Estimating Local Beckmann Roughness for Complex BSDFs

International audienceMany light transport related techniques require an analysis of the blur width of light scattering at a path vertex, for instance a Beck-mann roughness. Such use cases are for instance analysis of expected variance (and potential biased countermeasures in production rendering), radiance caching or directionally dependent virtual point light sources, or determination of step sizes in the path space Metropolis light transport framework: recent advanced mutation strategies for Metropolis Light Transport [Veach 1997], such as Manifold Exploration [Jakob 2013] and Half Vector Space Light Transport [Kaplanyan et al. 2014] employ local curvature of the BSDFs (such as an average Beckmann roughness) at all interactions along the path in order to determine an optimal mutation step size. A single average Beckmann roughness, however, can be a bad fit for complex measured materials (such as [Matusik et al. 2003]) and, moreover, such curvature is completely undefined for layered materials as it depends on the active scattering layer. We propose a robust estimation of local curvature for BSDFs of any complexity by using local Beckmann approximations, taking into account additional factors such as both incident and outgoing direction

A perceptual model of motion quality for rendering with adaptive refresh-rate and resolution

Limited GPU performance budgets and transmission bandwidths mean that real-time rendering often has to compromise on the spatial resolution or temporal resolution (refresh rate). A common practice is to keep either the resolution or the refresh rate constant and dynamically control the other variable. But this strategy is non-optimal when the velocity of displayed content varies. To find the best trade-off between the spatial resolution and refresh rate, we propose a perceptual visual model that predicts the quality of motion given an object velocity and predictability of motion. The model considers two motion artifacts to establish an overall quality score: non-smooth (juddery) motion, and blur. Blur is modeled as a combined effect of eye motion, finite refresh rate and display resolution. To fit the free parameters of the proposed visual model, we measured eye movement for predictable and unpredictable motion, and conducted psychophysical experiments to measure the quality of motion from 50 Hz to 165 Hz. We demonstrate the utility of the model with our on-the-fly motion-adaptive rendering algorithm that adjusts the refresh rate of a G-Sync-capable monitor based on a given rendering budget and observed object motion. Our psychophysical validation experiments demonstrate that the proposed algorithm performs better than constant-refresh-rate solutions, showing that motion-adaptive rendering is an attractive technique for driving variable-refresh-rate displays.</jats:p

Robust and Efficient Monte Carlo Light Transport Simulation using Regularizations and the Half Vector Integration Domain

Computer-generated imagery has become a central point in large and quickly growing areas such as computer animation, visual effects, architecture, high-quality visualization, and product design. A common requirement to all these areas is the efficient simulation of light transport from the light source to the sensor for image synthesis. This is a challenging task, as light can travel along all kinds of paths, and some paths are hard to find when computing the result numerically. The process of light propagation in a virtual scene is described by a path integral equation. This path integral is defined over the path space, the space of all possible light paths, on which photons travel from a light source to a virtual sensor. Monte Carlo approaches are employed for computing the path integral, where paths are stochastically generated (sampled). The integration, to compute an image of a virtual scene, in this space is challenging because of the mathematical properties of the integrand. In addition, the demand for both image quality and quantity requires shorter image rendering times, while imposing higher scene complexity at the same time. We provide several solutions to the problems arising with difficult light paths, especially these involving specular and glossy materials, as well as improve the rendering convergence with difficult illumination effects. Some physical phenomena, such as perfectly specular reflection, cause singularities in the path integral, and thus are hard or even impossible to sample. We propose an approach to adaptively smooth difficult components of light transport, which makes them samplable while still ensuring convergence to the exact solution. It uses regularization for paths which cannot be sampled in an unbiased way. To introduce as little bias as possible, we selectively regularize individual interactions along paths, and also derive the regularization consistency conditions. Additionally, in order to control the bias introduced by kernel smoothing, which is another means to sample complex light transport, we developed an adaptive bandwidth selection method. We have also analyzed the convergence rate for kernel smoothing methods in light transport. On the other hand, even not perfectly specular, i.e., glossy, materials can be as difficult to sample. We propose a new way for representing light paths for more efficient exploration of the path space. Based on this, we propose a new mutation strategy, to be used with Markov chain Monte Carlo methods such as Metropolis light transport, which is well-suited for all configurations of surface scattering. A different yet related problem is the creation of virtual scenes with photorealistic lighting. It can be challenging because it often requires interactive feedback and edits to the scene that are propagated consistently. To alleviate this process, we propose a set of visualization and manipulation tools, which allow the designers to grasp virtual illumination and to alter the physical laws of light transport for convenient illumination setup and analysis, as well as for achieving artistic goals