Publicaciones de Unity

En Unity realizamos investigación sobre gráficos IA, rendimiento y mucho más, y compartimos los resultados contigo y la comunidad mediante charlas, conferencias y revistas. A continuación, verás las publicaciones más recientes.

Probando la distribución de normales visibles GGX

Eric Heitz - JCGT 2018

Los ejemplos importantes de microfacetas de BSDF usando su distribución de normales visibles (VNDF) producen una reducción de variante significativa en el renderizado Monte Carlo. En este artículo, describimos una rutina de pruebas eficiente y exacta para el VNDF de la distribución de microfacetas de GGX. Esta rutina aprovecha la propiedad de que GGX es la distribución de normales de una elipsoide truncada y las pruebas de GGX VNDF son equivalentes a las pruebas de proyección 2D de esta elipsoide truncada. Para hacerlo, simplificamos el problema usando la transformación linear que mapea la elipsoide truncada a un hemisferio. Ya que las transformaciones lineares preservan la uniformidad de las áreas proyectadas, las pruebas en la configuración de hemisferio y transformar las pruebas de nuevo en una configuración de elipsoide produce ejemplos válidos de GGX VNDF.

Papel
Peto
Código de ejemplo

Analytical Calculation of the Solid Angle Subtended by an Arbitrarily Positioned Ellipsoid to a Point Source

Eric Heitz -  Nuclear Instruments and Methods in Physics Research 2018

We present a geometric method for computing an ellipse that subtends the same solid-angle domain as an arbitrarily positioned ellipsoid. With this method we can extend existing analytical solid-angle calculations of ellipses to ellipsoids. Our idea consists of applying a linear transformation on the ellipsoid such that it is transformed into a sphere from which a disk that covers the same solid-angle domain can be computed. We demonstrate that by applying the inverse linear transformation on this disk we obtain an ellipse that subtends the same solid-angle domain as the ellipsoid. We provide a MATLAB implementation of our algorithm and we validate it numerically.

Publisher’s Version
Author's free print

A note on track-length sampling with non-exponential distributions

Eric Heitz, Laurent Belcour - Tech Report 2018

Track-length sampling is the process of sampling random intervals according to a distance distribution. It means that, instead of sampling a punctual distance from the distance distribution, track-length sampling generates an interval of possible distances.The track-length sampling process is correct if the expectation of the intervals is the target distance distribution. In other words, averaging all the sampled intervals should converge towards the distance distribution as their number increases. In this note, we emphasize that the distance distribution that is used for sampling punctual distances and the track-length distribution that is used for sampling intervals are not the same in general. This difference can be surprising because, to our knowledge, track-length sampling has been mostly studied in the context of transport theory where the distance distribution is typically exponential: in this special case, the distance distribution and the track-length distribution happens to be both the same exponential distribution. However, they are not the same in general when the distance distribution is non-exponential.

Paper

Combining Analytic Direct Illumination and Stochastic Shadows

Eric Heitz, Stephen Hill (Lucasfilm), Morgan McGuire (NVIDIA)  - I3D 2018 (short paper) (Best Paper Presentation Award)

In this paper, we propose a ratio estimator of the direct-illumination equation that allows us to combine analytic illumination techniques with stochastic raytraced shadows while maintaining correctness. Our main contribution is to show that the shadowed illumination can be split into the product of the unshadowed illumination and the illumination-weighted shadow. These terms can be computed separately — possibly using different techniques — without affecting the exactness of the final result given by their product. This formulation broadens the utility of analytic illumination techniques to raytracing applications, where they were hitherto avoided because they did not incorporate shadows. We use such methods to obtain sharp and noise-free shading in the unshadowed-illumination image and we compute the weighted-shadow image with stochastic raytracing. The advantage of restricting stochastic evaluation to the weighted-shadow image is that the final result exhibits noise only in the shadows. Furthermore, we denoise shadows separately from illumination so that even aggressive denoising only overblurs shadows, while high-frequency shading details (textures, normal maps, etc.) are preserved.

Paper
Supplemental
Slides
Code + demo

Teselación no periódica de las funciones de sonido procesales

Aleksandr Kirillov - HPG 2018

Las funciones de sonido procesales tienen muchas aplicaciones en los gráficos de computadora, que van desde síntesis de textura hasta simulaciones de efecto atmosférico o hasta la especificación de la geometría del paisaje. El sonido ya puede estar precomputarizado y almacenado en una textura o evaluado directamente en el tiempo de ejecución de la aplicación. Esta opción ofrece una compensación entre la discrepancia de imágen, el consumo de memoria y el rendimiento.

Los algoritmos de teselación avanzados se pueden usar para disminuir la repetición visual. Los cuadros de Wang permiten que un plano sea teselado de un modo no periódico, usando un conjunto de texturas relativamente pequeño. Los mosaicos pueden estar distribuidos en un único mapa de textura para permitirle al GPU que utilice el filtrado de hardware.

En este artículo, presentamos modificaciones para varias funciones de sonido procesales populares que producen directamente mapas de textura que contienen el conjunto de cuadros de Wang completo más pequeño. Las conclusiones presentadas en este artículo permite el teselado no periódico de estas funciones de sonido y texturas basadas en estos, tanto en tiempo de ejecución como en forma de un paso de preprocesamiento. Estas conclusiones también permiten disminuir la repetición de los efectos basados en sonidos en imágenes generadas en computadoras a un costo de rendimiento bajo, mientras se mantiene o reduce el consumo de memoria.

Artículo

Complementario

High-Performance By-Example Noise using a Histogram-Preserving Blending Operator

Eric Heitz, Fabrice Neyret (Inria) - HPG 2018 (Best Paper Award)

We propose a new by-example noise algorithm that takes as input a small example of a stochastic texture and synthesizes an infinite output with the same appearance. It works on any kind of random-phase inputs as well as on many non-random-phase inputs that are stochastic and non-periodic, typically natural textures such as moss, granite, sand, bark, etc. Our algorithm achieves high-quality results comparable to state-of-the-art procedural-noise techniques but is more than 20 times faster

Paper
Supplemental
Slides
Video

Unsupervised Deep Single-Image Intrinsic Decomposition using Illumination-Varying Image Sequences

Louis Lettry (ETH Zürich), Kenneth Vanhoey, Luc Van Gool (ETH Zürich) - Pacific Graphics 2018 / Computer Graphics Forum

Intrinsic Decomposition decomposes a photographed scene into albedo and shading. Removing shading  allows to "delight" images, which can then be reused in virtually relit scenes. We propose an unsupervised learning method to solve this problem.

Recent techniques use supervised learning: it requires a large set of known decompositions, which are hard to obtain. Instead, we train on unannotated images by using time lapse imagery gained from static webcams. We exploit the assumption that albedo is static by definition, and shading varies with lighting. We transcribe this into a siamese training for deep learning.

Paper
Supplemental
Supplemental code

Bib
Slides
 

Efficient Rendering of Layered Materials using an Atomic Decomposition with Statistical Operators

Laurent Belcour - ACM SIGGRAPH 2018

We derive a novel framework for the efficient analysis and computation of light transport within layered materials. Our derivation consists of two steps. First, we decompose light transport into a set of atomic operators that act on its directional statistics. Speci€cally, our operators consist of reflƒection, refraction, scaŠttering, and absorption, whose combinations are sufficient to describe the statistics of light scattŠering multiple times within layered structures. We show that the €first three directional moments (energy, mean and variance) already provide an accurate summary. Second, we extend the adding-doubling method to support arbitrary combinations of such operators eciently. During shading, we map the directional moments to BSDF lobes. We validate that the resulting BSDF closely matches the ground truth in a lightweight and efficient form. Unlike previous methods, we support an arbitrary number of textured layers, and demonstrate a practical and accurate rendering of layered materials with both an offl„ine and real-time implementation that are free from per-material precomputation.

Paper
Supplemental
Supplemental code
Bib
Video
Slides

An Adaptive Parameterization for Material Acquisition and Rendering

Jonathan Dupuy and Wenzel Jakob (EPFL) - ACM SIGGRAPH Asia 2018

One of the key ingredients of any physically based rendering system is a detailed specification characterizing the interaction of light and matter of all materials present in a scene, typically via the Bidirectional Reflectance Distribution Function (BRDF). Despite their utility, access to real-world BRDF datasets remains limited: this is because measurements involve scanning a four-dimensional domain at sufficient resolution, a tedious and often infeasible time-consuming process. We propose a new parameterization that automatically adapts to the behavior of a material, warping the underlying 4D domain so that most of the volume maps to regions where the BRDF takes on non-negligible values, while irrelevant regions are strongly compressed. This adaptation only requires a brief 1D or 2D measurement of the material’s retro-reflective properties. Our parameterization is unified in the sense that it combines several steps that previously required intermediate data conversions: the same mapping can simultaneously be used for BRDF acquisition, storage, and it supports efficient Monte Carlo sample generation.

Paper
Video
Isotropic BRDF Dataset
Anisotropic BRDF Dataset
MERL Database Validation
C++ & Python code
Material Database

Sombras estocásticas

Eric Heitz, Stephen Hill (Lucasfilm), Morgan McGuire (NVIDIA) 

En este artículo, proponemos un estimador de proporción de la ecuación de iluminación directa que nos permite combinar técnicas analíticas de iluminación con sombras estocásticas trazadas por rayos sin perder precisión. Nuestro principal aporte es demostrar que la iluminación con sombra se puede dividir en el producto de la iluminación sin sombra y la sombra con iluminación. Estos datos se pueden calcular por separado (posiblemente mediante técnicas diferentes) sin afectar la exactitud del resultado final que se obtiene cuando están combinados.

Esta fórmula aumenta el grado de utilidad de las técnicas de iluminación analíticas en las aplicaciones de trazado de rayos, donde, hasta el momento, se evitaban porque no incorporaban las sombras. Utilizamos estos métodos para obtener un sombreado con efecto nítido y sin ruido en la imagen con ilimunación sin sombras, y calculamos la imagen con sombras mediante el trazado de rayos estocástico. La ventaja de limitar la evaluación estocástica a la imagen con sombras es que el resultado final presenta ruido solo en las sombras. Además, quitamos el ruido de las sombras por fuera de la iluminación, para que las grandes reducciones de ruido solo hagan borrosas las sombras, sin perder los detalles de sombreado de alta frecuencia (texturas, mapas normales, etc.).

Suplemento

Adaptive GPU Tessellation with Compute Shaders

Jad Khoury, Jonathan Dupuy, and Christophe Riccio - GPU Zen 2 (to appear)

GPU rasterizers are most efficient when primitives project into more than a few pixels. Below this limit, the Z-buffer starts aliasing, and shading rate decreases dramatically [Riccio 12]; this makes the rendering of geometrically-complex scenes challenging, as any moderately distant polygon will project to sub-pixel size. In order to minimize such sub-pixel projections, a simple solution consists in procedurally refining coarse meshes as they get closer to the camera. In this chapter, we are interested in deriving such a procedural refinement technique for arbitrary polygon meshes.

Paper
Code

Real-Time Line- and Disk-Light Shading with Linearly Transformed Cosines

Eric Heitz (Unity Technologies) and Stephen Hill (Lucasfilm) - ACM SIGGRAPH Courses 2017

We recently introduced a new real-time area-light shading technique dedicated to lights with polygonal shapes. In this talk, we extend this area-lighting framework to support lights shaped as lines, spheres and disks in addition to polygons.

Slides
Demo code
WebGL demo for quad, line and disk lights

Mapeo normal basado en microfacets para el trazado preciso de rutas Monte Carlo

Vincent Schüssler (KIT), Eric Heitz (Unity Technologies), Johannes Hanika (KIT) and Carsten Dachsbacher (KIT) - ACM SIGGRAPH ASIA 2017

Normal mapping imitates visual details on surfaces by using fake shading normals. However, the resulting surface model is geometrically impossible and normal mapping is thus often considered a fundamentally flawed approach with unavoidable problems for Monte Carlo path tracing: it breaks either the appearance (black fringes, energy loss) or the integrator (different forward and backward light transport). In this paper, we present microfacet-based normal mapping, an alternative way of faking geometric details without corrupting the robustness of Monte Carlo path tracing such that these problems do not arise.

Paper

A Spherical Cap Preserving Parameterization for Spherical Distributions

Jonathan Dupuy, Eric Heitz and Laurent Belcour - ACM SIGGRAPH 2017

We introduce a novel parameterization for spherical distributions that is based on a point located inside the sphere, which we call a pivot. The pivot serves as the center of a straight-line projection that maps solid angles onto the opposite side of the sphere. By transforming spherical distributions in this way, we derive novel parametric spherical distributions that can be evaluated and importance-sampled from the original distributions using simple, closed-form expressions. Moreover, we prove that if the original distribution can be sampled and/or integrated over a spherical cap, then so can the transformed distribution. We exploit the properties of our parameterization to derive efficient spherical lighting techniques for both real-time and offline rendering. Our techniques are robust, fast, easy to implement, and achieve quality that is superior to previous work.

Paper
Video

A Practical Extension to Microfacet Theory for the Modeling of Varying Iridescence

Laurent Belcour (Unity), Pascal Barla (Inria) - ACM SIGGRAPH 2017

Thin film iridescence permits to reproduce the appearance of leather. However, this theory requires spectral rendering engines (such as Maxwell Render) to correctly integrate the change of appearance with respect to viewpoint (known as goniochromatism). This is due to aliasing in the spectral domain as real-time renderers only work with three components (RGB) for the entire range of visible light. In this work, we show how to anti-alias a thin-film model, how to incorporate it in microfacet theory, and how to integrate it in a real-time rendering engine. This widens the range of reproducible appearances with microfacet models.

Paper
Supplemental
Bib
Video
Code
Slides

Linear-Light Shading with Linearly Transformed Cosines

Eric Heitz, Stephen Hill (Lucasfilm) - GPU Zen (book)

In this book chapter, we extend our area-light framework based on Linearly Transformed Cosines to support linear (or line) lights. Linear lights are a good approximation for cylindrical lights with a small but non-zero radius. We describe how to approximate these lights with linear lights that have similar power and shading, and discuss the validity of this approximation.

Paper

A Practical Introduction to Frequency Analysis of Light Transport

Laurent Belcour - ACM SIGGRAPH Courses 2016

Frequency Analysis of Light Transport expresses Physically Based Rendering (PBR) using signal processing tools. It is thus tailored to predict sampling rate, perform denoising, perform anti-aliasing, etc. Many method have been proposed to deal with specific cases of light transport (motion, lenses, etc). This course aims to introduce concepts and present practical application scenario of frequency analysis of light transport in a unified context. To ease the understanding of theoretical elements, frequency analysis will be introduced in pair with an implementation.

Course

Real-Time Polygonal-Light Shading with Linearly Transformed Cosines

Eric Heitz, Jonathan Dupuy, Stephen Hill (Ubisoft), David Neubelt (Ready at Dawn Studios) - ACM SIGGRAPH 2016

Shading with area lights adds a great deal of realism to CG renders. However, it requires solving spherical equations that make it challenging for real-time rendering. In this project, we develop a new spherical distribution that allows us to shade physically based materials with polygonal lights in real-time.

Paper
Slides
MATLAB
Plots and Validation
Comparison against Technicolor's Technique
Demo
WebGL Demo
BRDF fitting code
Video

Additional Progress Towards the Unification of Microfacet and Microflake Theories

Jonathan Dupuy and Eric Heitz - EGSR 2016 (E&I)

We study the links between microfacet and microflake theories from the perspective of linear transport theory. In doing so, we gain additional insights, find several simplifications and touch upon important open questions as well as possible paths forward in extending the unification of surface and volume scattering models. First, we introduce a semi-infinite homogeneous exponential-free-path medium that (a) produces exactly the same light transport as the Smith microsurface scattering model and the inhomogeneous Smith medium that was recently introduced by Heitz et al, and (b) allows us to rederive all the Smith masking and shadowing functions in a simple way. Second, we investigate in detail what new aspects of linear transport theory enable a volume to act like a rough surface. We show that this is mostly due to the use of non-symmetric distributions of normals and explore how the violation of this symmetry impacts light transport within the microflake volume without breaking global reciprocity. Finally, we argue that the surface profiles that would be consistent with very rough Smith microsurfaces have geometrically implausible shapes. To overcome this, we discuss an extension of Smith theory in the volume setting that includes NDFs on the entire sphere in order to produce a single unified reflectance model capable of describing everything from a smooth flat mirror all the way to a semi-infinite isotropically scattering medium with both low and high roughness regimes in between.

Paper
Slides

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