International audienceTone Mapping Operators (TMOs) aim at converting real world high dynamic range (HDR) images captured withHDR cameras, into low dynamic range (LDR) images that can be displayed on LDR displays. Several TMOshave been proposed over the last decade, from the simple global mapping to the more complex one simulating thehuman vision system. While these solutions work generally well for still pictures, they are usually less efficient forvideo sequences as they are source of visual artifacts. Only few of them can be adapted to cope with a sequenceof images. In this paper we present a major problem that a static TMO usually encounters while dealing withvideo sequences, namely the temporal coherency. Indeed, as each tone mapper deals with each frame separately,no temporal coherency is taken into account and hence the results can be quite disturbing for high varyingdynamics in a video. We propose a temporal coherency algorithm that is designed to analyze a video as a whole,and from its characteristics adapts each tone mapped frame of a sequence in order to preserve the temporalcoherency. This temporal coherency algorithm has been tested on a set of real as well as Computer GraphicsImage (CGI) content and put in competition with several algorithms that are designed to be time-dependent.Results show that temporal coherency preserves the overall contrast in a sequence of images. Furthermore, thistechnique is applicable to any TMO as it is a post-processing that only depends on the used TMO

Multiple importance sampling (MIS) is a provably good way to combine a finite set of sampling techniques to reduce variance in Monte Carlo integral estimation. However, there exist integration problems for which a continuum of sampling techniques is available. To handle such cases we establish a continuous MIS (CMIS) formulation as a generalization of MIS to uncountably infinite sets of techniques. Our formulation is equipped with a base estimator that is coupled with a provably optimal balance heuristic and a practical stochastic MIS (SMIS) estimator that makes CMIS accessible to a broad range of problems. To illustrate the effectiveness and utility of our framework, we apply it to three different light transport applications, showing improved performance over the prior state-of-the-art techniques.

Fig. 1. Equal-time (5 minutes) renderings of a smoky Kitchen scene. Gradient-domain volumetric rendering techniques with L1 reconstruction converge faster than primal-domain volumetric rendering technique. The relMSE error metric has a unitless scale of 10 −2 .Gradient-domain rendering can improve the convergence of surface-based light transport by exploiting smoothness in image space. Scenes with participating media exhibit similar smoothness and could potentially benefit from gradient-domain techniques. We introduce the first gradient-domain formulation of image synthesis with homogeneous participating media, including four novel and efficient gradient-domain volumetric density estimation algorithms. We show that naïve extensions of gradient domain path-space and density estimation methods to volumetric media, while functional, can result in inefficient estimators. Focussing on point-, beam-and plane-based gradient-domain estimators, we introduce a novel shift mapping that eliminates redundancies in the naïve formulations using spatial relaxation within the volume. We show that gradient-domain volumetric rendering improve convergence compared to primal domain state-of-the-art, across a suite of scenes. Our formulation and algorithms support progressive estimation and are easy to incorporate atop existing renderers.Authors' addresses: Adrien Gruson, adrien

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Layered materials capture subtle, realistic reflection behaviors that traditional single‐layer models lack. Much of this is due to the complex subsurface light transport at the interfaces of – and in the media between – layers. Rendering with these materials can be costly, since we must simulate these transport effects at every evaluation of the underlying reflectance model. Rendering an image requires thousands of such evaluations, per pixel. Recent work treats this complexity by introducing significant approximations, requiring large precomputed datasetsper material, or simplifying the light transport simulations within the materials. Even the most effective of these methods struggle with the complexity induced by high‐frequency variation in reflectance parameters and micro‐surface normal variation, as well as anisotropic volumetric scattering between the layer interfaces. We present a more efficient, unbiased estimator for light transport in such general, complex layered appearance models. By conducting an analysis of the types of transport paths that contribute most to the aggregate reflectance dynamics, we propose an effective and unbiased path sampling method that reduces variance in the reflectance evaluations. Our method additionally supports reflectance importance sampling, does not rely on any precomputation, and so integrates readily into existing renderers. We consistently outperform the state‐of‐the‐art by ~2 – 6× in equal‐quality (i.e., equal error) comparisons.

Gradient Ours L2 Recons. EV+3 Throughput Primal L2 Recons Manzi et al. [2015] L1 Recons. L1 Recons PrimalFigure 1: Equal-time rendering (five minutes) of a bathroom scene with gradient-domain photon density estimation and bi-directional path tracing [MKA * 15]. Our method more efficiently handles regions dominated by complex transport paths. AbstractThe most common solutions to the light transport problem rely on either Monte Carlo (MC) integration or density estimation methods, such as uni-& bi-directional path tracing or photon mapping. Recent gradient-domain extensions of MC approaches show great promise; here, gradients of the final image are estimated numerically (instead of the image intensities themselves) with coherent paths generated from a deterministic shift mapping. We extend gradient-domain approaches to light transport simulation based on density estimation. As with previous gradient-domain methods, we detail important considerations that arise when moving from a primal-to gradient-domain estimator. We provide an efficient and straightforward solution to these problems. Our solution supports stochastic progressive density estimation, so it is robust to complex transport effects. We show that gradient-domain photon density estimation converges faster than its primal-domain counterpart, as well as being generally more robust than gradient-domain uni-& bi-directional path tracing for scenes dominated by complex transport.

Markov chain Monte Carlo (MCMC) sampling is a powerful approach to generate samples from an arbitrary distribution. The application to light transport simulation allows us to efficiently handle complex light transport such as highly occluded scenes. Since light transport paths in MCMC methods are sampled according to the path contributions over the sampling domain covering the whole image, bright pixels receive more samples than dark pixels to represent differences in the brightness. This variation in the number of samples per pixel is a fundamental property of MCMC methods. This property often leads to uneven convergence over the image, which is a notorious and fundamental issue of any MCMC method to date. We present a novel stratification method of MCMC light transport methods. Our stratification method, for the first time, breaks the fundamental limitation that the number of samples per pixel is uncontrollable. Our method guarantees that every pixel receives a specified number of samples by running a single Markov chain per pixel. We rely on the fact that different MCMC processes should converge to the same result when the sampling domain and the integrand are the same. We thus subdivide an image into multiple overlapping tiles associated with each pixel, run an independent MCMC process in each of them, and then align all of the tiles such that overlapping regions match. This can be formulated as an optimization problem similar to the reconstruction step for gradient‐domain rendering. Further, our method can exploit the coherency of integrands among neighboring pixels via coherent Markov chains and replica exchange. Images rendered with our method exhibit much more predictable convergence compared to existing MCMC methods.

Monte Carlo methods for physically‐based light transport simulation are broadly adopted in the feature film production, animation and visual effects industries. These methods, however, often result in noisy images and have slow convergence. As such, improving the convergence of Monte Carlo rendering remains an important open problem. Gradient‐domain light transport is a recent family of techniques that can accelerate Monte Carlo rendering by up to an order of magnitude, leveraging a gradient‐based estimation and a reformulation of the rendering problem as an image reconstruction. This state of the art report comprehensively frames the fundamentals of gradient‐domain rendering, as well as the pragmatic details behind practical gradient‐domain uniand bidirectional path tracing and photon density estimation algorithms. Moreover, we discuss the various image reconstruction schemes that are crucial to accurate and stable gradient‐domain rendering. Finally, we benchmark various gradient‐domain techniques against the state‐of‐the‐art in denoising methods before discussing open problems.