34-Issue 4

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https://diglib.eg.org/handle/10.2312/12634

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Consistent Scene Editing by Progressive Difference Images
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Günther, Tobias; Grosch, Thorsten; Jaakko Lehtinen and Derek Nowrouzezahrai
Even though much research was dedicated to the acceleration of consistent, progressive light transport simulations, the computation of fully converged images is still very time-consuming. This is problematic, as for the practical use in production pipelines, the rapid editing of lighting effects is important. While previous approaches restart the simulation with every scene manipulation, we make use of the coherence between frames before and after a modification in order to accelerate convergence of the context that remained similar. This is especially beneficial if a scene is edited that has already been converging for a long time, because much of the previous result can be reused, e.g., sharp caustics cast or received by the unedited scene parts. In its essence, our method performs the scene modification stochastically by predicting and accounting for the difference image. In addition, we employ two heuristics to handle cases in which stochastic removal is likely to lead to strong noise. Typical scene interactions can be broken down into object adding and removal, material substitution, camera movement and light editing, which we all examine in a number of test scenes both qualitatively and quantitatively. As we focus on caustics, we chose stochastic progressive photon mapping as the underlying light transport algorithm. Further, we show preliminary results of bidirectional path tracing and vertex connection and merging.
Extracting Microfacet-based BRDF Parameters from Arbitrary Materials with Power Iterations
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Dupuy, Jonathan; Heitz, Eric; Iehl, Jean-Claude; Poulin, Pierre; Ostromoukhov, Victor; Jaakko Lehtinen and Derek Nowrouzezahrai
We introduce a novel fitting procedure that takes as input an arbitrary material, possibly anisotropic, and automatically converts it to a microfacet BRDF. Our algorithm is based on the property that the distribution of microfacets may be retrieved by solving an eigenvector problem that is built solely from backscattering samples. We show that the eigenvector associated to the largest eigenvalue is always the only solution to this problem, and compute it using the power iteration method. This approach is straightforward to implement, much faster to compute, and considerably more robust than solutions based on nonlinear optimizations. In addition, we provide simple conversion procedures of our fits into both Beckmann and GGX roughness parameters, and discuss the advantages of microfacet slope space to make our fits editable. We apply our method to measured materials from two large databases that include anisotropic materials, and demonstrate the benefits of spatially varying roughness on texture mapped geometric models.
Illumination-driven Mesh Reduction for Accelerating Light Transport Simulations
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Reich, Andreas; Günther, Tobias; Grosch, Thorsten; Jaakko Lehtinen and Derek Nowrouzezahrai
Progressive light transport simulations aspire a physically-based, consistent rendering to obtain visually appealing illumination effects, depth and realism. Thereby, the handling of large scenes is a difficult problem, as in typical scene subdivision approaches the parallel processing requires frequent synchronization due to the bouncing of light throughout the scene. In practice, however, only few object parts noticeably contribute to the radiance observable in the image, whereas large areas play only a minor role. In fact, a mesh simplification of the latter can go unnoticed by the human eye. This particular importance to the visible radiance in the image calls for an output-sensitive mesh reduction that allows to render originally out-of-core scenes on a single machine without swapping of memory. Thus, in this paper, we present a preprocessing step that reduces the scene size under the constraint of radiance preservation with focus on high-frequency effects such as caustics. For this, we perform a small number of preliminary light transport simulation iterations. Thereby, we identify mesh parts that contribute significantly to the visible radiance in the scene, and which we thus preserve during mesh reduction.
Improved Half Vector Space Light Transport
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Hanika, Johannes; Kaplanyan, Anton; Dachsbacher, Carsten; Jaakko Lehtinen and Derek Nowrouzezahrai
In this paper, we present improvements to half vector space light transport (HSLT) [KHD14], which make this approach more practical, robust for difficult input geometry, and faster. Our first contribution is the computation of half vector space ray differentials in a different domain than the original work. This enables a more uniform stratification over the image plane during Markov chain exploration. Furthermore, we introduce a new multi chain perturbation in half vector space, which, if combined appropriately with half vector perturbation, makes the mutation strategy both more robust to geometric configurations with fine displacements and faster due to reduced number of ray casts. We provide and analyze the results of improved HSLT and discuss possible applications of our new half vector ray differentials.
Manifold Next Event Estimation
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Hanika, Johannes; Droske, Marc; Fascione, Luca; Jaakko Lehtinen and Derek Nowrouzezahrai
We present manifold next event estimation (MNEE), a specialised technique for Monte Carlo light transport simulation to render refractive caustics by connecting surfaces to light sources (next event estimation) across transmissive interfaces. We employ correlated sampling by means of a perturbation strategy to explore all half vectors in the case of rough transmission while remaining outside of the context of Markov chain Monte Carlo, improving temporal stability. MNEE builds on differential geometry and manifold walks. It is very lightweight in its memory requirements, as it does not use light caching methods such as photon maps or importance sampling records. The method integrates seamlessly with existing Monte Carlo estimators via multiple importance sampling.
Path-space Motion Estimation and Decomposition for Robust Animation Filtering
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Zimmer, Henning; Rousselle, Fabrice; Jakob, Wenzel; Wang, Oliver; Adler, David; Jarosz, Wojciech; Sorkine-Hornung, Olga; Sorkine-Hornung, Alexander; Jaakko Lehtinen and Derek Nowrouzezahrai
Renderings of animation sequences with physics-based Monte Carlo light transport simulations are exceedingly costly to generate frame-by-frame, yet much of this computation is highly redundant due to the strong coherence in space, time and among samples. A promising approach pursued in prior work entails subsampling the sequence in space, time, and number of samples, followed by image-based spatio-temporal upsampling and denoising. These methods can provide significant performance gains, though major issues remain: firstly, in a multiple scattering simulation, the final pixel color is the composite of many different light transport phenomena, and this conflicting information causes artifacts in image-based methods. Secondly, motion vectors are needed to establish correspondence between the pixels in different frames, but it is unclear how to obtain them for most kinds of light paths (e.g. an object seen through a curved glass panel). To reduce these ambiguities, we propose a general decomposition framework, where the final pixel color is separated into components corresponding to disjoint subsets of the space of light paths. Each component is accompanied by motion vectors and other auxiliary features such as reflectance and surface normals. The motion vectors of specular paths are computed using a temporal extension of manifold exploration and the remaining components use a specialized variant of optical flow. Our experiments show that this decomposition leads to significant improvements in three image-based applications: denoising, spatial upsampling, and temporal interpolation.
Portal-Masked Environment Map Sampling
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Bitterli, Benedikt; Novák, Jan; Jarosz, Wojciech; Jaakko Lehtinen and Derek Nowrouzezahrai
We present a technique to e ciently importance sample distant, all-frequency illumination in indoor scenes. Standard environment sampling is ine cient in such cases since the distant lighting is typically only visible through small openings (e.g. windows). This visibility is often addressed by manually placing a portal around each window to direct samples towards the openings; however, uniformly sampling the portal (its area or solid angle) disregards the possibly high frequency environment map. We propose a new portal importance sampling technique which takes into account both the environment map and its visibility through the portal, drawing samples proportional to the product of the two. To make this practical, we propose a novel, portal-rectified reparametrization of the environment map with the key property that the visible region induced by a rectangular portal projects to an axis-aligned rectangle. This allows us to sample according to the desired product distribution at an arbitrary shading location using a single (precomputed) summed-area table per portal. Our technique is unbiased, relevant to many renderers, and can also be applied to rectangular light sources with directional emission profiles, enabling e cient rendering of non-di use light sources with soft shadows.
Stochastic Soft Shadow Mapping
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Liktor, Gabor; Spassov, Stanislav; Mückl, Gregor; Dachsbacher, Carsten; Jaakko Lehtinen and Derek Nowrouzezahrai
In this paper, we extend the concept of pre-filtered shadow mapping to stochastic rasterization, enabling real-time rendering of soft shadows from planar area lights. Most existing soft shadow mapping methods lose important visibility information by relying on pinhole renderings from an area light source, providing plausible results only for small light sources. Since we sample the entire 4D shadow light field stochastically, we are able to closely approximate shadows of large area lights as well. In order to efficiently reconstruct smooth shadows from this sparse data, we exploit the analogy of soft shadow computation to rendering defocus blur, and introduce a multiplane pre-filtering algorithm. We demonstrate how existing pre-filterable approximations of the visibility function, such as variance shadow mapping, can be extended to four dimensions within our framework.
Unifying Color and Texture Transfer for Predictive Appearance Manipulation
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Okura, Fumio; Vanhoey, Kenneth; Bousseau, Adrien; Efros, Alexei A.; Drettakis, George; Jaakko Lehtinen and Derek Nowrouzezahrai
Recent color transfer methods use local information to learn the transformation from a source to an exemplar image, and then transfer this appearance change to a target image. These solutions achieve very successful results for general mood changes, e.g., changing the appearance of an image from ''sunny'' to ''overcast''. However, such methods have a hard time creating new image content, such as leaves on a bare tree. Texture transfer, on the other hand, can synthesize such content but tends to destroy image structure. We propose the first algorithm that unifies color and texture transfer, outperforming both by leveraging their respective strengths. A key novelty in our approach resides in teasing apart appearance changes that can be modeled simply as changes in color versus those that require new image content to be generated. Our method starts with an analysis phase which evaluates the success of color transfer by comparing the exemplar with the source. This analysis then drives a selective, iterative texture transfer algorithm that simultaneously predicts the success of color transfer on the target and synthesizes new content where needed. We demonstrate our unified algorithm by transferring large temporal changes between photographs, such as change of season - e.g., leaves on bare trees or piles of snow on a street - and flooding.

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Browsing 34-Issue 4 by Subject "I.3.7 [Computer Graphics]"

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