l * l);
    return md;

The topology of the meshes are pretty strange, since the car model came as a triangle mesh, which I then subdivided:

Figure 4: Procedural wireframe texture.

The material in the wireframe render only uses the lambertian diffuse lobe in Takua’s standard material; as such, the adaptive sampling heatmap for the wireframe render is interesting to compare to Figure 2. Overall the sample distribution is much more even, and areas where diffuse inter-reflections are present got more samples:

Figure 5: Adaptive sampling heatmap for Figure 4. Brighter areas indicate more samples. Compare with Figure 2.

Takua’s shading model supports layering different materials through parameter blending, similar to how the Disney Brdf (and, at this point, most other shading systems) handles material layering. I wanted to make an even more outrageous looking version of the Aventador than the orange-red version, so I used the procedural wireframe texture as a layer mask to drive parameter blending between a black paint and a metallic gold paint:

Figure 6: An outrageous Aventador paint scheme using a procedural wireframe texture to blend between black and metallic gold car paint.

Olaf's Frozen Adventure

After an amazing 2016, we’re having a bit of a break year this year at Walt Disney Animation Studios. We don’t have a feature film this year; instead, we made a half-hour featurette called Olaf’s Frozen Adventure, which will be released in front of Pixar’s Coco during Thanksgiving. I think this is the first time a Disney Animation short/featurette has accompanied a Pixar film. Olaf’s Frozen Adventure is a fun little holiday story set in the world of Frozen, and I had the privilege of getting to play a small role in making Olaf’s Frozen Adventure! I got an official credit as part of a handful of engineers that did some specific, interesting technology development for Olaf’s Frozen Adventure.

Olaf’s Frozen Adventure is really really funny; because Olaf is the main character, the entire story takes on much more of a self-aware, at times somewhat absurdist tone. The featurette also has a bunch of new songs- there are six new songs in total, which is somehow pretty close to the original film’s count of eight songs, but in a third of the runtime. Olaf’s Frozen Adventure was originally announced as a TV special, but the wider Walt Disney Company was so happy with the result that they decided to give us a theatrical release instead!

Something I personally find fascinating about Olaf’s Frozen Adventure is comparing it visually with the original Frozen. Olaf’s Frozen Adventure is rendered entirely with Disney’s Hyperion Renderer, compared with Frozen, which was rendered using pre-RIS Renderman. While both films used our Disney BRDF, Olaf’s Frozen Adventure benefits from all of the improvements and advancements that have been made during Big Hero 6, Zootopia, and Moana. The original Frozen used dipole subsurface scattering, radiosity caching, and generally had fairly low geometric complexity relative to Hyperion-era films. In comparison, Olaf’s Frozen Adventure uses brute force subsurface scattering, uses path-traced global illumination, uses the full Disney BSDF (which is significantly extended from the Disney BRDF), uses our advanced fur/hair shader developed during Zootopia, and has much greater geometric complexity. Some shots even utilize an extended version of the photon mapped caustics we developed during Moana. Extensions to our photon mapping system is one of the things I worked on for Olaf’s Frozen Adventure. Even the water in Arendelle Harbor looks way better than in Frozen, since we were able to make use of the incredible water systems developed for Moana. All of these advancements are discussed in our SIGGRAPH 2017 Course Notes.

As usual with Disney Animation projects I get to work on, here are some stills in no particular order, pulled from marketing material. Even though Olaf’s Frozen Adventure was originally meant for TV, we still put the same level of effort into it that we do with theatrical features, and I think it shows!

All images in this post are courtesy of and the property of Walt Disney Animation Studios.

SIGGRAPH 2017 Course Notes- Recent Advances in Disney's Hyperion Renderer

This year at SIGGRAPH 2017, Luca Fascione and Johannes Hanika from Weta Digital organized a Path Tracing in Production course. The course was split into two halves: a first half about production renderers, and a second half about using production renderers to make movies. Brent Burley presented our recent work on Disney’s Hyperion Renderer as part of the first half of the course. To support Brent’s section of the course, the entire Hyperion team worked together to put together some course notes describing recent work in Hyperion done for Zootopia, Moana, and upcoming films.

Image from course notes Figure 8: a production frame from Zootopia, rendered using Disney's Hyperion Renderer.

Here is the abstract for the course notes:

Path tracing at Walt Disney Animation Studios began with the Hyperion renderer, first used in production on Big Hero 6. Hyperion is a custom, modern path tracer using a unique architecture designed to efficiently handle complexity, while also providing artistic controllability and efficiency. The concept of physically based shading at Disney Animation predates the Hyperion renderer. Our history with physically based shading significantly influenced the development of Hyperion, and since then, the development of Hyperion has in turn influenced our philosophy towards physically based shading.

The course notes and related materials can be found at:

The course wasn’t recorded due to proprietary content from various studios, but the overall course notes for the entire course cover everything that was presented. The major theme of our part of the course notes (and Brent’s presentation) is replacing multiple scattering approximations with accurate brute-force path-traced solutions. Interestingly, the main motivator for this move is primarily a desire for better, more predictable and intuitive controls for artists, as opposed to simply just wanting better visual quality. In the course notes, we specifically discuss fur/hair, path-traced subsurface scattering, and volume rendering.

The Hyperion team also had two other presentations at SIGGRAPH 2017: