Practical Pigment Mixing for Digital Painting

About a year ago at SIGGRAPH Asia 2021 (which took place as a hybrid conference both online and on site at the Tokyo International Forum) one of the technical papers that caught my attention was the publication by Šárka Sochorová and Ondřej Jamriška on color mixing.

Color mixing in most digital painting tools is infamously unsatisfying, often limited to a linear interpolation in RBG space, resulting in unpleasing gradients very different from what one would expect. Ten years ago I mentioned this article that presented the color mixing of the application Paper, which tried to solve this very problem.

This time, the core idea is to model colors as pigments: estimate the pigment concentration based on the color, so in a way, move from RGB space to “pigment space”, and interpolate the pigment concentration, before converting back to RGB space.

The paper uses the Kubelka-Munk model for estimating colors from pigment concentration. The problem however is to find a transformation between the two spaces. A first assumption is made on the available pigments: essentially restricting them to CMYK. Then two problems are addressed: RGB colors that cannot be represented with those pigments, and likewise pigment colors that cannot be represented in RGB.
The paper proposes a remapping that enables a transform and its inverse, thus allowing to move from RGB space to pigment space, interpolate in pigment space, and move back to RGB space.

You could argue this is therefore a physically based diffuse color mixing.

Finally, the implementation of the proposed model, Mixbox, is available under a CC BY-NC license:
https://github.com/scrtwpns/mixbox

Two Minute Papers did a video on this paper as well:

Building an artificial window

Several years ago, I mentioned the Italian company CoeLux, which specializes in making artificial windows: light fixtures that look like sunlight in a clear blue sky.

The price of their products is apparently in the range of several tens of thousands of dollars (I’ve heard prices like $20k to 50k), which makes it out of reach for most individuals. Not many details about their invention are available either (from the promotion material: LED powered, several hundred watts of electrical power, a solid diffuse material, and a thickness around 1 meter), and I was left wondering what was the secret sauce to their intriguing technology. The YouTube channel DIY Perks has been working on day light projects for a while now, improving at each iteration. Yesterday they published a video explaining how to build a light that seems to give very similar results as CoeLux’s product, from some basic materials that are fairly simple to find. Since their solution takes roughly the same volume, it’s tempting to think it uses the same technique It’s extremely satisfying to finally see how this works and, despite the practical aspects, quite tempting to try if only to see how it looks in real life. Physically based shading references, at the end of 2019 A lot has happened in the graphics community in the last ten years, especially when it comes to physically based rendering (PBR). It started to become popular around 2009, as hardware made more powerful models affordable, and really took over between 2010 and 2014. Real-time engines started replacing Phong and Blinn-Phong with a normalized Blinn-Phong, until pretty much everyone switched to GGX and its long trailing reflections. Researchers explored how to make it work with image based lighting (IBL), area lights, and it seems that nowadays everyone is looking at the secondary bounce problem. I am not sure why adoption also happened at the same in the film industry (instead of much earlier), despite having different constraints than real-time. Films made before 2010 were mostly ad hoc, until a wave converted nearly the entire industry to unbiased path tracing. I gathered a first PBR reading list back in 2011, but since then, the community has collectively made strides of progress. I also have a better understanding of the topic myself. So I think it is time to revisit it with a new, updated (and unfortunately, longer) reading list. However, covering the entire PBR pipeline would be way too vast, so I am going to focus on physically based shading instead, and ignore topics like physical lighting units, physically based camera or photogrammetry, even though some of the links cover those topics. Note: If you see mistakes, inaccuracies or missing important pieces, please let me know. I expect to update this article accordingly during the next few weeks. Courses and tutorials • Physically Based Shading in Theory and Practice (formerly “Practical Physically Based Shading in Film and Game Production”) 2010, (no 2011?), 2012, 2013, 2014, 2015, 2016, 2017. This recurring SIGGRAPH course by the leading actors of the field is a fantastic resource and a must see for anyone interested in the topic. Naty Hoffman then Stephen Hill have been hosting on their websites the course material for several years. Some of the presentations are also available on Youtube. • Physically Based Rendering: From Theory To Implementation, Third edition, 2016, Matt Pharr, Wenzel Jakob, and Greg Humphreys As of 2018, the content of this reference book is entirely available online. • Implementation Notes: Runtime Environment Map Filtering for Image Based Lighting, 2015, Padraic Hennessy. Details how to implement the environment map filtering described in Karis and Lagarde publications (see below), then how to optimize it by reducing the number of samples thanks to importance sampling and rejecting samples that don’t contribute. • Image Based Lighting, 2015, Chetan Jaggi. Focused on specular reflections, the article presents the implementation of image based lighting (IBL) using the split sum approximation from Unreal Engine 4 (described below), and how to improve quality for several cases. • Physically Based Rendering Algorithms: A Comprehensive Study In Unity3D, 2017?, Jordan Stevens. This tutorial explains what the different parts of the Bidirectional Reflection Distribution Function (BRDF) mean, lists many available bricks, and shows them in isolation. It is directed at Unity, but translates easily to other environments. • LearnOpenGL’s PBR series (theory, lighting, diffuse irradiance, specular IBL), 2017, Joey de Vries. An excellent introduction that explains the basics and walks the reader through the implementation of a shader based on the same model as Unreal Engine 4 (detailed below). There seems to be a confusion between albedo and base colour, but it’s otherwise clear and well structured. Real world references In depth overviews The following publications all describe the work done by teams who had to do an inventory of the existing options, and choose a model for their particular needs. • Physically-Based Shading at Disney (slides), 2012, Brent Burley et al. What came to be known as the “Disney BRDF” was a milestone in PBR literature, and a reference many other works are built upon. It compares different existing models to the MERL database (see previous section), notes their strengths and weaknesses, discusses in length the observed behaviour, especially the diffuse response at grazing angles, and proceeds to define their own empirical shading model to mimic that behaviour. The Disney BRDF is designed to be robust and expressive but also simple and intuitive for artists. In the annex, a brief overview of the history of BRDF is given. The publication proposes a tool, the BRDF Explorer, to visualize and compare analytic BRDF models or measured ones. (In 2015, a follow up publication extended their model to a full BSDF in order to support refraction and scattering, but this falls out of the scope of this already long list.) • Real Shading in Unreal Engine 4 (slides), 2013, Brian Karis. Strongly inspired by the Disney BRDF, it presents a similar shading model. It prefers a simple Lambert diffuse BRDF due to both the cost and the integration with spherical harmonics, and uses other approximations for realtime. The course notes mention that a lot of work was done to compare the various available bricks, but doesn’t list them. Karis lists them in a separate publication, listed next. For image based lighting, the famous “split sum” approximation of the integral is introduced, allowing to convolve a part of the integral, and precompute the rest in a 2D look-up table (LUT). When explaining how the workflow adapted to this new model, the course notes stress the importance of having linear parameters for material interpolation. Warning: I was told a year ago by Yusuke Tokuyoshi that there was an error in a derivation, but my understanding of it is not sufficient to spot it. Apparently the error is only in the publication, and was fixed in the actual code though. • Specular BRDF Reference, 2013, Brian Karis. Lists various available bricks for the Cook-Torrance BRDF, using the same naming convention. • Moving Frostbite to Physically Based Rendering 3.0, 2014, Sébastien Lagarde. The biggest publication in this list, with over 120 pages of course notes. I haven’t finished reading it yet, but this is an outstanding piece of work, that goes deep into details for many of the aspects involved. • Physically Based Rendering in Filament, 2018, Romain Guy et al. This documentation presents the shading model used in Filament, the choices that were made, which are similar to Frostbite in many ways, and the alternatives that were available. The quality of this document is outstanding, and it seems it is becoming a reference for PBR implementations. • MaterialX Physically-Based Shading Nodes, 2019, Niklas Harrysson, Doug Smythe and Jonathan Stone. This specification is meant as a transfer format in the VFX industry. It describes a wide range of materials, not limited to BRDF, but also including emissive and volumetric materials, and allows to choose between a variety of such functions. Reading this document can help solidify or confirm the understanding of how all these different functions contribute to the rendering ecosystem. However I would only recommend it to readers who already have a fairly good understanding of the PBR models. Disney BRDF and BRDF Explorer implementations The Disney BRDF is so popular that many implementations can be found in the wild. Here are a few of them. Diffuse It seems that in litterature, diffuse BRDF are a lot less covered than specular ones. I suppose this is because it is harder to solve, while the low frequency nature of the diffuse component makes its quality less noticeable. Therefore, many realtime implementations consider the Lambert model sufficient. However, the following publications explore the topic. • Physically-Based Shading at Disney (slides), 2012, Brent Burley et al. One of the contribution of the “Disney BRDF” is its diffuse model. It compares several existing diffuse models with the measured data of the MERL database (see earlier section) but, unsatisfied with their response, it proposes its own, empirical one. One of the features of that model is the retroreflection at grazing angles. I have read multiple times that this model is not energy conserving. Yet Disney uses it for offline rendering, which I assume is path tracing (?), so I am not sure of what is the impact of that decision. • Moving Frostbite to Physically Based Rendering 3.0, 2014, Sébastien Lagarde. The diffuse BRDF described is a normalized version of the Disney BRDF to make it energy conserving. • Designing Reflectance Models for New Consoles (slides), 2014, Yoshiharu Gotanda. Gotanda explains here several weaknesses of the Oren-Nayar model for PBR (its geometry term is different than the one used for the specular term, and it’s not energy conserving), and proceeds to propose a modified version. Since there is no analytic solution, he suggests a fitted approximation. He also reminds his own improvement over Schlick’s Fresnel approximation, but concludes that both models fail for complex indices of refraction. • PBR Diffuse Lighting for GGX+Smith Microsurfaces, 2017, Earl Hammon, Jr. This presentation tries to combine the Oren-Nayar diffuse model (originally a Gaussian normal distribution) with the GGX normal distribution. It studies the Smith geometry fonction (G), proposes a BRDF to use for testing with path tracing, and concludes with an approximation for a diffuse GGX. On a side note, a few final slides give some identities that are useful for shader optimization. Energy conservation One of the pillars of PBR is to make sure to respect the energy conservation law. When designing a BRDF, there shouldn’t be more energy out than comes in. This is especially important for path tracing to converge. The following links explain how to take that constraint into account. • Energy Conservation In Games, 2009, Rory Driscoll. Explains briefly the problem of energy conservation, and details how to obtain the normalization factor for diffuse Lambert. It’s a good example of how to get started. The comments discuss the case of the Phong and Blinn-Phong specular lobe. • Phong Normalization Factor derivation, 2009, Fabian Giesen. Demonstrates the derivation to obtain the normalization factor for the Phong and Blinn-Phong specular lobe. • The Blinn-Phong Normalization Zoo, 2011, Christian Schüler. Lists various normalization factors that exist for variants of Phong and Blinn-Phong. Also proposes a crude approximation for Cook-Torrance. • How Is The NDF Really Defined?, 2013, Nathan Reed. Explains conceptually what the Normal Distribution Function (NDF) is, and how this affects the region to integrate over for normalization. • Adopting a physically based shading model, 2011, Sébastien Lagarde et al. Starts by reminding a few normalization factors (Lambert, Phong and Blinn-Phong). Includes a quick paragraph on the factor to use to combine diffuse with specular. • How to properly combine the diffuse and specular terms?, 2016, CG Stack Exchange. A question I candidly asked on how to combine diffuse and specular so the energy lost to specular is taken into account in the diffuse term. • Designing Reflectance Models for New Consoles (slides), 2014, Yoshiharu Gotanda. The third section explains that a Fresnel term should be taken into account for the diffuse part, but also why this is problematic. This Fresnel term should take into account all microfacets, not only the perfect reflection ones that contribute to the specular component. This is an answer to my Stack Exchange question above. • PBR Diffuse Lighting for GGX+Smith Microsurfaces, 2017, Earl Hammon, Jr. Among the other topics it covers, this presentation shows the derivation to normalize a BRDF. • Physically Based Shading at DreamWorks Animation, 2017, Feng Xie and Jon Lanz. In the appendix of these course notes, the derivation to normalize their fabric BRDF is shown. Wrapped diffuse It is a common trick in video games to represent certain diffuse materials that have a lot of scattering with a custom diffuse that “wraps” around and brings light in the shadowed part. When PBR became popular, several people looked into how to make their wrapped diffuse PBR compliant. Multiple scattering A recently tackled problem is the energy loss due to ignoring multiple scattering. In many BRDF models, rays occluded by geometry are simply discarded. This tends to cause a noticeable darkening as roughness increases, visible in many of the charts showing material appearance for various roughnesses. However the trend is changing and this is why we see more and more references to the “furnace test”, which is a way to highlight energy loss. • Multiple-Scattering Microfacet BSDFs with the Smith Model, 2016, Eric Heitz et al. I haven’t read that paper except for the abstract, but the reception it received indicates that it’s an important publication. Recently, Morgan McGuire even said about it: “It is such a beautifully complete piece of work, a short, careful, and clear book on microfacets of the form that typically only arrives out of a complete Ph.D. thesis.”. If I understand correctly, they extended the Smith model to take multiple scattering into account, and compared their results with a simulation, by raytracing a surface at the micro-facet level. Károly Zsolnai-Fehér of Two Minute Papers did a video abstract of their paper. • A Multi-Faceted Exploration, part 1, part 2, part 3, part 4 2018-2019, Stephen Hill. This series of articles explores the feasibility of using in real-time rendering a model used by Sony Pictures Imageworks for offline rendering. The first part explains and illustrates what the problem is. The second part presents the solution from Heitz, and uses it as a ground truth reference, before presenting the Sony Pictures Imageworks solution and comparing the two. It then proposes an improvement of the latter. The third part gives a brief and clear reminder of the idea behind the split integral technique from UE4 and others, and uses it to propose a further improvement by precomputing a 2D LUT (instead of a 3D one). The fourth part details the precomputation step and shows the results in a WebGL demo. The series is not concluded yet, so I imagine one or more articles are coming. • Advances in Rendering, Graphics Research and Video Game Production (PDF version, video), 2019, Steve McAuley. This presentation shows the steps that were involved in implementing multiscattering BRDF and area lights for diffuse and specular, in FarCry, which uses a complex rendering engine that has to support a variety of combination of cases. It’s a reminder that such task can become more involved that expected. It’s also a case for academic papers that highlight their main insight, and have code available. Area lights I haven’t explored this topic yet, but I bookmarked some publications that spent time on the topic. • Lighting of KillZone Shadowfall, 2013, Michal Drobot. A part of the presentation is dedicated to area lights. It observes that point lights are inadequate for artists and that they tend to tweak roughness to compensate. It then briefly explains the technique, which consist in analytically integrating over the area light. Unfortunately the full derivation is not shown. • Real Shading in Unreal Engine 4 (slides), 2013, Brian Karis. A part covers the area lights. Like Drobot, Karis observes a tendency of artists to use roughness to compensate the small reflection highlight of point lights. The course notes list their requirements, some solutions that were considered (including Drobot’s) and why they were rejected. They then present a method based on a “representative point”, and how it applies to spherical lights and tube lights. • Real-Time Polygonal-Light Shading with Linearly Transformed Cosines, 2016, Eric Heitz et al. The current state of the art. This technique approximates physically based lighting from polygonal lights by transforming a cosine distribution (which is simpler to integrate) so it matches the BRDF properties. A demo with the code as well as a WebGL demo showing the result are provided. Acknowledgements Many thanks to Calvin Simpson, Dimitri Diakopoulos, Jeremy Cowles, Jonathan Stone, Julian Fong, Sébastien Lagarde, Stefan Werner, Yining Karl Li and the computer graphics community at large for your contributions and suggestions of material to read. A list of important graphics research papers This is an announcement that got all my attention. Since Twitter is a mess to find anything older than a day, here is the list so far: 1. A Characterization of Ten Hidden-Surface Algorithms, Sutherland et al., ACM Computing Surveys, 1974 2. Survey of Texture Mapping, Paul Heckbert, IEEE Computer Graphics and Applications, 1986 3. Rendering Complex Scenes with Memory-Coherent Ray Tracing, Matt Pharr et al., proceedings of SIGGRAPH, 1997 4. An Efficient Representation for Irradiance Environment Maps, Ramamoorthi & Hanrahan, proceedings of SIGGRAPH, 2001 5. Decoupled Sampling for Graphics Pipelines, Ragan-Kelley et al. ACM Transactions on Graphics, 2011 6. The Aliasing Problem in Computer-Generated Shaded Images, Franklin C. Crow, Communications of the ACM, 1977 7. Ray Tracing Complex Scenes, Kay & Kajiya, proceedings of SIGGRAPH, 1986 8. Hierarchical Z-buffer Visibility, Greene et al., proceedings of SIGGRAPH, 1993 9. Geometry Images, Gu et al., ACM Transactions on Graphics, 2002 10. A Hidden-Surface Algorithm with Anti-Aliasing, Edwin Catmull, proceedings of SIGGRAPH, 1978 11. Modeling the Interaction of Light Between Diffuse Surfaces, Goral et al., proceedings of SIGGRAPH, 1984 “The first radiosity paper, with the real physical Cornell box (which I’ve actually have seen in real life!)” 12. Pyramidal Parametrics, Lance Williams, proceedings of SIGGRAPH, 1983 13. Rendering synthetic objects into real scenes: bridging traditional and image-based graphics with global illumination and high dynamic range photography, Paul Debevec, proceedings of SIGGRAPH 2008 “Influence on gfx proportional to title length!” 14. A parallel algorithm for polygon rasterization, Juan Pineda, proceedings of SIGGRAPH, 1988 15. Rendering from compressed textures, Beers et al., proceedings of SIGGRAPH 1996 “This one (out of 3) of the 1st texture compression papers ever! Uses VQ so probably not something you want today, but major eye opener!” 16. A general version of Crow’s shadow volumes, P. Bergeron, IEEE Computer Graphics and Applications, 1986 “Generalized SV. Nice trick” 17. Reality engine graphics, Kurt Akeley, proceedings of SIGGRAPH 1993 “Paper describes MSAA, guard bands, etc etc” 18. The design and analysis of a cache architecture for texture mapping, Hakura and Gupta, proceedings of ISCA 1997 “Classic texture$ paper!”
19. Deep shadow maps, Lokovic and Veach, proceedings of SIGGRAPH 2000
“Lots of inspiration here!”
20. The Reyes image rendering architecture, Cook et al., proceedings of SIGGRAPH 1987
“Sooo good & mega-influential!”
21. A practical model for subsurface light transport, Jensen et al., proceedings of SIGGRAPH 2001
22. Casting curved shadows on curved surfaces, Lance Williams, proceedings of SIGGRAPH 1978
23. On the design of display processors, Myer and Sutherland, Communications of the ACM 1968
“Wheel of reincarnation”
24. Ray tracing Jell-O brand gelatin, Paul S. Heckbert, Communications of the ACM 1988
25. Talisman: Commodity realtime 3D graphics for the PC, Torborg and Kajiya, Proceedings of SIGGRAPH 1996
26. A Frequency Analysis of Light Transport, Durand et al., Proceedings of SIGGRAPH 2005
“Very influential!!”
27. An Ambient Light Illumination Mode (behind a paywall), S. Zhukov, A. Iones, G. Kronin, Eurographics 1998
“First paper on ambient occlusion, AFAIK. Not that old…”

A GLSL version of smallpt

smallpt is a bare minimum path tracer written under 100 lines of C++, featuring diffuse, and specular reflection, and refraction. Using the detailed explanation slides by David Cline, I experimented porting it to GLSL on Shadertoy.

This proved to be an interesting experiment that brought a few lessons.

You can see the shader and tweak it here. By default it uses 6 samples per pixel, and 3 bounces, which allows it to run smoothly on average hardware. I found 40 samples per pixel and 5 bounces to give nice results while maintaining interactive framerate.

Path tracing, 40 samples per pixel, 5 bounces

Update: since GLSL Sandbox has a feature, reading from the previous frame buffer, that Shadertoy is missing at the moment, I thought it’d be interesting try it to have the image converging over time. A little hacking later, a minute or so worth of rendering got me this kind of result: Given the effort, I am really pleased by the result.

Path tracing, unknown number of samples per pixel, 7 bounces

Short reading list on diffuse lighting

This is going to be a short one: I haven’t done any extensive search, but I still want to make a back of the envelope note with a couple of links.