Disney Animation’s fall 2023 release is Wish, which is the studio’s 62nd animated feature and also the studio’s film celebrating the 100th anniversary of Disney Animation and The Walt Disney Company. Wish is a brand new story but is also steeped in the past century of Disney storytelling; while Asha and Star and Valentino’s adventure is a new musical story, the themes and setting Wish draws upon are timeless and classic. As part of this theme of modern Disney with throwback elements, Wish also has a unique beautiful visual style that combines the latest of our computer graphics animation with a classic watercolor style. Creating this style presented an interesting set of new challenges for our artists, TDs, engineers, and for Disney’s Hyperion Renderer.

At Disney Animation, every one of our films is a new opportunity for us to push our filmmaking art and technology forward. On most films this advancement takes place across many different aspects of the film, but on Wish, there is one obvious challenge that stood out above everything else: the film’s visual style. Of course we still made large improvements in other areas, such as major pipeline optimizations [Li et al. 2024a], but on Wish a large proportion of technology development was focused on achieving the target visual style. One could be forgiven for thinking that stylization is mostly a rendering problem, but on Wish it really was a challenge that reached into every department and every corner or our production process. Stylization on Wish meant stylization in modeling, lookdev, animation, effects, lighting, everything else in between, and even new pipeline challenges!

The decision to give Wish a unique style came from pretty much the very beginning of the project; the studio wanted to do something special for the 100th anniversary film to tie together our modern way of making animated films with the studio’s rich hand-drawn heritage. The look of Wish is especially influenced by early 20th century Disney traditional watercolor animation, with Snow White and the Seven Dwarfs (1937) and Sleeping Beauty (1959) being the largest guideposts. This influence extends all the way to the very shape of the film, so to speak- Wish is the first CG film that Disney Animation has made in the ultrawide 2.55:1 Cinemascope-style aspect ratio, matching the aspect ratio that was used on Sleeping Beauty and Lady and the Tramp (1955). This aspect ratio choice meant that stylization on Wish even impacted layout, since they had to think about how to frame for such an ultrawide image!

Disney Animation has a long history of stylizing 3D CG to resemble and fit in with hand-drawn animation, going all the way back to the studio’s traditional hand-drawn era [Meier 1996, Daniels 1999, Tamstorf et al. 2001, Odermatt and Springfield 2002, Teece 2003]. In the 3D CG era, Disney Animation has continually experimented with stylizing CG as well with a number of different approaches. Paperman focused on integrating 2D linework with 3D rendered characters [Kahrs et al. 2012 ,Whited et al. 2012], while Feast experimented with driving 2D lighting entirely in compositing on top of flat unlit/unshaded 3D renders [Osborne and Staub 2014]. The studio’s recent Short Circuit experimental shorts program had many shorts that experimented with a variety of different stylized looks, ranging from Chinese ink brush watercolors to graphic 2D illustrations to stop motion and wood carved looks [Newfield and Staub 2020]. Disney Research has also worked closely with Disney Animation in the past decade plus to develop various experimental stylization techniques [Schmid et al. 2011, Sýkora et al. 2014]. Both Strange World and Raya and the Last Dragon had small amounts of stylized sequences as well, with Strange World doing a 1950s pulp scifi comic book look and Raya and the Last Dragon doing a more graphic digital mixed media sort of look. Most recently, the short Far From the Tree utilized a look with cel-shaded characters on watercolor backgrounds. All of these were animated using our standard 3D pipeline, with much of the look being built using a combination of render passes from the renderer (Hyperion for everything except Paperman, which preceded Hyperion’s existence by a few years), various tricks in lighting, and a lot of work in compositing; how much of each was used varied widely per show and per target style.

Wish builds upon all of these predecessors. Wish’s stylization system is a vastly expanded version of what was used on Far From the Tree, which in turn was built on top of everything that was developed for Short Circuit, which in turn drew upon lessons from both Feast and Paperman. One of the biggest challenges came from having to scale up a stylization pipeline from a short film to a full feature length project, while also trying to hit a new target style. Early tests on the show were able to reproduce in CG the target visdev paintings essentially exactly, but through entirely ad-hoc and mostly manual approaches, which we then had to systematically take apart and figure out how to apply to the whole movie. My wife, Harmony Li, was an Associate Technical Supervisor on the show and (among a ton of other things) oversaw the development of the entire technical backend that was built out to support stylization on Wish [Li et al. 2024b]; as a member of the rendering team, I got to work with her on this, which was great fun! Meanwhile, much of the development for the actual techniques used was led primarily by lighting and lookdev artists.

An early breakthrough in achieving Wish’s style was finding that combining Kuwahara filters¹ [Kuwahara et al. 1976] with linework generated from the renderer created a convincing starting point for a line-on-watercolor look that could use the renderer’s physical lighting as a starting point, instead of needing to construct stylized lighting entirely from scratch in comp on top of flat-shaded unlit renders. To help really tie together the watercolor look, early tests put the entire image on a watercolor paper texture background, but once we tested the watercolor paper texture background in motion, some issues became apparent. With just a static watercolor paper texture background, the illusion of motion broke as animation looked like it was “swimming” through the texture, but simply texturing everything with watercolor textures in 3D space looked downright bizarre since it looked less like the frame was a watercolor painting and more like all of the characters and the environment were made out of paper. To solve this problem, the Hyperion team invented a new dynamic screen space texture technique [Burley et al. 2024] where the renderer would project screen space textures onto 3D surfaces while tracking motion vectors. The result is that Wish’s watercolor backgrounds look like just a flat sheet of watercolor paper when still, but under motion convincingly move with the characters while neither looking like they’re actually in 3D space nor looking disconnected from motion.

One interesting question I worked on for Wish was making Hyperion robustly handle shading normals that are really dramatically disconnected from the underlying “physically correct” geometric normal. Extreme bending of normals was used extensively on Wish to simplify shapes and art-direct lighting detail and shadows. In a normal physically based path traced render, shading normals coming from bump mapping and normal mapping usually have at least some relationship with the underlying true geometric normal, meaning that the ways shading normals modify light transport are relatively constrained to a plausible range. However, on Wish, extreme shading normals were used for things like simplifying the lighting on an entire complex tree canopy to match what the lighting would be on a simple sphere. Making Hyperion handle these cases both from an authoring perspective and making Hyperion’s light transport robust against these cases took some work!

There were actually also some more traditional physically based rendering problems to solve on Wish too, which one might not necessarily expect for such a stylized film. For some of Magnifico’s magic, the art-direction called for a sort of prismatic look where white light would get split into different colors. We decided to try to achieve this effect through physically based shading, since the starting point for Wish’s entire stylization pipeline was renders with physical lighting in order to provide consistency. To achieve this effect through physically based shading, I extended the Disney BSDF with spectral dispersion support (retrofitting a spectral effect into a non-spectral renderer was a fun challenge worth discussing on its own someday). Once our lookdev artists had access to dispersion within the Disney BSDF, it was fun seeing all of the other places where they started sprinkling the effect in, such as in various glass objects.

Stylization on Wish didn’t just mean new renderer effects and lighting and compositing work; in order to make characters read correctly in a watercolor look, stylization had to be incorporated into all of the characters at a geometric and design level as well, and had to be incorporated into animation and simulation. As an example: a core story device in Wish is the collective wishes of Rosas, which take the form of orbs containing entire small worlds set inside of swirling volumetrics. Creating these wishes required clever pipeline solutions to embed entire stylized animated scenes inside of the orbs in 3D space, which was used instead of a usual compositing-based insert-shot workflow or the teleport-based solution [Butler et al. 2022] used on Encanto; this approach was taken in order to provide animators with the ability to sync and see fully combined shots interactively and to simplify the rendering setup needed for stylization in lighting and compositing [Karanam et al. 2024]. On top of creating the individual wishes, huge numbers of wishes then had to be choreographed into tight, closely synchronized formations to meet the art-direction and shape language of the songs they were a part of, which required developing new crowd rigs and animation controls. The rendering aspect of the wishes was in a lot of ways actually the easy part! Each wish was also an internally emissive object, so when thousands of wishes are massed together in key sequences in the movie, we initially had some concerns about efficiently rendering all of the wishes, but our long-standing cache points many-light selection strategy [Li et al. 2024c] proved to be more than capable for the task.

Another example of stylization far upstream of lighting is in the project’s entire approach to character stylization. Character hair and fur grooms required a different approach from our usual process; normally in more photoreal Disney Animation films, hair and fur grooms are built to be highly detailed to support the rich detailed look of the film, but Wish’s watercolor style meant using a more simplified and graphic shape language across the board, where detail is traded off for a stronger focus on silhouette and overall massing. Hair and fur grooms had to be adjusted to match, and hair and fur simulation had to be adjusted to keep art-directed shapes intact instead of operating on a more individual strand-based level [Kaur and Stratton 2024]. Asha’s braids, with their North African inspired long box braids, required additional attention to create and simulate [Kaur et al. 2024]. The braids themselves were already a major technical challenge; even using our state-of-the-art in-house grooming system Tonic [Simmons and Whited 2014], the braids still required a final groom two orders of magnitude more complex than our average groom. Once Asha’s groom was figured out, her hair then had to also be put through the same stylized simulation setup mentioned earlier, with extensive 2D drawovers being used to art-direct simulations. Character animation then also had to take into account the fact that Wish does not have motion blur and how that impacts how viewers perceive character performances.

Speaking of 2D drawovers, one particularly interesting use of 2D drawings to art-direct stylization on Wish is in Magnifico’s magic and in various effects like flames and torches. Normal volumetric effects created from simulations tend to be highly physical and detailed, but Wish’s style called for these effects to harken back to the much more graphic shape language of magic effects from Disney Animation’s hand-drawn era. To do this, our effects artists built on top of the neural volume style transfer work from Raya and the Last Dragon [Navarro and Rice 2021] and Strange World [Navarro 2023] to develop a new system where effects animation would begin with hand-drawn 2D elements, which were then projected and extruded into the 3D space to provide a guide for neural volume style transfer on top of volume simulations [Tollec and Navarro 2024]. The result is that Wish’s volumetric effects combine the movement and interactions of physical simulations while retaining the shape and style of traditional hand-drawn effects.

Everything I’ve written about here is just what I was familiar with on this film; vastly more work went into every single frame of Wish than even I know. The final look of Wish is something that I think really is unique and beautiful. Wish’s 3D watercolor look speaks to the entire history of Disney Animation and simultaneously roots itself in the studio’s rich traditional hand-drawn legacy while also exemplifying the studio’s long history of innovating and driving filmmaking craft forward. Walt Disney never stopped seeking to innovate in animation, and 100 years after he founded the studio, the animation studio that carries his name today continues to embody that same bold spirit on every new film. As someone who’s a lifelong fan and student of animation, I feel incredibly humbled and fortunate to get to contribute towards that legacy every day.

Below are some frames from Wish, pulled from the Blu-ray and presented in no particular order. As always, I’d highly recommend seeing Wish on the biggest screen you can find!

Here are two credits frames from Wish; the first is the fancy hero-credit card for my wife Harmony Li and her fellow Associate Technical Supervisors, and the second is for the Hyperion team, along with several of the Hyperion’s sister technology teams that all support lighting and lookdev. Also, Wish has a lovely post-credits scene that I’d encourage sticking around for!

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

References

Brent Burley, Brian Green, and Daniel Teece. 2024. Dynamic Screen Space Textures for Coherent Stylization. In ACM SIGGRAPH 2024 Talks. Article 50.

Corey Butler, Brent Burley, Wei-Feng Wayne Huang, Yining Karl Li, and Benjamin Huang. 2022. “Encanto” - Let’s Talk About Bruno’s Visions. In ACM SIGGRAPH 2022 Talks. Article 8.

Eric Daniels. 1999. Deep Canvas in Disney’s Tarzan. In ACM SIGGRAPH 1999 Sketches & Applications. 200.

Avneet Kaur, Jennifer Stratton, David Hutchins, and Nikki Mull. 2024. Art-Directing Asha’s Braids in Disney’s Wish. In ACM SIGGRAPH 2024 Talks. Article 4.

Avneet Kaur and Jennifer Stratton. 2024. Character Stylization in Disney’s Wish. In ACM SIGGRAPH 2024 Talks. Article 5.

John Kahrs, Patrick Osborne, Amol Sathe, Jeff Turley, Brian Whited, and Darrin Butters. 2012. The Art and Science Behind Walt Disney Animation Studios’ “Paperman”. In ACM SIGGRAPH 2012 Production Sessions.

Neelima Karanam, Joel Einhorn, Emily Vo, and Harmony M. Li. 2024. Creating the Wishes of Rosas. In ACM SIGGRAPH 2024 Talks. Article 6.

Michiyoshi Kuwahara, Kozaburo Hachimura, Shigeru Eiho, and Masato Kinoshita. 1976. Processing of RI-Angiocardiographic Images. In Digital Processing of Biomedical Images. 187-202.

Harmony M. Li, George Rieckenberg, Neelima Karanam, Emily Vo, and Kelsey Hurley. 2024a. Optimizing Assets for Authoring and Consumption in USD. In ACM SIGGRAPH 2024 Talks. Article 30.

Harmony M. Li, Angela McBride, Sari Rodrig, and Gregory Culp. 2024b. A Pipeline for Effective and Extensible Stylization. In ACM SIGGRAPH 2024 Talks. Article 51.

Yining Karl Li, Charlotte Zhu, Gregory Nichols, Peter Kutz, Wei-Feng Wayne Huang, David Adler, Brent Burley, and Daniel Teece. 2024c. Cache Points for Production-Scale Occlusion-Aware Many-Lights Sampling and Volumetric Scattering. In Proc. of Digital Production Symposium (DigiPro 2024). Article 6.

Barbara J. Meier. 1996. Painterly Rendering for Animation. In SIGGRAPH 1996: Proceedings of the 23rd Annual Conference on Computer Graphics and Interactive Techniques. 477-484.

Mike Navarro and Jacob Rice. 2021. Stylizing Volumes with Neural Networks. In ACM SIGGRAPH 2021 Talks. Article 54.

Mike Navarro. 2023. Diving Deeper Into Volume Style Transfer. In ACM SIGGRAPH 2023 Talks. Article 39.

Jennifer Newfield and Josh Staub. 2020. How Short Circuit Experiments: Experimental Filmmaking at Walt Disney Animation Studios. In ACM SIGGRAPH 2020 Talks. Article 72.

Kyle Odermatt and Chris Springfield. 2002. Creating 3D Painterly Environments for Disney’s “Treasure Planet”. In ACM SIGGRAPH 2002 Sketches & Applications. 160.

Patrick Osborne and Josh Staub. 2014. Feast – A Look at Walt Disney Animation Studios’ Newest Short. In ACM SIGGRAPH 2014 Production Sessions.

Johannes Schmid, Martin Sebastian Senn, Markus Gross, and Robert W. Sumner. 2011. OverCoat: An Implicit Canvas for 3D Painting. ACM Transactions on Graphics (Proc. of SIGGRAPH) 30, 4 (Jul. 2011), Article 28.

Maryann Simmons and Brian Whited. 2014. Disney’s Hair Pipeline: Crafting Hair Styles From Design to Motion. In Eurographics 2014 Industrial Presentation.

Daniel Sýkora, Ladislav Kavan, Martin Čadik, Ondrej Jamriška, Alec Jacobson, Brian Whited, Maryann Simmons, and Olga Sorkine-Hornung. 2014. Ink-and-Ray: Bas-relief Meshes for Adding Global Illumination Effects to Hand-Drawn Characters. ACM Transactions on Graphics 33, 2 (Apr. 2016), Article 16.

Rasmus Tamstorf, Ramón Montoya-Vozmediano, Daniel Teece, and Patrick Dalton. 2001. Hybrid Ink-Line Rendering in a Production Environment. In ACM SIGGRAPH 2001 Sketches & Applications. 201.

Daniel Teece. 2003. Sable - a Painterly Renderer for Film Animation. In ACM SIGGRAPH 2003 Sketches & Applications.

Marie Tollec and Mike Navarro. 2024. Making Magic with 3D Volume Style Transfer. In ACM SIGGRAPH 2024 Talks. Article 48.

Brian Whited, Eric Daniels, Michael Kaschalk, Patrick Osborne, and Kyle Odermatt. 2012. Computer-Assisted Animation of Line and Paint in Disney’s Paperman. In ACM SIGGRAPH 2012 Talks. Article 19.

Footnotes

¹ I recently learned that the Kuwahara filter originated from completely outside of graphics; it was originally invented at Kyoto University’s medical school and at Shiga University of Medical Science for medical imaging purposes. Specifically, it was invented for reducing noise in radioisotopic heart scans without blurring sharp features, and much later graphics people realized it made for a great edge-preserving blur for painting-like effects. I love when graphics intersects with other fields to produce interesting results! keyboard_return

This year at SIGGRAPH 2023, we have a conference-track technical paper in collaboration with Zackary Misso and Wojciech Jarosz from Dartmouth College! The paper is titled “Progressive Null-tracking for Volumetric Rendering” and is the result of work that Zackary did while he was a summer intern with the Hyperion development team last summer. On the Disney Animation side, Brent Burley, Dan Teece, and I oversaw Zack’s internship work, while on the the Dartmouth side, Wojciech was involved in the project as both Zack’s PhD advisor and as a consultant to Disney Animation.

Here is the paper abstract:

Null-collision approaches for estimating transmittance and sampling free-flight distances are the current state-of-the-art for unbiased rendering of general heterogeneous participating media. However, null-collision approaches have a strict requirement for specifying a tightly bounding total extinction in order to remain both robust and performant; in practice this requirement restricts the use of null-collision techniques to only participating media where the density of the medium at every possible point in space is known a-priori. In production rendering, a common case is a medium in which density is defined by a black-box procedural function for which a bounding extinction cannot be determined beforehand. Typically in this case, a bounding extinction must be approximated by using an overly loose and therefore computation- ally inefficient conservative estimate. We present an analysis of how null-collision techniques degrade when a more aggressive initial guess for a bounding extinction underestimates the true maximum density and turns out to be non-bounding. We then build upon this analysis to arrive at two new techniques: first, a practical, efficient, consistent progressive algorithm that allows us to robustly adapt null-collision techniques for use with procedural media with unknown bounding extinctions, and second, a new importance sampling technique that improves ratio-tracking based on zero-variance sampling.

The paper and related materials can be found at:

• Project Page (Author’s Version and Supplementary Material)
• Wojciech Jarosz’s Project Page (Author’s Version and Supplementary Material)
• Official Print Version (ACM Library)

One cool thing about this project is that this project both served as a direct extension of Zack’s PhD research area and served as a direct extension of the approach we’ve been taking to volume rendering in Disney’s Hyperion Renderer over the past 6 years. Hyperion has always used unbiased transmittance estimators for volume rendering (as opposed to biased ray marching) [Fong et al. 2017], and Hyperion’s modern volume rendering system is heavily based on null-collision theory [Woodcock et al. 1965]. We’ve put significant effort into making a null-collision based volume rendering system robust and practical in production, which led to projects such as residual ratio tracking [Novák et al. 2014], spectral and decomposition tracking [Kutz et al. 2017] and approaches for unbiased emission and scattering importance sampling in heterogeneous volumes [Huang et al. 2021]. Over the past decade, many other production renderers [Christensen et al. 2018, Gamito 2018, Novák et al. 2018] have similarly made the shift to null-collision based volume rendering because of the many benefits that the null-collision framework brings, such as unbiased volume rendering and efficient handling of volumes with lots of high-order scattering due to the null-collision framework’s ability to cheaply perform distance sampling. Vanilla null-collision volume rendering does have shortcomings, such as difficulty in efficiently sampling optically thin volumes due to the fact that null-collision tracking techniques produce a binary transmittance estimate that is super noisy. A lot of progress has been made in improving null-collision volume rendering’s efficiency and robustness in these thin volumes cases [Villemin and Hery 2013, Villemin et al. 2018, Herholz et al. 2019, Miller et al. 2019]; the intro to the paper goes into much more extensive detail about these advancements.

However, one major limitation of null-collision volume rendering that remained unsolved until this paper is that the null-collision framework requires knowing the maximum density, or bounding majorant of a heterogeneous volume beforehand. This is a fundamental requirement of null-collision volume rendering that makes using procedurally defined volumes difficult, since the maximum possible density value of a procedurally defined volume cannot be known a-priori without either putting into place a hard clamp or densely evaluating the procedural function. As a result, renderers that use null-collision volume rendering typically only support procedurally defined volumes by pre-rasterizing the procedural function onto a fixed voxel grid, à la the volume pre-shading in Manuka [Fascione et al. 2018]. The need to pre-rasterize procedural volumes negates a lot of the workflow and artistic benefits of using procedural volumes; this is one of several reasons why other renderers continue to use ray-marching based integrators for volumes despite the bias and loss of efficiency at handling high-order scattering. Inspired by ongoing challenges we were facing with rendering huge volume-scapes on Strange World at the time, we gave Zack a very open-ended challenge for his internship: brainstorm and experiment with ways to lift this limitation in null-collision volume rendering.

Zack’s PhD research coming into this internship revolved around deeply investigating the math behind modern volume rendering theory, and from these investigations, Zack had previously found deep new insights into how to formulate volumetric transmittance [Georgiev et al. 2019] and cool new ways to de-bias previously biased techniques such as ray marching [Misso et al. 2022]. Zack’s solution to the procedural volumes in null-collision volume rendering problem very much follows in the same trend as his previous papers; after initially attempting to find ways to adapt de-biased ray marching to fit into a null-collision system, Zack went back to first principles and had the insight that a better solution was to find a way to de-bias the result that one gets from clamping the majorant of a procedural function. This idea really surprised me when he first proposed it; I had never thought about the problem from this perspective before. Dan, Brent, and I were highly impressed!

In addition to the acknowledgements in the paper, I wanted to acknowledge here Henrik Falt and Jesse Erickson from Disney Animation, who spoke with Zack and us early in the project to help us better understand how better procedural volumes support in Hyperion could benefit FX artist workflows. We are also very grateful to Disney Animation’s CTO, Nick Cannon, for granting us permission to include example code implemented in Mitsuba as part of the paper’s supplemental materials.

A bit of a postscript: during the Q&A session after Zack’s paper presentation at SIGGRAPH, Zack and I had a chat with Wenzel Jakob, Merlin Nimier-David, Delio Vicini, and Sébastien Speierer from EPFL’s Realistic Graphics Lab. Wenzel’s group brought up a potential use case for this paper that we hadn’t originally thought of. Neural radiance fields (NeRFs) [Mildenhall et al. 2020, Takikawa et al. 2023] are typically rendered using ray marching, but this is often inefficient. Rendering NeRFs using null tracking instead of ray marching is an interesting idea, but the neural networks that underpin NeRFs are essentially similar to procedural functions as far as null-collision tracking is concerned because there’s no way to know a tight bounding majorant for a neural network a-priori without densely evaluating the neural network. Progressive null tracking solves this problem and potentially opens the door to more efficient and interesting new ways to render NeRFs! If you happen to be interested in this problem, please feel free to reach out to Zack, Wojciech, and myself.

Getting to work with Zack and Wojciech on this project was an honor and a blast; I count myself as very lucky that working at Disney Animation continues to allow me to meet and work with rendering folks from across our field!

References

Brent Burley, David Adler, Matt Jen-Yuan Chiang, Hank Driskill, Ralf Habel, Patrick Kelly, Peter Kutz, Yining Karl Li, and Daniel Teece. 2018. The Design and Evolution of Disney’s Hyperion Renderer. ACM Transactions on Graphics 37, 3 (Jul. 2018), Article 33.

Per Christensen, Julian Fong, Jonathan Shade, Wayne Wooten, Brenden Schubert, Andrew Kensler, Stephen Friedman, Charlie Kilpatrick, Cliff Ramshaw, Marc Bannister, Brenton Rayner, Jonathan Brouillat, and Max Liani. 2018. RenderMan: An Advanced Path-Tracing Architeture for Movie Rendering. ACM Transactions on Graphics 37, 3 (Jul. 2018), Article 30.

Luca Fascione, Johannes Hanika, Mark Leone, Marc Droske, Jorge Schwarzhaupt, Tomáš Davidovič, Andrea Weidlich and Johannes Meng. 2018. Manuka: A Batch-Shading Architecture for Spectral Path Tracing in Movie Production. ACM Transactions on Graphics 37, 3 (Jul. 2018), Article 31.

Julian Fong, Magnus Wrenninge, Christopher Kulla, and Ralf Habel. 2017. Production Volume Rendering. In ACM SIGGRAPH 2017 Courses. Article 2.

Manuel Gamito. 2018. Path Tracing the Framestorian Way. In ACM SIGGRAPH 2018 Course Notes: Path Tracing in Production. 52-61.

Sebastian Herholz, Yangyang Zhao, Oskar Elek, Derek Nowrouzezahrai, Hendrik P A Lensch, and Jaroslav Křivánek. 2019. Volume Path Guiding Based on Zero-Variance Random Walk Theory. ACM Transactions on Graphics 38, 3 (Jun. 2019), Article 25.

Wei-Feng Wayne Huang, Peter Kutz, Yining Karl Li, and Matt Jen-Yuan Chiang. 2021. Unbiased Emission and Scattering Importance Sampling For Heterogeneous Volumes. In ACM SIGGRAPH 2021 Talks. Article 3.

Peter Kutz, Ralf Habel, Yining Karl Li, and Jan Novák. 2017. Spectral and Decomposition Tracking for Rendering Heterogeneous Volumes. ACM Transactions on Graphics (Proc. of SIGGRAPH) 36, 4 (Aug. 2017), Article 111.

Ben Mildenhall, Pratul P. Srinivasan, Matthew Tancik, Jonathan T. Barron, Ravi Ramamoorthi, and Ren Ng. 2020. NeRF: Representing Scenes as Neural Radiance Fields for View Synthesis. In Proc. of European Conference on Computer Vision (ECCV 2020). 405-421.

Bailey Miller, Iliyan Georgiev, and Wojciech Jarosz. 2019. A Null-Scattering Path Integral Formulation of Light Transport. ACM Transactions on Graphics (Proc. of SIGGRAPH) 38, 4 (Jul. 2019), Article 44.

Jan Novák, Iliyan Georgiev, Johannes Hanika, and Wojciech Jarosz. 2018. Monte Carlo Methods for Volumetric Light Transport Simulation. Computer Graphics Forum (Proc. of Eurographics) 37, 2 (May 2018), 551-576.

Jan Novák, Andrew Selle and Wojciech Jarosz. 2014. Residual Ratio Tracking for Estimating Attenuation in Participating Media. ACM Transactions on Graphics (Proc. of SIGGRAPH Asia) 33, 6 (Nov. 2014), Article 179.

Towaki Takikawa, Shunsuke Saito, James Tompkin, Vincent Sitzmann, Srinath Sridhar, Or Litany, and Alex Yu. 2023. Neural Fields for Visual Computing. In ACM SIGGRAPH 2023 Courses. Article 10.

Ryusuke Villemin and Christophe Hery. 2013. Practical Illumination from Flames. Journal of Computer Graphics Techniques 2, 2 (Dec. 2013), 142-155.

Ryusuke Villemin, Magnus Wrenninge, and Julian Fong. 2018. Efficient Unbiased Rendering of Thin Participating Media. Journal of Computer Graphics Techniques 7, 3 (Sep. 2018), 50-65.

E. R. Woodcock, T. Murphy, P. J. Hemmings, and T. C. Longworth. 1965. Techniques used in the GEM code for Monte Carlo neutronics calculations in reactors and other systems of complex geometry. In Applications of Computing Methods to Reactor Problems. Argonne National Laboratory.

Disney Animation’s fall 2022 release is Strange World, which is the studio’s 61st animated feature, and third original story in as many years. Strange World takes us to the land of Avalonia, a realm surrounded by impenetrable mountains and home to a society that blends elements of early 20th century pulp fiction, steampunk, and environmental solarpunk. The core story of Strange World revolves around father-son relationships and is exactly the type of story that Disney Animation excels at: something personal and relatable but set in a fantastic world we’ve never seen before. That “never seen before” aspect made my two years of working on rendering technology for Strange World an interesting time indeed!

In my writeups about our films, two recurring themes always are: 1. with each film, we build upon advancements made and lessons learned from the previous film, and 2. one of the greatest advantages that having in-house tools gives us is the ability to customize and build exactly what each film’s story and art direction requires. Strange World’s production exemplifies both of these themes; so much of what we had to do on Strange World builds upon things we learned from and developed for previous films that I’m not entirely sure we would have been able to make Strange World a few years ago, and much of what we learned we have only been able to apply as effectively as we have because we have the ability to extend and improve our own tools.

As an example: a large part of Strange World takes place on the airship Venture, which from a production pipeline perspective has to function as both a set/environment in which characters move and interact, and as a sort of character of its own as it moves around in the larger surrounding environment. In CG production pipelines, various pipeline optimizations are often built around the reasonable assumption that sets are relatively static; sets typically don’t need complex animation rigs and can be used as a stationary frame of reference for all kinds of different things. The Venture, of course, breaks all of these expectations. Disney Animation had to deal with this type of scenario before on Moana, where much of the movie is set on a canoe out at sea, so handling the “sets as characters” challenge wasn’t new. Instead of having to solve this problem from scratch, our artists and TDs were able to build on top of what they had learned before to enable the Venture to be a far more complex “set as a character” than anything we had done before. In fact, the Venture isn’t the only case of this type of challenge in Strange World! Huge parts of Strange World follow this “set as a character” pattern; entire chunks of terrain get up and walk around in this movie! All of these complex sets were made possible by advancements [Vo et al. 2023] in our USD based pipeline, which in turn built upon all of the lessons learned [Miller et al. 2022] from our previous pipeline.

Things got even more complex once crowds were brought into the mix too. Strange World has some of the most massive crowd simulation ever made by Disney Animation [Devlin et al. 2023], and these huge crowds had to interact with the Venture and complex terrain. One of the main tools our crowds team used to guide giant swarms of creatures traveling through Strange World’s massive environments and around the Venture originated as a tool made for a single character on Frozen 2, and had to be turbocharged to massive scales to go from handling the requirements for one character to handling thousands upon thousands of creatures [Lin et al. 2023]. Challenges involving simulating collisions in huge crowds like this, along with similar challenges in hair simulation, helped inspire further research work [Zhang et al. 2023] for future films as well.

Once the story moves into the subterranean world, the environment of the film ratchets up in production complexity on multiple different axes. Essentially every single surface in the subterranean world has significant subsurface scattering since everything is made up of organic gummy materials, and of course all of the giant crowds also all have subsurface scattering. Many of our previous films already were beginning to push the use of subsurface scattering in environments for things like plants and plastics and other materials, all thanks to the work that the rendering team put into making path traced subsurface scattering efficient and controllable enough for large-scale production usage [Chiang et al. 2016], but Strange World saw the widest usage of subsurface scattering in environments yet, by far.

Everything we’ve learned about controlling subsurface scattering also proved to be extremely important for creating the look for the Splat character, who is essentially a giant immune cell. Splat wound up requiring a unique custom one-off shader with custom functionality in Disney’s Hyperion Renderer combining subsurface scattering, a custom faux volumetric emission technique, our multiple-scattering sheen solution [Zeltner et al. 2022], and more in order to achieve the target art-direction in a single render pass [Litaker et al. 2023]. Splat’s challenges weren’t limited to just rendering though; rigging and animating Splat also required novel solutions in order to handle how varied and multi-purpose Splat’s limbs are [Black and Pederson 2023]. Splat’s rig was only made possible through a combination of new novel techniques and a decade of experience and continuous improvement in Disney Animation’s DRig modular rig building system [Smith et al. 2012].

Splat wasn’t the only character that provided interesting technical challenges though; in fact, our entire character asset workflow got an upgrade on Strange World. Our standard character asset workflow saw three major improvements on Strange World: eyes, skin, and curves.

Strange World’s character art direction called for eyes to use a bit of a different look from Disney Animation’s usual style; eyes on Strange World have more of an oblong oval shape. Over the past several shows, we introduced a new eye shading model that incorporates manifold next event estimation for physically accurate iris caustics and limbal arcs [Chiang et al. 2018]; one of my smaller projects on Strange World was to help work out the minor modifications to this system that were required to support Strange World’s eye shapes.

For skin shading, Strange World uses the same fully path traced subsurface scattering approach [Chiang et al. 2016] (as opposed to older diffusion-based approaches [Burley 2015]) that we have now used for all of our movies over the past few years. However, Strange World has one of the most diverse casts of any of our recent films in terms of skin tones, and our lighting and look dev artists took special care to make sure all of the different skin types were depicted accurately and beautifully. Doing so required rebuilding our entire skin material from the ground up and radically rethinking our entire approach to lighting characters to better handle contrasting skin tones and high specularity skin [Khoo et al. 2023].

Previously on Encanto, our look artists started to replace triangle mesh-based geometric representations for cloth with curve-based fiber level representations [Velasquez et al. 2022]. This authoring approach was pushed to new limits on Strange World, where curve-based garments were extended to incorporate custom weave patterns and widely varying fiber thicknesses ranging from fine threads to thick yarns [Lipson and Velasquez 2023]. Humans weren’t the only type of characters on Strange World to see upgraded curve geometry though; the Clade family’s lovable dog, Legend, also required upgraded curve grooming techniques to produce one of the most complex animal grooms the studio has ever made [Chun et al. 2023]. Of course all of these improved authoring techniques meant increased curve rendering complexity, but interestingly, we didn’t actually need to improve anything in the renderer to handle the increased curve rendering demand. After having spent many prior shows improving Hyperion’s ability to chew through vast geometric complexity, on Strange World we found that Hyperion was able to just handle all of the meshes and curves that we threw at it!

The hardest rendering challenge I worked on for Strange World was volume rendering. Strange World’s environments have some of the largest scale and most ambitious use of volumes in any of our films to date. Strange World extensively utilizes mist and atmospherics and low cloud cover to help convey a sense of mystery and to sell the sheer scale of the environments. Frozen 2 was the first movie that really extensively leveraged Hyperion’s modern volume rendering system (which we rewrote essentially from scratch during the early production of Ralph Breaks the Internet) and the first movie that introduced our modern volumes authoring workflow. This workflow, which is heavily based around quickly set-dressing atmospherics and clouds around environments by kitbashing together volumes from a large pre-made in-house library of VDBs, was further fleshed out on Raya and the Last Dragon and saw its largest and most complex usage yet on Strange World. Strange World also further extended our volume workflows with an evolved version [Navarro 2023] of the neural volume stylization tech we first introduced on Raya and the Last Dragon [Navarro and Rice 2021].

During Raya and the Last Dragon we consolidated various different experiments and techniques in our volume rendering system into a single unified volume integrator [Huang et al. 2022] that can efficiently handle every imaginable type of volume effect, so the challenge presented by Strange World’s volumes wasn’t so much light transport as it was simply a problem of efficiency at scale. When volumes are simultaneously highly detailed but also span kilometers of world space, massive memory usage becomes challenging, even with instancing. Also, super large and detailed volumes coverage means that average path length in volumes can get very long, exposing any potential performance issues in the volume integrator. A huge part of my time on Strange World was spent optimizing our volume integrator. There were no clever shortcuts or brilliant solutions here, just tons of profiling and careful analysis of the existing system architecture and hard low level optimization work.

We also noticed during Strange World that artists sometimes had to overauthor volume details in areas as a way to work around the lack of true procedural volumes evaluation support in our renderer. While Hyperion does support authoring procedural volumes, these procedural volumes are not actually evaluated at render time but instead are pre-evaluated and baked into a required underlying VDB grid at renderer startup. The reason for this limitation is fundamental to null collision-based volume rendering theory [Novák 2018]; null collision approaches only work if the bounding majorant (AKA max density) for all volumes in a region of space is known upfront. In theory we could just require artists to input a max density value that we would clamp all higher values down to, but such a value isn’t easy for artists to estimate in practice; too low of a value clamps away detail, while too high of value results in an overly loose bounding majorant, which in null collision theory-based volume rendering can result in significantly slower performance. Inspired by what we were seeing on Strange World, we kicked off a research project in collaboration with the Visual Computing Lab at Dartmouth College to solve this problem, with promising results [Misso et al. 2023]!

As usual, I’ve only written about the parts of making Strange World that I know a bit more about; hundreds of artists, TDs, and engineers worked to craft every frame of this movie and solve many many more problems. For the entire history of Disney Animation, one of the studio’s primary driving purposes has been to push the limits of animation as an art form, and Strange World is no exception to this rule. Strange World is the latest example of how each of our films builds upon what we’ve learned on previous films to push our filmmaking process forward, and as always, getting to be a part of this process is a lot of work but also a lot of fun!

Below are some frames from the strange but gorgeous world of Strange World, pulled from the Blu-ray and presented in semi-randomized order to prevent giving away too much of the story. Go see Strange World on the biggest screen you can find!

Here is the credits frame for the Hyperion team, which is listed as part of the larger Rendering & Visualization group at Disney Animation. In addition to the Hyperion team, this group also includes our sister render translation pipeline and interactive visualization teams:

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

References

Cameron Black and Christoffer Pedersen. 2023. The Versatile Rigging of Splat in ‘Strange World’. In ACM SIGGRAPH 2023 Talks. Article 29.

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

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

Matt Jen-Yuan Chiang and Brent Burley. 2018. Plausible Iris Caustics and Limbal Arc Rendering. In ACM SIGGRAPH 2018 Talks. Article 15.

Courtney Chun, Jose Velasquez, and Haixiang Liu. 2023. Creating the Art-directed Groom for Legend in Disney’s Strange World. In ACM SIGGRAPH 2023 Talks. Article 7.

Nathan Devlin, Yasser Hamed, Alberto J Luceño Ros, Jeff Sullivan, and D’Lun Wong. 2023. Creating Creature Chaos: The Methods That Brought Crowds to the Forefront on Disney’s ‘Strange World’. In ACM SIGGRAPH 2023 Talks. Article 34.

Wei-Feng Wayne Huang, Peter Kutz, Yining Karl Li, and Matt Jen-Yuan Chiang. 2021. Unbiased Emission and Scattering Importance Sampling for Heterogeneous Volumes. In ACM SIGGRAPH 2021 Talks. Article 3.

Mason Khoo, Dan Lipson, and Jose Velasquez. 2023. Lighting and Look Dev for Skin Tones in Disney’s “Strange World”. In Proc. of Digital Production Symposium (DigiPro 2023). Article 5.

Andy Lin, Hannah Swan, Justin Walker, Cathy Lam, and Ricky Arietta. 2023. Swoop: Animating Characters Along a Path. In ACM SIGGRAPH 2023 Talks. Article 45.

Dan Lipson and Jose Velasquez. 2023. Creating Curve-based Garments With Custom Weave Patterns. In ACM SIGGRAPH 2023 Talks. Article 18.

Kendall Litaker, Brent Burley, Dan Lipson, and Mason Khoo. 2023. Splat: Developing a ‘Strange’ Shader. In ACM SIGGRAPH 2023 Talks. Article 28.

Tad Miller, Harmony M. Li, Neelima Karanam, Nadim Sinno, and Todd Scopio. 2022. Making Encanto with USD: Rebuilding a Production Pipeline Working from Home. In ACM SIGGRAPH 2022 Talks. Article 12.

Zackary Misso, Yining Karl Li, Brent Burley, Daniel Teece, and Wojciech Jarosz. 2023. Progressive Null-tracking for Volumetric Rendering. In Proc. of SIGGRAPH (SIGGRAPH 2023). Article 31.

Mike Navarro and Jacob Rice. 2021. Stylizing Volumes with Neural Networks. In ACM SIGGRAPH 2021 Talks. Article 54.

Mike Navarro. 2023. Diving Deeper Into Volume Style Transfer. In ACM SIGGRAPH 2023 Talks. Article 39.

Jan Novák, Iliyan Georgiev, Johannes Hanika, and Wojciech Jarosz. 2018. Monte Carlo Methods for Volumetric Light Transport Simulation. Computer Graphics Forum (Proc. of Eurographics) 37, 2 (May 2018), 551-576.

Greg Smith, Mark McLaughlin, Andy Lin, Evan Goldberg, and Frank Hanner. 2012. DRig: An Artist-Friendly, Object-Oriented Approach to Rig Building. In ACM SIGGRAPH 2012 Talks. Article 18.

Jose Velasquez, Alexander Alvarado, Ying Liu, and Maryann Simmons. 2022. Embroidery and Cloth Fiber Workflows on Disney’s “Encanto”. In ACM SIGGRAPH 2022 Talks. Article 22.

Emily Vo, George Rieckenberg, and Ernest Petti. 2023. Honing USD: Lessons Learned and Workflow Enhancements at Walt Disney Animation Studios. In ACM SIGGRAPH 2023 Talks. Article 13.

Tizian Zeltner, Brent Burley, and Matt Jen-Yuan Chiang. 2022. Practical Multiple-Scattering Sheen Using Linearly Transformed Cosines. In ACM SIGGRAPH 2022 Talks. Article 7.

Paul Zhang, Zoë Marschner, Justin Solomon, and Rasmus Tamstorf. 2023. Sum-of-squares Collision Detection for Curved Shapes and Paths. In Proc. of SIGGRAPH (SIGGRAPH 2023). Article 76.

This year at SIGGRAPH 2022, Corey Butler, Brent Burley, Wei-Feng Wayne Huang, Benjamin Huang, and I have a talk that presents the technical and artistic challenges and solutions that went into creating the holographic look for Bruno’s visions in Encanto. In Encanto, Bruno is a character who has a magical gift of being able to see into the future, and the visions he sees of the future get crystalized into a sort of glassy emerald tablet with the vision embedded in the glassy surface with a holographic effect. Coming up with this unique look and an efficient and robust authoring workflow required a tight collaboration between visual development, lookdev, lighting, and the Hyperion rendering team to develop a custom solution in Disney’s Hyperion Renderer. On the artist side, Corey was the main lighter and Benjamin was the main lookdev artist for this project, while on the rendering team side, Wayne and I worked closely together to develop a series of prototype shaders that were instrumental in defining how the effect should look and then Brent came up with the implementation approach for the final production version of the shader. This project was a lot of fun to be a part of and in my opinion really demonstrates the benefits of having an in-house rendering team that works closely with and embedded within a production context.

Here is the paper abstract:

In Walt Disney Animation Studios’ “Encanto”, Mirabel discovers the remnants of her Uncle Bruno’s mysterious visions of the future. Developing the look and lighting for the emerald shards required close collaboration between our Visual Development, Look Development, Lighting, and Technology departments to create a holographic effect. With an innovative new teleporting holographic shader, we were able to bring a unique and unusual effect to the screen.

The paper and related materials can be found at:

• Project Page (Author’s Version and Supplementary Material)
• Official Print Version (ACM Library)

When Corey first came to the rendering team with the request for a more efficient way to create the hologram effect that lighting had prototyped using camera mapping, our initial instinct actually wasn’t to develop a new shader at all. Hyperion has an existing “hologram” shader that was developed for use on Big Hero 6 [Joseph et al. 2014], and our initial instinct was to tell Corey that they should use the hologram shader. The way the Big Hero 6 era hologram shader works is: upon hitting a surface that has the hologram shader applied, the ray is moved into a virtual space containing a bunch of imaginary parallel planes, with each plane textured with a 2D slice of a 3D interior. In some ways the hologram shader can be thought of as raymarching through a sparse volumetric representation of a 3D interior, but the sparse volumetric interior really is just a stack of 2D slices. This technique works really well for things like building interiors seen through glass windows. However, our artists… really dislike using the hologram shader, to put things lightly. The problem with the hologram shader is that setting up the 2D slices that are inputs to the shader is an incredibly annoying and difficult process, and since the 2D slice baker has to be run as an offline process before the shader can be authored and rendered, making changes and iterating on the contents of the hologram shader is a slow process. Furthermore, if the inside of the hologram shader has to be animated, the slice baker needs to be run for every frame. We were told in no uncertain terms that the hologram shader was likely more work to set up and iterate on than the already painful manual camera mapping approach that the artists had prototyped the effect with. This request also came to us fairly late in Encanto’s production schedule, so easy setup and fast iteration times along with an extremely accelerated development timeline were hard requirements for whatever approach we took.

Upon receiving this feedback, Wayne and I set out to prototype a version of the teleportation shader that Pixar came up with for the portals in Incredibles 2 [Coleman et al. 2014]. This process was a lot of fun; Wayne and I spent a few days rapidly iterating on several different ideas for both how to implement ray teleportation in Hyperion and on how the artist workflow and interface for this new teleportation system should work. At the same time that we were prototyping, we started giving test builds of our latest prototypes to Corey to try out, which produced a feedback loop where Corey would use our prototypes to further iterate on how the final effect would look and go back and forth with the movie’s production designer and we would use Corey’s feedback to further improve the prototype. One example of where our prototype directly informed the final look was in how the prophecies fade away towards the edges of the emerald tablet- Wayne and I threw in a feature where artists could use a map to paint in the ratio of teleportation effect versus normal surface BSDF that would be applied at each surface point, and this feature wound up driving the faded edges.

The key thing that made our new approach work better than the old hologram shader was in simplicity of setup. Instead of having to run a pre-bake process and then wire up a whole bunch of texture slices into the renderer, our new approach was designed so that all an artist had to do was set up the 3D geometry that they wanted to put inside of the hologram in a target space hidden somewhere in the overall scene (typically below the ground plane in a black box or something), and then select the geometry in the main scene that they wanted to act as the “entrance” portal, select the geometry in the target space that they wanted to act as the “exit” portal, and link the two using the teleportation shader. The renderer then did all of the rest of the work of figuring out how each point on the entrance portal corresponded to the surface of the exit portal, how transforms needed to be calculated, and so on and so forth. Multiple portal pairs could be set up in a single scene too, and the contents of a world seen through a portal could contain more portals, all of which was important because in the movie, Mirabel initially finds Bruno’s prophecy broken into shards, which had to be set up as a separate entrance portal per shard all into the same interior world. Since all of this just piggy-backed off of the normal way artists set up scenes, things like animation just worked out-of-the-box with no additional code or effort.

The last piece of the puzzle fell into place when Wayne and I discussed our progress with Brent. One of the big remaining challenges for us was that tracking correspondences between entrance and exit geometry and transforms was prone to easy breakage if input geometry wasn’t set up exactly the way we expected. At the time Brent was working on a new fracture-aware tessellation system for subdivision surfaces in Hyperion [Burley and Rodriguez 2022], and Brent quickly realized that the approach we were using for figuring out the transform from the entrance to the exit portal could be replaced with something he had already developed for the fracture-aware tessellation system. Specifically, the fracture-aware tessellation system has to be able to calculate correspondences between undeformed unfractured reference points and corresponding points in a deformed fractured fragment space; this is done using a best-fit process to find orthonormal transforms [Horn et al. 1998]. Brent realized that the problem we were trying to solve was actually the same problem he that he had already solved in the fracture system, so he took our latest prototype and reworked the internals to use the same best-fit orthonormal transform solution as in the fracturing system. With Brent’s improvements, we arrived at the final production version of the teleportation shader used on Encanto.

Going from the start of brainstorming and prototyping to delivering the final production version of the shader took us a little over a week, which anyone who has worked in an animation/VFX production setting before will know is very fast for a large new rendering feature. Working tightly with Corey and Benjamin to simultaneously iterate on the art and the software and inform each other was key to this project’s fast development time and key to achieving an amazing looking effect in the film. At Disney Animation, we have a mantra that goes “art challenges technology and technology inspires the art”- this project was a case that exemplifies how we carry out that mantra in real-world filmmaking and demonstrates the amazing results that come out of such a process. Bruno’s visions in Encanto are every bit a case where the artistic vision challenged us to develop new technology, and the process of iterating on the new technology between engineers and artists in turn informed the final artwork that made it into the movie; for me, projects like these are one of the things that makes Disney Animation such a fun and amazing place to be.

References

Brent Burley, David Adler, Matt Jen-Yuan Chiang, Hank Driskill, Ralf Habel, Patrick Kelly, Peter Kutz, Yining Karl Li, and Daniel Teece. 2018. The Design and Evolution of Disney’s Hyperion Renderer. ACM Transactions on Graphics 37, 3 (Jul. 2018), Article 33.

Brent Burley and Francisco Rodriguez. 2022. Fracture-Aware Tessellation of Subdivision Surfaces. In ACM SIGGRAPH 2022 Talks. Article 10.

Patrick Coleman, Darwyn Peachey, Tom Nettleship, Ryusuke Villemin, and Tobin Jones. 2018. Into the Voyd: Teleportation of Light Transport in Incredibles 2. In Proc. of Digital Production Symposium (DigiPro 2018). Article 12.

Berthold K. P. Horn, Hugh M. Hilden, and Shahriar Negahdaripour. 1988. Close-Form Solution of Absolute Orientation using Orthonormal Matrices. Journal of the Optical Society of America A 5, 7 (Jul. 1988), 1127–1135.

Norman Moses Joseph, Brett Achorn, Sean D. Jenkins, and Hank Driskill. Visualizing Building Interiors Using Virtual Windows. In ACM SIGGRAPH Asia 2014 Technical Briefs. Article 18.