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Item Line Integration for Rendering Heterogeneous Emissive Volumes(The Eurographics Association and John Wiley & Sons Ltd., 2017) Simon, Florian; Hanika, Johannes; Zirr, Tobias; Dachsbacher, Carsten; Zwicker, Matthias and Sander, PedroEmissive media are often challenging to render: in thin regions where only few scattering events occur the emission is poorly sampled, while sampling events for emission can be disadvantageous due to absorption in dense regions. We extend the standard path space measurement contribution to also collect emission along path segments, not only at vertices. We apply this extension to two estimators: extending paths via scattering and distance sampling, and next event estimation. In order to do so, we unify the two approaches and derive the corresponding Monte Carlo estimators to interpret next event estimation as a solid angle sampling technique. We avoid connecting paths to vertices hidden behind dense absorbing layers of smoke by also including transmittance sampling into next event estimation. We demonstrate the advantages of our line integration approach which generates estimators with lower variance since entire segments are accounted for. Also, our novel forward next event estimation technique yields faster run times compared to previous next event estimation as it penetrates less deeply into dense volumes.Item 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 NowrouzezahraiIn 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.Item Graphical Interface Models for Procedural Mesh Growing(The Eurographics Association, 2010) Menz, Stefan; Dammertz, Holger; Hanika, Johannes; Weber, Michael; Lensch, Hendrik P. A.; Reinhard Koch and Andreas Kolb and Christof Rezk-SalamaProcedural modeling allows to create highly complex 3D scenes from a small set of construction rules, which has several advantages over storing the full data of an object. The most important ones are a very small memory footprint and the ability to generate infinite variations of one prototype object by using the same set of rules. However, the problem that procedural modeling imposes on the user is to define a reasonable set of rules to generate a specific object. To simplify this task, we present new interaction metaphors for a graphical user interface and a minimal set of geometric operations that allow the user to efficiently create such rules and the respective models. These metaphors are then implemented in a prototype system and are evaluated by user tests with regard to usability and user performance. The results show that the system enables even inexperienced users to create complex 3D objects via procedural modeling using the presented approach.Item Temporal Sample Reuse for Next Event Estimation and Path Guiding for Real-Time Path Tracing(The Eurographics Association, 2020) Dittebrandt, Addis; Hanika, Johannes; Dachsbacher, Carsten; Dachsbacher, Carsten and Pharr, MattGood importance sampling is crucial for real-time path tracing where only low sample budgets are possible. We present two efficient sampling techniques tailored for massively-parallel GPU path tracing which improve next event estimation (NEE) for rendering with many light sources and sampling of indirect illumination. As sampling densities need to vary spatially, we use an octree structure in world space and introduce algorithms to continuously adapt the partitioning and distribution of the sampling budget. Both sampling techniques exploit temporal coherence by reusing samples from the previous frame: For NEE we collect sampled, unoccluded light sources and show how to deduplicate, but also diffuse this information to efficiently sample light sources in the subsequent frame. For sampling indirect illumination, we present a compressed directional quadtree structure which is iteratively adapted towards high-energy directions using samples from the previous frame. The updates and rebuilding of all data structures takes about 1ms in our test scenes, and adds about 6ms at 1080p to the path tracing time compared to using state-of-the-art light hierarchies and BRDF sampling. We show that this additional effort reduces noise in terms of mean squared error by at least one order of magnitude in many situations.Item Sparse High-degree Polynomials for Wide-angle Lenses(The Eurographics Association and John Wiley & Sons Ltd., 2016) Schrade, Emanuel; Hanika, Johannes; Dachsbacher, Carsten; Elmar Eisemann and Eugene FiumeRendering with accurate camera models greatly increases realism and improves the match of synthetic imagery to real-life footage. Photographic lenses can be simulated by ray tracing, but the performance depends on the complexity of the lens system, and some operations required for modern algorithms, such as deterministic connections, can be difficult to achieve. We generalise the approach of polynomial optics, i.e. expressing the light field transformation from the sensor to the outer pupil using a polynomial, to work with extreme wide angle (fisheye) lenses and aspherical elements. We also show how sparse polynomials can be constructed from the large space of high-degree terms (we tested up to degree 15). We achieve this using a variant of orthogonal matching pursuit instead of a Taylor series when computing the polynomials. We show two applications: photorealistic rendering using Monte Carlo methods, where we introduce a new aperture sampling technique that is suitable for light tracing, and an interactive preview method suitable for rendering with deep images.Item Improving the Dwivedi Sampling Scheme(The Eurographics Association and John Wiley & Sons Ltd., 2016) Meng, Johannes; Hanika, Johannes; Dachsbacher, Carsten; Elmar Eisemann and Eugene FiumeDespite recent advances in Monte Carlo rendering techniques, dense, high-albedo participating media such as wax or skin still remain a difficult problem. In such media, random walks tend to become very long, but may still lead to a large contribution to the image. The Dwivedi sampling scheme, which is based on zero variance random walks, biases the sampling probability distributions to exit the medium as quickly as possible. This can reduce variance considerably under the assumption of a locally homogeneous medium with constant phase function. Prior work uses the normal at the Point of Entry as the bias direction. We demonstrate that this technique can fail in common scenarios such as thin geometry with a strong backlight. We propose two new biasing strategies, Closest Point and Incident Illumination biasing, and show that these techniques can speed up convergence by up to an order of magnitude. Additionally, we propose a heuristic approach for combining biased and classical sampling techniques using Multiple Importance Sampling.Item Polynomial Optics: A Construction Kit for Efficient Ray-Tracing of Lens Systems(The Eurographics Association and Blackwell Publishing Ltd., 2012) Hullin, Matthias B.; Hanika, Johannes; Heidrich, Wolfgang; Fredo Durand and Diego GutierrezSimulation of light transport through lens systems plays an important role in graphics. While basic imaging properties can be conveniently derived from linear models (like ABCD matrices), these approximations fail to describe nonlinear effects and aberrations that arise in real optics. Such effects can be computed by proper ray tracing, for which, however, finding suitable sampling and filtering strategies is often not a trivial task. Inspired by aberration theory, which describes the deviation from the linear ray transfer in terms of wavefront distortions, we propose a ray-space formulation for nonlinear effects. In particular, we approximate the analytical solution to the ray tracing problem by means of a Taylor expansion in the ray parameters. This representation enables a construction-kit approach to complex optical systems in the spirit of matrix optics. It is also very simple to evaluate, which allows for efficient execution on CPU and GPU alike, including the computation of mixed derivatives of any order. We evaluate fidelity and performance of our polynomial model, and show applications in high-quality offline rendering and at interactive frame rates.Item A Study of Observer Metamerism for Reflectance-induced Stimuli(The Eurographics Association, 2024) Fascione, Luca; Hanika, Johannes; Hardeberg, Jon Yngve; Rushmeier, HollyCameras make images collecting per-pixel measurements of light reflected by the objects in the world. Commonly, these measurements undergo a transformation so that they become values in a standardized color space, such as the sRGB space. This makes it possible to send the values to a display device and produce in a human a visual sensation as close as possible to what would have been caused by the original scene. In this work we aim to explore the difficulties and opportunities that arise in devising such non-bijective transformations, visualizing differences between device vision and human vision. In particular we are interested in the practical impact of observer metamerism: different camera devices and human observers can distinguish a different set of spectral stimuli presented to them. When characterizing a camera, this is usually ignored, missing potential to increase chromatic acuity where the camera can see more than the human observer. A question that arises is whether the metameric stimuli involved here do actually appear in practice in relevant cases. We run numeric experiments to investigate these questions.Item A Low-Dimensional Function Space for Efficient Spectral Upsampling(The Eurographics Association and John Wiley & Sons Ltd., 2019) Jakob, Wenzel; Hanika, Johannes; Alliez, Pierre and Pellacini, FabioWe present a versatile technique to convert textures with tristimulus colors into the spectral domain, allowing such content to be used in modern rendering systems. Our method is based on the observation that suitable reflectance spectra can be represented using a low-dimensional parametric model that is intrinsically smooth and energy-conserving, which leads to significant simplifications compared to prior work. The resulting spectral textures are compact and efficient: storage requirements are identical to standard RGB textures, and as few as six floating point instructions are required to evaluate them at any wavelength. Our model is the first spectral upsampling method to achieve zero error on the full sRGB gamut. The technique also supports large-gamut color spaces, and can be vectorized effectively for use in rendering systems that handle many wavelengths at once.Item Edge-Optimized À-Trous Wavelets for Local Contrast Enhancement with Robust Denoising(The Eurographics Association and Blackwell Publishing Ltd., 2011) Hanika, Johannes; Dammertz, Holger; Lensch, Hendrik; Bing-Yu Chen, Jan Kautz, Tong-Yee Lee, and Ming C. LinIn this paper we extend the edge-avoiding à-trous wavelet transform for local contrast enhancement while avoiding common artifacts such as halos and gradient reversals. We show that this algorithm is a highly efficient and robust tool for image manipulation based on multi-scale decompositions. It can achieve comparable results to previous high-quality methods while being orders of magnitude faster and simpler to implement. Our method is much more robust than previously known fast methods by avoiding aliasing and ringing which is achieved by introducing a data-adaptive edge weight. Operating on multi-scale, our algorithm can directly include the BayesShrink method for denoising. For moderate noise levels our edge-optimized technique consistently improves separation of signal and noise.