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.

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:

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!

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:

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:

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:

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.

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.

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.

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.

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.

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:

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:

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:

After an amazing 2016, Walt Disney Animation Studios is having a bit of a break year this year. Disney Animation doesn’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 Olaf’s Frozen Adventure 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 [Burley 2012] and Ptex [Burley and Lacewell 2008], 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) [Burley 2015], uses our advanced fur/hair shader developed during Zootopia [Chiang et al. 2016], and has much greater geometric complexity. A great example of the greater geometric complexity is the knitted scarf sequence [Staub et al. 2018], where 2D animation was brought into Hyperion as a texture map to drive the colors on a knitted scarf that was modeled and rendered down to the fiber level. Some shots even utilize an extended version of the photon mapped caustics we developed during Moana; the photon mapped caustics system on Moana only supported distant lights as a photon source, but for Olaf’s Frozen Adventure, the photon mapping system was extended to support all of Hyperion’s existing light types as photon sources. These extensions to our photon mapping system is one of the things I worked on for Olaf’s Frozen Adventure, and was used for lighting the ice crystal tree that Elsa creates at the end of the film. Even the water in Arendelle Harbor looks way better than in Frozen, since the FX artists were able to make use of the incredible water systems developed for Moana [Palmer et al. 2017]. Many of these advancements are discussed in our SIGGRAPH 2017 Course Notes [Burley et al. 2017].

One of the huge advantages to working on an in-house production rendering team in a vertically integrated studio is being able to collaborate and partner closely with productions on executing long-term technical visions. Because of the show leadership’s confidence in our long-term development efforts targeted at later shows, the artists on Olaf’s Frozen Adventure were willing to take on and try out early versions of a number of new features in Hyperion that were originally targeted at later shows. Some of these “preview” features wound up making a big difference on Olaf’s Frozen Adventure, and lessons learned on Olaf’s Frozen Adventure were instrumental in making these features much more robust and complete on Ralph Breaks the Internet.

One major feature was brute force path-traced subsurface scattering; Peter Kutz, Matt Chiang, and Brent Burley had originally started development during Moana’s production on brute force path-traced subsurface scattering [Chiang 2016] as a replacement for Hyperion’s existing normalized diffusion based subsurface scattering [Burley 2015]. This feature wasn’t completed in time for use on Moana (although some initial testing was done using Moana assets), but was far enough along by Olaf’s Frozen Adventure was in production that artists started to experiment with it. If I remember correctly, the characters in Olaf’s Frozen Adventure are still using normalized diffusion, but path-traced subsurface wound up finding extensive use in rendering all of the snow in the show, since the additional detail that path-traced subsurface brings out helped highlight the small granular details in the snow. A lot of lessons learned from using path-traced subsurface scattering on the snow were then applied to making path-traced subsurface scattering more robust and easier to use and control. These experiences gave us the confidence to go ahead with full-scale deployment on Ralph Breaks the Internet, which uses path-traced subsurface scattering for everything including characters.

Another major development effort that found experimental use on Olaf’s Frozen Adventure were some large overhauls to Hyperion’s ray traversal system. During the production of Moana, we started running into problems with how large instance groups are structured in Hyperion. Moana’s island environments featured vast quantities of instanced vegetation geometry, and because of how the instancing was authored, Hyperion’s old strategy for grouping instances in the top-level BVH wound up producing heavily overlapping BVH leaves, which in extreme cases could severely degrade traversal performance. On Moana, the solution to this problem was to change how instances were authored upstream in the pipeline, but the way that the renderer wanted instances organized was fairly different from how artists and our pipeline like to think about instances, which made authoring more difficult. This problem motivated Peter Kutz and I to develop a new traversal system that would be less sensitive to how instance groups were authored; the system we came up with allows Hyperion to internally break up top-level BVH nodes with large overlapping bounds into smaller, tighter subbounds based on the topology of the lower-level BVHs. It turns out this system is conceptually essentially identical to BVH rebraiding [Benthin et al. 2017], but we developed and deployed this system independently before Benthin 2017 was published. As part of this effort, we also wound up revisiting Hyperion’s original cone-based packet traversal strategy [Eisenacher et al. 2013] and, motivated by extensive testing and statistical performance analysis, developed a new, simpler, higher performance multithreading strategy for handling Hyperion’s ultra-wide batched ray traversal. Olaf’s Frozen Adventure has a sequence where Olaf and Sven are being pulled down a mountainside through a forest by a burning sled; the enormous scale of the groundplane and large quantities of instanced trees proved to be challenging for Hyperion’s old traversal system. We were able to partner with the artists to deploy a mid-development prototype of our new traversal system on this sequence, and were able to cut traversal times by up to close to an order of magnitude in some cases. As a result, the artists were able to render this sequence with reasonable render times, and we were able to field-test the new traversal system prior to studio-wide deployment and iron out various kinks that were found along the way.

The last major mid-development Hyperion feature that saw early experimental use on Olaf’s Frozen Adventure was our new, next-generation spectral and decomposition tracking [Kutz et al. 2017] based null-collision volume rendering system, which was written with the intention of eventually completely replacing Hyperion’s existing residual ratio tracking [Novák 2014] based volume rendering system [Fong 2017]. Artists on Olaf’s Frozen Adventure ran into some difficulties with rendering loose, fluffy white snow, where the bright white appearance is the result of high-order scattering requiring large numbers of bounces. We realized that this problem is essentially identical to the problem of rendering white puffy clouds, which also have an appearance dominated by energy from high-order scattering. Since null-collision volume integration is specifically very efficient at handling high-order scattering, we gave the artists an early prototype version of Hyperion’s new volume rendering system to experiment with rendering loose fluffy snow as a volume. The initial results looked great; I’m not sure if this approach wound up being used in the final film, but this experiment gave both us and the artists a lot of confidence in the new volume rendering system and provided valuable feedback.

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

Here is a credits frame with my name! I wasn’t actually expecting to get a credit on Olaf’s Frozen Adventure, but because I had spent a lot of time supporting the show and working with artists on deploying experimental Hyperion features to solve particularly difficult shots, the show decided to give me a credit! I was very pleasantly surprised by that; my teammate Matt Chiang got a credit as well for similar reasons.

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

References

Carsten Benthin, Sven Woop, Ingo Wald, and Attila T. Áfra. 2017. Improved Two-Level BVHs using Partial Re-Braiding. In HPG ‘17 (Proceedings of High Performance Graphics). 7:1-7:8.

Brent Burley. Physically Based Shading at Disney. 2012. In ACM SIGGRAPH 2012 Course Notes: Practical Physically-Based Shading in Film and Game Production.

Brent Burley. Extending the Disney BRDF to a BSDF with Integrated Subsurface Scattering. 2015. In ACM SIGGRAPH 2015 Course Notes: Physically Based Shading in Theory and Practice.

Brent Burley, David Adler, Matt Jen-Yuan Chiang, Ralf Habel, Patrick Kelly, Peter Kutz, Yining Karl Li, and Daniel Teece. 2017. Recent Advances in Disney’s Hyperion Renderer. Path Tracing in Production Part 1, ACM SIGGRAPH 2017 Course Notes.

Brent Burley and Dylan Lacewell. 2008. Ptex: Per-face Texture Mapping for Production Rendering. Computer Graphics Forum. 27, 4 (2008), 1155-1164.

Matt Jen-Yuan Chiang, Benedikt Bitterli, Chuck Tappan, and Brent Burley. 2016. A Practical and Controllable Hair and Fur Model for Production Path Tracing. Computer Graphics Forum. 35, 2 (2016), 275-283.

Matt Jen-Yuan Chiang, Peter Kutz, and Brent Burley. 2016. Practical and Controllable Subsurface Scattering for Production Path Tracing. In ACM SIGGRAPH 2016 Talks. 49:1-49:2.

Christian Eisenacher, Gregory Nichols, Andrew Selle, and Brent Burley. 2013. Sorted Deferred Shading for Production Path Tracing. Computer Graphics Forum. 32, 4 (2013), 125-132.

Julian Fong, Magnus Wrenninge, Christopher Kulla, and Ralf Habel. 2017. Production Volume Rendering. In ACM SIGGRAPH 2017 Courses.

Peter Kutz, Ralf Habel, Yining Karl Li, and Jan Novák. 2017. Spectral and Decomposition Tracking for Rendering Heterogeneous Volumes. ACM Transactions on Graphics. 36, 4 (2017), 111:1-111:16.

Jan Novák, Andrew Selle, and Wojciech Jarosz. 2014. Residual Ratio Tracking for Estimating Attenuation in Participating Media. ACM Transactions on Graphics. 33, 6 (2014), 179:1-179:11.

Sean Palmer, Jonathan Garcia, Sara Drakeley, Patrick Kelly, and Ralf Habel. 2017. The Ocean and Water Pipeline of Disney’s Moana. In ACM SIGGRAPH 2017 Talks. 29:1-29:2.

Josh Staub, Alessandro Jacomini, Dan Lund. 2018. The Handiwork Behind “Olaf’s Frozen Adventure”. In ACM SIGGRAPH 2018 Talks. 26:1-26:2.

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.

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:

• Official Course Resources Page (Full course notes and supplemental materials)
• Project Page (Author’s Version)
• Official Print Version (ACM Library)

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:

• Ralf Habel presented several sections of the “Production Volume Rendering”” course, which was jointly put together by Julian Fong and Magnus Wrenninge from Pixar Animation Studios, Christophe Kulla from Sony Imageworks, and Ralf Habel from Walt Disney Animation Studios.
• Peter Kutz presented our “Spectral and Decomposition Tracking for Rendering Heterogeneous Volumes” technical paper in the “Rendering Volumes” papers session.

Some recent work I was part of at Walt Disney Animation Studios has been published in the July 2017 issue of ACM Transactions on Graphics as part of SIGGRAPH 2017! The paper is titled “Spectral and Decomposition Tracking for Rendering Heterogeneous Volumes”, and the project was a collaboration between the Hyperion development team at Walt Disney Animation Studios (WDAS) and the rendering group at Disney Research Zürich (DRZ). From the WDAS side, the authors are Peter Kutz (who was at Penn at the same time as me), Ralf Habel, and myself. On the DRZ side, our collaborator was Jan Novák, the head of DRZ’s rendering research group.

Here is the paper abstract:

We present two novel unbiased techniques for sampling free paths in heterogeneous participating media. Our decomposition tracking accelerates free-path construction by splitting the medium into a control component and a residual component and sampling each of them separately. To minimize expensive evaluations of spatially varying collision coefficients, we define the control component to allow constructing free paths in closed form. The residual heterogeneous component is then homogenized by adding a fictitious medium and handled using weighted delta tracking, which removes the need for computing strict bounds of the extinction function. Our second contribution, spectral tracking, enables efficient light transport simulation in chromatic media. We modify free-path distributions to minimize the fluctuation of path throughputs and thereby reduce the estimation variance. To demonstrate the correctness of our algorithms, we derive them directly from the radiative transfer equation by extending the integral formulation of null-collision algorithms recently developed in reactor physics. This mathematical framework, which we thoroughly review, encompasses existing trackers and postulates an entire family of new estimators for solving transport problems; our algorithms are examples of such. We analyze the proposed methods in canonical settings and on production scenes, and compare to the current state of the art in simulating light transport in heterogeneous participating media.

The paper and related materials can be found at:

• Official WDAS Project Page (Preprint paper and supplemental materials)
• Project Page (Author’s Version)
• Official Print Version (ACM Library)

Peter Kutz will be presenting the paper at SIGGRAPH 2017 in Log Angeles as part of the Rendering Volumes Technical Papers session.

Instead of repeating the contents of the paper here (which is pointless since the paper already says everything we want to say), I thought instead I’d use this blog post to talk about some of the process we went through while writing this paper. Please note that everything stated in this post are my own opinions and thoughts, not Disney’s.

This project started over a year ago, when we began an effort to significantly overhaul and improve Hyperion’s volume rendering system. Around the same time that we began to revisit volume rendering, we heard a lecture from a visiting professor on multilevel Monte Carlo (MLMC) methods. Although the final paper has nothing to do with MLMC methods, the genesis of this project was in initial conversations we had about how MLMC methods might be applied to volume rendering. We concluded that MLMC could be applicable, but weren’t entirely sure how. However, these conversations eventually gave Peter the idea to develop the technique that would eventually become decomposition tracking (importantly, decomposition tracking does not actually use MLMC though). Further conversations about weighted delta tracking then led to Peter developing the core ideas behind what would become spectral tracking. After testing some initial implementations of these prototype version of decomposition and spectral tracking, Peter, Ralf, and I shared the techniques with Jan. Around the same time, we also shared the techniques with our sister teams, Pixar’s RenderMan development group in Seattle and the Pixar Research Group in Emeryville, who were able to independently implement and verify our techniques. Being able to share research between Walt Disney Animation Studios, Disney Research, the Renderman group, Pixar Animation Studios, Industrial Light & Magic, and Imagineering is one of the reasons why Disney is such an amazing place to be for computer graphics folks.

At this point we had initial rudimentary proofs for why decomposition and spectral tracking worked separately, but we still didn’t have a unified framework that could be used to explain and combine the two techniques. Together with Jan, we began by deep-diving into the origins of delta/Woodcock tracking in neutron transport and reactor physics papers from the 1950s and 1960s and working our way forward to the present. All of the key papers we dug up during this deep-dive are cited in our paper. Some of these early papers were fairly difficult to find. For example, the original delta tracking paper, “Techniques used in the GEM code for Monte Carlo neutronics calculations in reactors and other systems of complex geometry” (Woodcock et al. 1965), is often cited in graphics literature, but a cursory Google search doesn’t provide any links to the actual paper itself. We eventually managed to track down a copy of the original paper in the archives of the United States Department of Commerce, which for some reason hosts a lot of archive material from Argonne National Laboratory. Since the original Woodcock paper has been in the public domain for some time now but is fairly difficult to find, I’m hosting a copy here for any researchers that may be interested.

Several other papers we were only able to obtain by requesting archival microfilm scans from several university libraries. I won’t host copies here, since the public domain status for several of them isn’t clear, but if you are a researcher looking for any of the papers that we cited and can’t find it, feel free to contact me. One particularly cool find was “The Relativistic Doppler Problem” (Zerby et al. 1961), which Peter obtained by writing to the Oak Ridge National Laboratory’s research library. Their staff were eventually able to find the paper in their records/archives, and subsequently scanned and uploaded the paper online. The paper is now publicly available here, on the United States Department of Energy’s Office of Scientific and Technical Information website.

Eventually, through significant effort from Jan, we came to understand Galtier et al.’s 2013 paper, “Integral Formulation of Null-Collision Monte Carlo Algorithms”, and were able to import the integral formulation into computer graphics and demonstrate how to derive both decomposition and spectral tracking directly from the radiative transfer equation using the integral formulation. This step also allowed Peter to figure out how to combine spectral and decomposition tracking into a single technique. With all of these pieces in place, we had the framework for our SIGGRAPH paper. We then put significant effort into working out remaining details, such as finding a good mechanism for bounding the free-path-sampling coefficient in spectral tracking. Producing all of the renders, results, charts, and plots in the paper also took an enormous amount of time; it turns out that producing all of this stuff can take significantly longer than the amount of time originally spent coming up with and implementing the techniques in the first place!

One major challenge we faced in writing the final paper was finding the best order in which to present the three main pieces of the paper: decomposition tracking, spectral tracking, and the integral formulation of null-collision algorithms. At one point, we considered first presenting decomposition tracking, since on a general level decomposition tracking is the easiest of the three contributions to understand. Then, we planned to use the proof of decomposition tracking to expand out into the integral formulation of the RTE with null collisions, and finally derive spectral tracking from the integral formulation. The idea was essentially to introduce the easiest technique first, expand out to the general mathematical framework, and then demonstrate the flexibility of the framework by deriving the second technique. However, this approach in practice felt disjointed, especially with respect to the body of prior work we wanted to present, which underpinned the integral framework but wound up being separated by the decomposition tracking section. So instead, we arrived on the final presentation order, where we first present the integral framework and derive out prior techniques such as delta tracking, and then demonstrate how to derive out new decomposition tracking and spectral tracking techniques. We hope that presenting the paper in this way will encourage other researchers to adopt the integral framework and derive other, new techniques from the framework. For Peter’s presentation at SIGGRAPH, however, Peter chose to go with the original order since it made for a better presentation.

Since our final paper was already quite long, we had to move some content into a separate supplemental document. Although the supplemental content isn’t necessary for implementing the core algorithms presented, I think the supplemental content is very useful for gaining a better understanding of the techniques. The supplemental content contains, among other things, an extended proof of the minimum-of-exponents mechanism that decomposition tracking is built on, various proofs related to choosing bounds for the local collision weight in spectral tracking, and various additional results and further analysis. We also provide a nifty interactive viewer for comparing our techniques against vanilla delta tracking; the interactive viewer framework was originally developed by Fabrice Rousselle, Jan Novák and Benedikt Bitterli at Disney Research Zürich.

One of the major advantages of doing rendering research at a major animation or VFX studio is the availability of hundreds of extremely talented artists, who are always eager to try out new techniques and software. Peter, Ralf, and I worked closely with a number of artists at WDAS to test our techniques and produce interesting scenes with which to generate results and data for the paper. Henrik Falt and Alex Nijmeh had created a number of interesting clouds in the process of testing our general volume rendering improvements, and worked with us to adapt a cloud dataset for use in Figure 11 of our paper. The following is one of the renders from Figure 11:

Henrik and Alex also constructed the cloudscape scene used as the banner image on the first page of the paper. After we submitted the paper, Henrik and Alex continued iterating on this scene, which eventually resulted in the more detailed version seen in our SIGGRAPH Fast Forward video. The version of the cloudscape used in our paper is reproduced below:

To test out spectral tracking, we wanted an interesting, dynamic, colorful dataset. After describing spectral tracking to Jesse Erickson, we arrived at the idea of a color explosion similar in spirit to certain visuals used in recent Apple and Microsoft ads, which in turn were inspired by the Holi festival celebrated in India and Nepal. Jesse authored the color explosion in Houdini and provided a set of VDBs for each color section, which we were then able to shade, light, and render using Hyperion’s implementation of spectral tracking. The final result was the color explosion from Figure 12 of the paper, seen at the top of this post. We were honored to learn that the color explosion figure was chosen to be one of the pictures on the back cover of this year’s conference proceedings!

At one point we also remembered that brute force path-traced subsurface scattering is just volume rendering inside of a bounded surface, which led to the translucent heterogeneous Stanford dragon used in Figure 15 of the paper:

For our video for the SIGGRAPH 2017 Fast Forward, we were able to get a lot of help from a number of artists. Alex and Henrik and a number of other artists significantly expanded and improved the cloudscape scene, and we also rendered out several more color explosion variants. The final fast forward video contains work from Alex Nijmeh, Henrik Falt, Jesse Erickson, Thom Wickes, Michael Kaschalk, Dale Mayeda, Ben Frost, Marc Bryant, John Kosnik, Mir Ali, Vijoy Gaddipati, and Dimitre Berberov. The awesome title effect was thought up by and created by Henrik. The final video is a bit noisy since we were severely constrained on available renderfarm resources (we were basically squeezing our renders in between actual production renders), but I think the end result is still really great:

Here are a couple of cool stills from the fast forward video:

We owe an enormous amount of thanks to fellow Hyperion teammate Patrick Kelly, who played an instrumental role in designing and implementing our overall new volume rendering system, and who discussed with us extensively throughout the project. Hyperion teammate David Adler also helped out a lot in profiling and instrumenting our code. We also must thank Thomas Müller, Marios Papas, Géraldine Conti, and David Adler for proofreading, and Brent Burley, Michael Kaschalk, and Rajesh Sharma for providing support, encouragement, and resources for this project.

I’ve worked on a SIGGRAPH Asia paper before, but working on a large scale publication in the context of a major animation studio instead of in school was a very different experience. The support and resources we were given and the amount of talent and help that we were able to tap into made this project possible. This project is also an example of the incredible value that comes from companies maintaining in-house industrial research labs; this project absolutely would not have been possible without all of the collaboration from DRZ, in both the form of direct collaboration from Jan and indirect collaboration from all of the DRZ researchers that provided discussions and feedback. Everyone worked really hard, but overall the whole process was immensely intellectually satisfying and fun, and seeing our new techniques in use by talented, excited artists makes all of the work absolutely worthwhile!