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[Display omitted] • This special section includes papers on computer graphics education. • These papers are the best papers from the Eurographics 2022 Education Papers Program. • Specific topics include ray-tracing, the Vulkan API, and virtual reality. [ABSTRACT FROM AUTHOR]

Abstract: The popularity of augmented reality (AR) applications on mobile devices is increasing, but there is as yet little research on their use in real-settings. We review data from two pioneering field trials where MapLens, a magic lens that augments paper-based city maps, was used in small-group collaborative tasks. The first study compared MapLens to a digital version akin to Google Maps, the second looked at using one shared mobile device vs. using multiple devices. The studies find place-making and use of artefacts to communicate and establish common ground as predominant modes of interaction in AR-mediated collaboration with users working on tasks together despite not needing to. [Copyright &y& Elsevier]

New technology such as virtual reality can help computer graphics education, for example, by providing the opportunity to illustrate challenging 3D procedures. RePiX VR is a virtual reality tool for computer graphics education that focuses on teaching the core ideas of the rendering pipeline. This paper describes the development and two initial evaluations, which aimed to strengthen the usability, review requirements for different stakeholders, and build infrastructure for learning analytics and research. The integration of learning analytics raises the question of appropriate indicators to be approached through exploratory data analysis. In addition to learning analytics, the evaluation includes quantitative techniques to get insights about usability, and didactical feedback. This paper discusses advanced aspects of learning in VR and looks specifically at movement behavior. According to the evaluations, even learners without prior experience can utilize the VR tool to pick up the fundamentals of computer graphics. [Display omitted] • Evaluated educational VR environment for teaching Computer Graphics. • Teaching Computer Graphics in Virtual Reality is promising. • Learners have various movement and teleportation pattern and different interaction behavior. • A comparison of Desktop and VR users shows differences between groups, as well as a comparison of novices and experts. • The evaluation contains Multimodal Learning Analytics, Quantitative Feedback, and Usability aspects. [ABSTRACT FROM AUTHOR]

3D meshes are commonly utilized and may be considered to be the most popular surface representation in computer graphics due to their simplicity, efficiency and flexibility. However, the explicit storage of mesh vertices and connectivity, as in widely-used PLY and OBJ file formats, leads to substantial memory consumption. This, in turn, directly affects the processing and transmission in downstream applications. Though mesh simplification and mesh compression are common strategies to lessen memory consumption, they exhibit inherent limitations either in maintaining a balance between accuracy, efficiency, memory usage and mesh quality, or breaking the simplicity of explicit storage and struggling with optimizing the trade-off between compression performance and computational resource consumption. To overcome these limitations, inspired by the Gaussian Mixture Model (GMM), this paper proposes a GMM vertex-optimization-based implicitly-connected quadrilateral format for 3D mesh storage, named GMM-ICQ. Extensive qualitative and quantitative evaluations demonstrate that the GMM-ICQ format achieves efficient compression by retaining only a small amount of vertex information, while preserving sharp features and maintaining relatively high mesh quality. It also exhibits a certain degree of robustness in the presence of noise interference. Furthermore, benefiting from the inherent grid-based connectivity, the GMM-ICQ format maintains the simplicity of explicit storage and can be implemented as a progressive variant without incurring additional computational overhead. [Display omitted] • We present a GMM vertex-optimization-based implicitly-connected quadrilateral format for 3D mesh storage. • Simultaneously balances accuracy, efficiency, memory usage, and mesh quality. • Preserves the simplicity of explicit storage (such as PLY and OBJ). • No additional computational overhead needed for progressive variant implementation. [ABSTRACT FROM AUTHOR]

Search and retrieval remains a major research topic in several domains, including computer graphics, computer vision, engineering design, etc. A search engine requires primarily an input search query and a database of items to search from. In engineering, which is the primary context of this paper, the database consists of 3D CAD models, such as washers, pistons, connecting rods, etc. A query from a user is typically in the form of a sketch, which attempts to capture the details of a 3D model. However, sketches have certain typical defects such as gaps, over-drawn portions (multi-strokes), etc. Since the retrieved results are only as good as the input query, sketches need cleaning-up and enhancement for better retrieval results. In this paper, a deep learning approach is proposed to improve or clean the query sketches. Initially, sketches from various categories are analysed in order to understand the many possible defects that may occur. A dataset of cleaned-up or enhanced query sketches is then created based on an understanding of these defects. Consequently, an end-to-end training of a deep neural network is carried out in order to provide a mapping between the defective and the clean sketches. This network takes the defective query sketch as the input and generates a clean or an enhanced query sketch. Qualitative and quantitative comparisons of the proposed approach with other state-of-the-art techniques show that the proposed approach is effective. The results of the search engine are reported using both the defective and enhanced query sketches, and it is shown that using the enhanced query sketches from the developed approach yields improved search results. [Display omitted] • The first learning-based strategy to clean rough query sketches of 3D CAD models • Introduces SketchCleanNet — an end-to-end image translation scheme • SketchCleanNet aims to understand the mapping between rough sketches and clean query images • A novel scheme to calculate the loss is introduced • Dataset Contribution: The resulting enhanced query sketch dataset is made available publicly. • This paper will significantly contribute to the research community and give researchers opportunities to develop new algorithms for search and retrieval of 3D mechanical components. [ABSTRACT FROM AUTHOR]

Raw point clouds captured by sensing devices are often contaminated with noise, which perturbs the fidelity of the original geometric information. Point cloud denoising is therefore an inseparable post-processing step, aiming to remove the noise in the point clouds. Existing point cloud denoising approaches are typically trained on datasets that have uniform point distributions and densities, making them unsuitable for effectively denoising point clouds with severe noise or irregular point distributions. In this paper, we introduce a novel random screening-based feature aggregation method for point cloud denoising. Our key insight is that merging features of dense and sparse points assists with enhancing the quality of point cloud denoising results. In specific, our approach involves randomly screening the features of local point patches and fusing richer geometric information of denser points into sparser point representations. Comprehensive experiments demonstrate that our method achieves state-of-the-art performance in the point cloud denoising task on both synthetic and real-world datasets. [Display omitted] • Random screening-based point feature aggregation is useful for point cloud denoising. • Merging features of dense points into sparser points boosts point cloud denoising. • Our pipeline demonstrates robustness to noise and saves processing time. • Our technique achieves impressive results on severe noise and irregular points. [ABSTRACT FROM AUTHOR]

Reconstructing the shape of hand-held objects from single-view color images is a long-standing problem in computer vision and computer graphics. The task is complicated by the ill-posed nature of single-view reconstruction, as well as potential occlusions due to both the hand and the object. Previous works mostly handled the problem by utilizing known object templates as priors to reduce the complexity. In contrast, our paper proposes a novel approach without knowing the object templates beforehand but by exploiting prior knowledge of contacts in hand-object interactions to train an attention-based network that can perform precise hand-held object reconstructions with only a single forward pass in inference. The network we propose encodes visual features together with contact features using a multi-head attention module as a way to condition the training of a neural field representation. This neural field representation outputs a Signed Distance Field representing the reconstructed object and extensive experiments on three well-known datasets demonstrate that our method achieves superior reconstruction results even under severe occlusion compared to the state-of-the-art techniques. • Conditioning on contact, we reconstruct the hand-held object from single-view images. • We use an end-to-end attention-based network to better encode contact priors. • We achieve state-of-the-art results on Obman, HO3D, and MOW datasets. [ABSTRACT FROM AUTHOR]

Weathering phenomena are ubiquitous in urban environments, where it is easy to observe severely degraded old buildings as a result of water penetration. Despite being an important part of any realistic city, this kind of phenomenon has received little attention from the Computer Graphics community compared to stains resulting from biological or flow effects on the building exteriors. In this paper, we present physically-inspired deep weathering effects, where the penetration of humidity (i.e., water particles) and its interaction with a building's internal structural elements result in large, visible degradation effects. Our implementation is based on a particle-based propagation model for humidity propagation, coupled with a spring-based interaction simulation that allows chemical interactions, like the formation of rust, to deform and destroy a building's inner structure. To illustrate our methodology, we show a collection of deep degradation effects applied to urban models involving the creation of rust or of ice within walls. [Display omitted] • Physically-inspired simulation of the penetration of water particles in walls, dealing with multiple types of materials. • Approximate simulation of the interactions of water with the inner structures of the wall. • Expansion and deformation, such as water into ice, rusting, or plaster deformation. • Simulation of the formation of cracks inside the wall, leading to broken concrete, loose plaster, etc. • Cracks can accelerate previous steps if increased exposure (e.g., rain). [ABSTRACT FROM AUTHOR]

Developable surfaces are widely used in Computer-Aided Design (CAD), computer graphics, architecture geometry and manufacturing. In this paper, we present an approach to segmentation and approximation of an input triangular mesh with developable patches. First, exact developable regions are detected and extracted from the input mesh. Then the non-developable region is further segmented and approximated also by developable patches. The parametric equations for all obtained developable patches are simultaneously computed, whose iso-parameter curves can serve directly as the milling paths in CNC machining. Examples and comparisons with existing works are provided. [Display omitted] • A novel method for segmenting and approximating an input mesh with developable patches is proposed. • The exact parametric equations for all segment patches are computed. • A new linearity and smoothness measures to detect curve-like features on Gauss Images is provided. [ABSTRACT FROM AUTHOR]

• An integrated visualization of scholarly data can support the academic reviewer search process. • The visualization of scholarly data helps to avoid conflicts of interest and to build a fairly distributed pool of reviewers. • A well-combined visualization of citations and co-authorship relations only, can reduce the need for complicated content analysis techniques. • The platform evaluation with members from the Computer Graphics community demonstrated the improvement on the traditional process for searching reviewers. • The evaluation confirmed that users were able to get acquainted with the system with a very limited training. We propose ReviewerNet, an online, interactive visualization system aimed to improve the reviewer selection process in the academic domain. Given a paper submitted for publication, we assume that good candidate reviewers can be chosen among the authors of a small set of pertinent papers; ReviewerNet supports the construction of such set of papers, by visualizing and exploring a literature citation network. The system helps journal editors and Program Committee members to select reviewers that do not have any conflict-of-interest and are representative of different research groups, by visualising the careers and co-authorship relations of candidate reviewers. The system is publicly available, and is demonstrated in the field of Computer Graphics. [ABSTRACT FROM AUTHOR]

• We present a novel particle resampling method for surface reconstruction of a liquid. • The first adaptive sampling tailored to level-sets defined by the boundary particles. • Our adaptive particle resampling preserves small and thin features of a liquid. • A new quality metric to evaluate the effectiveness of the resampling methods. In this paper, we present a novel adaptive particle resampling method tailored for surface reconstruction of level-sets defined by the boundary particles from a particle-based liquid simulation. The proposed approach is simple and easy to implement, and only requires the positions of the particles to identify and refine regions with small and thin fluid features accurately. The method comprises four main stages: boundary detection, feature classification, particle refinement, and surface reconstruction. For each simulation frame, firstly the free-surface particles are captured through a boundary detection method. Then, the boundary particles are classified and labeled according to the deformation and the stretching of the free-surface computed from the Principal Component Analysis (PCA) of the particle positions. The particles placed at feature regions are then refined according to their feature classification. Finally, we extract the free-surface of the zero level-set defined by the resampled boundary particles and its normals. In order to render the free-surface, we demonstrate how the traditional methods of surface fitting in Computer Graphics and Computational Physics literature can benefit from the proposed resampling method. Furthermore, the results shown in the paper attest the effectiveness and robustness of our method when compared to state-of-the-art adaptive particle resampling techniques. [ABSTRACT FROM AUTHOR]

Time-varying connectivity of triangle mesh sequences leads to substantial difficulties in their processing. Unlike editing sequences with constant connectivity, editing sequences with varying connectivity requires addressing the problem of temporal correspondence between the frames of the sequence. We present a method for time-consistent editing of triangle mesh sequences with varying connectivity using sparse temporal correspondence, which can be obtained using existing methods. Our method includes a deformation model based on the usage of the sparse temporal correspondence, which is suitable for the temporal propagation of user-specified deformations of the edited surface with respect to the shape and true topology of the surface while preserving the individual connectivity of each frame. Since there is no other method capable of comparable types of editing on time-varying meshes, we compare our method and the proposed deformation model with a baseline approach and demonstrate the benefits of our framework. [Display omitted] • This paper presents a method for editing mesh sequences with varying connectivity. • The proposed method exploits sparse temporal correspondences. • Dual quaternions are used to represent the transformations. [ABSTRACT FROM AUTHOR]

This paper tackles the volumetric representation of geophysical and geotechnical data, gathered during exploration surveys of the subsoil; in particular, we focus on the modeling and analysis of underwater deposits. The creation of a 3D model as support to geological interpretation has to take into account the heterogeneity of the input data, coming from offshore acquisition campaigns. Some data are massive, but cover the domain unevenly, e.g., along dense differently spaced lines, while others are very sparse, e.g., borehole locations with soil sampling and CPTU (Piezocone Penetration Test) locations. A automatic process is presented to generate the subsurfaces and volume defining a sub-seabed deposit, starting from the identification of relevant morphological features in seismic data. In particular, simplification and refinement based on geostatistics have been applied to generate regular 2D meshes from strongly anisotropic data, in order to improve the quality of the final 3D tetrahedral mesh. Furthermore, we also use geostatistics to predict geotechnical parameters from local surveys and estimate their distribution on the whole domain: in this way the 3D model will include relevant geological features of the deposit and allow extrapolating different geotechnical information with associated uncertainty. The volume characterization and its 3D inspection will support geological analysis and planning of future engineering activities. The developed methodology has been tested on two real case studies. • We implemented a complete pipeline for the 3D modeling of stratified deposits • Our research action follows the surface-based approach • We combine computer graphics methodologies with geophysics and geostatistics [ABSTRACT FROM AUTHOR]

Highlights • The Lagrangian-Eulerian method can handle violent liquid and spray with particle system and density field respectively. • Four different participating media are actively involved to enhance details and enrich rendering. • Vivid interaction and state transition among different media and tight coupling between particle and grid are guaranteed. Graphical abstract Abstract This paper proposes a hybrid Lagrangian–Eulerian method involving all participating media for high-speed fluid simulation and its accompanying phenomena. We take advantages of both particle- and grid-based solvers in our new approach by dividing all the physical states into two different parts based on their individual characteristics. Lagrangian-based solver gives rise to liquid, droplet, and foam creation and facilitates the drastic shape distortion and state transition during the multi-physical processes simulation from the perspective of individual particles. At the same time, Eulerian-based solver mainly focuses on the animation of spray, which enables a fog-like atmosphere. Essential to all of the aforementioned functionalities is the state transition among different media considering both physical and geometric features in this paper. The state transition enables to streamline the dis-association and generation of certain physical states. Moreover, to ensure the synchronization of hybrid models, we establish essential interaction and build tight coupling between the particle- and grid-based subsystems. With the comprehensive experimental results, we have manifested that our new modeling approach is both effective and highly modularized, and is capable of producing convincing visual effects for a wider range of graphics and animation tasks. [ABSTRACT FROM AUTHOR]

Digital images are everywhere, from social media to news and scientific papers. This paper describes an extensive user study to evaluate the ability of an average individual to spot edited images. By design, our study avoids lucky guesses. After observing an image, subjects were asked if it is authentic or not. Whenever a subject indicated that an image has been altered, (s)he had to provide evidence to support the answer by pointing at the suspected region in the image. We collected 17,208 individual answers from 393 volunteers, using 177 images selected from public forensic databases. Our results indicate that the average individual is not good at distinguishing original from edited images, answering correctly on 58% of all images, and only identifying the modified ones 46.5% of the time. This performance is superior to random guessing, but poor compared to results achieved by computational techniques. [ABSTRACT FROM AUTHOR]

Implicit curve and surface reconstruction has wide applications in computer graphics, which attracts great attention of many researchers. In this paper, we propose the relaxed implicit randomized algebraic reconstruction technique (RIR-ART) for curve and surface reconstruction. The RIR-ART method generates a sequence of curves and surfaces by adjusting the control coefficients with the relaxation parameters iteratively. It is proved that the sequence of curves and surfaces generated by the RIR-ART method with the suitable relaxation parameters converge to the least-norm results in terms of the mean square error. The RIR-ART method can eliminate the extra zero-level sets effectively during its iterative process, and converge faster than the implicit progressive-iterative approximation method (Hamza et al. 2020). Numerical examples present the efficiency and effectiveness of the proposed method. • The RIR-ART method converges in terms of the mean square error. • The extra zero-level sets are eliminated by the RIR-ART method effectively. • The RIR-ART method converges faster than the I-PIA method for the reconstruction. [ABSTRACT FROM AUTHOR]

Point set registration has been actively studied in computer vision and graphics. Optimization algorithms are at the core of solving registration problems. Traditional optimization approaches are mainly based on the gradient of objective functions. The derivation of objective functions makes it challenging to find optimal solutions for complex optimization models, especially for those applications where accuracy is critical. Learning-based optimization is a novel approach to address this problem, which learns the gradient direction from datasets. However, many learning-based optimization algorithms learn gradient directions via a single feature extracted from the dataset, which will cause the updating direction to be vulnerable to perturbations around the data, thus falling into a bad stationary point. This paper proposes the General Discriminative Optimization (GDO) method that updates a gradient path automatically through the trade-off among contributions of different features on updating gradients. We illustrate the benefits of GDO with tasks of 3D point set registrations and show that GDO outperforms the state-of-the-art registration methods in terms of accuracy and robustness to perturbations. [Display omitted] • Cast the point sets registration as a learning-based optimization problem. • Utilize features of point sets to update gradients without the Hessian matrix. • Collaborate the different features to reduce the influence of perturbations. • Outperform the other advanced registration methods on accuracy and robustness. [ABSTRACT FROM AUTHOR]

Acceleration techniques for Rendering in general, and Ray-Casting in particular, have been the subject of much research in Computer Graphics. Most efforts have been focused on new data structures for efficient ray/scene traversal and intersection. In this paper, we propose an acceleration technique that approximates rendering and that is built around a new feature-based clustering approach. The technique starts preprocessing the scene by grouping elements according to their features using a set of channels based on an information theory-based approach. Then, at run-time, a rendering strategy uses that clustering information to reconstruct the final image, by deciding which areas could take advantage of the coherence in the features and thus, could be interpolated; and which areas require more involved calculations. This process starts with a low-resolution render that is iteratively refined up to the desired resolution by reusing previously computed pixels. Our experimental results show a significant speedup of an order of magnitude, depending on the complexity of the per-pixel calculations, the screen size of the objects, and the number of clusters. Rendering quality and speed directly depend on the number of clusters and the number of steps performed during the reconstruction procedure, and both can easily be set by the user. Our findings show that feature-based clustering can significantly impact rendering speed if samples are chosen to enable interpolation of smooth regions. Our technique, thus, accelerates a range of popular and costly techniques, ranging from texture mapping up to complex ambient occlusion, soft and hard shadow calculations, and it can even be used in conjunction with more traditional acceleration methods. [Display omitted] • Mesh-clustering based Information Theory tools to exploit feature similarity. • User-definable parameters, decoupling them from the actual rendering process. • Multi-pass rendering based on reuse and interpolation of previous results. • Controlling mechanism to guarantee Ray-casting as a lower bound to rendering speed. • Our technique can accommodate both static and animated scenes as well. [ABSTRACT FROM AUTHOR]

The representation of consistent mixed reality (XR) environments requires adequate real and virtual illumination composition in real-time. Estimating the lighting of a real scenario is still a challenge. Due to the ill-posed nature of the problem, classical inverse-rendering techniques tackle the problem for simple lighting setups. However, those assumptions do not satisfy the current state-of-art in computer graphics and XR applications. While many recent works solve the problem using machine learning techniques to estimate the environment light and scene's materials, most of them are limited to geometry or previous knowledge. This paper presents a CNN-based model to estimate complex lighting for mixed reality environments with no previous information about the scene. We model the environment illumination using a set of spherical harmonics (SH) environment lighting, capable of efficiently represent area lighting. We propose a new CNN architecture that inputs an RGB image and recognizes, in real-time, the environment lighting. Unlike previous CNN-based lighting estimation methods, we propose using a highly optimized deep neural network architecture, with a reduced number of parameters, that can learn high complex lighting scenarios from real-world high-dynamic-range (HDR) environment images. We show in the experiments that the CNN architecture can predict the environment lighting with an average mean squared error (MSE) of 7.85 × 10−4 when comparing SH lighting coefficients. We validate our model in a variety of mixed reality scenarios. Furthermore, we present qualitative results comparing relights of real-world scenes. [Display omitted] • Automatic end-to-end method to estimate the environment lighting in XR applications. • A CNN architecture that learns a latent-space of the environment lighting. • A methodology to generate egocentric mixed-reality-views from HDR panoramas. • Real-time lighting estimation that does not make assumptions about the XR scene. [ABSTRACT FROM AUTHOR]

[Display omitted] Motion Blur is an important effect of photo-realistic rendering. Distribution ray tracing can simulate motion blur very well by integrating light, both over the spatial and the temporal domain. However, increasing the problem by the temporal dimension entails many challenges, particularly in cinematic multi-bounce path tracing of complex scenes, where heavy-weight geometry with complex lighting and even offscreen elements contribute to the final image. In this paper, we propose the Motion DAG (Directed Acyclic Graph), a novel data structure that filters an entire animation sequence of an object, both in the spatial and temporal domain. These Motion DAGs interleave a temporal interval binary tree for filtering time consecutive data and a sparse voxel octree (SVO), which simplifies spatially nearby data. Motion DAGs are generated in a pre-process and can be easily integrated in a conventional physically based path tracer. Our technique is designed to target motion blur of small objects, where coarse representations are sufficient. Specifically, in this scenario our results show that it is possible to significantly reduce both, memory consumption and render time. [ABSTRACT FROM AUTHOR]

Empty-region graphs are well-studied in Computer Graphics, Geometric Modeling, Computational Geometry, as well as in Robotics and Computer Vision. The vertices of these graphs are points in space, and two vertices are connected by an arc if there exists an empty region of a certain shape and size between them. In most of the graphs discussed in the literature, the empty region is assumed to be a circle or the union/intersection of circles. In this paper we propose a new type of empty-region graphs—the γ -visibility graph. This graph can accommodate a variety of shapes of empty regions and may be defined in any dimension. Interestingly, we will show that commonly-used shapes are a special case of our graph. In this sense, our graph generalizes some empty-region graphs. Though this paper is mostly theoretical, it may have practical implication—the numerous applications that make use of empty-region graphs would be able to select the best shape that suits the problem at hand. [ABSTRACT FROM AUTHOR]

• A sufficient and necessary condition for the injectivity of toric volumes is proposed. • The sign of mixed product of three vectors is used to check the compatibility between the sets of lattice points and control points. • An algorithm is proposed to check the injectivity of toric volumes for arbitrary positive weights. • The algorithm is improved based on the property of clean and empty tetrahedron. [Display omitted] Parameterizations, which map parametric domains into certain domains, are widely used in computer aided design, computer aided geometric design, computer graphics, isogeometric analysis, and related fields. The parameterizations of curves, surfaces, and volumes are injective means that they do not have self-intersections. A 3D toric volume is defined via a set of 3D control points with weights that correspond to a set of finite 3D lattice points. Rational tensor product or tetrahedral Bézier volumes are special cases of toric volumes. In this paper, we proved that a toric volume is injective for any positive weights if and only if the lattice points set and control points set are compatible. An algorithm is also presented for checking the compatibility of the two sets by the mixed product of three vectors. Some examples illustrate the effectiveness of the proposed method. Moreover, we improve the algorithm based on the properties and results of clean and empty tetrahedrons in combinatorics. [ABSTRACT FROM AUTHOR]

• Computer graphics. • Computer vision. • Deep learning. • 3D reconstruction. • Virtual human digitization. • Generative adversarial network. [Display omitted] With the development of AR/VR technologies, a reliable and straightforward way to digitize a three-dimensional human body is in high demand. Most existing methods use complex equipment and sophisticated algorithms, but this is impractical for everyday users. In this paper, we propose a pipeline that reconstructs a 3D human shape avatar from a single image. Our approach simultaneously reconstructs the three-dimensional human geometry and whole body texture map with only a single RGB image as input. We first segment the human body parts from the image and then obtain an initial body geometry by fitting the segment to a parametric model. Next, we warp the initial geometry to the final shape by utilizing a silhouette-based dense correspondence. Finally, to infer invisible back texture from a frontal image, we propose a network called InferGAN. Based on human semantic information, we also propose a method to handle partial occlusion by reconstructing the occluded body parts separately. Comprehensive experiments demonstrate that our solution is robust and effective on both public and our own datasets. Our human avatars can be easily rigged and animated using MoCap data. We have developed a mobile application that demonstrates this capability for AR applications. [ABSTRACT FROM AUTHOR]

The creation of a digital representation of the Earth and its associated data is a complex and difficult task. The incredible size of geospatial data and differences between data sets pose challenges related to big data, data creation, and data integration. Advances in globe representation and visualization have made use of Discrete Global Grid Systems (DGGSs) that discretize the globe into a set of cells to which data are assigned. DGGSs are well studied and important in the GIS, OGC, and Digital Earth communities but have not been well-introduced to the computer graphics community. In this paper, we provide an overview of DGGSs and their use in digitally representing the Earth, describe several current Digital Earth systems and their methods of Earth representation, and list a number of applications of Digital Earths with related works. Moreover, we discuss the key research areas and related papers from computer graphics that are useful for a Digital Earth system, such as advanced techniques for geospatial data creation and representation. [ABSTRACT FROM AUTHOR]

The paper reports on the 40 years of development of Computer Graphics and, more recently, Visual computing (VC) at the Technische Universität Darmstadt in Germany, from its beginning in 1975 to the leading “Computer Graphics and Visual Computing Hub” in Europe as of 2014. This development is described along three axes. First, the institutional development and its rational to establish Computer Graphics as a discipline of Computer Science and as an enabling technology for developing our Knowledge Society, are described. Second, the scientific and technological impact based on the teaching activities and the large number of theses submitted in Darmstadt for the area during these 40 years are addressed. Finally, the research road maps of the Computer Graphics and Visual Computing Hub in Darmstadt are presented relatively to the different stages of CG and VC research, relatively to a scientific view to the large number of projects implemented over these 40 years and, finally, also relatively to the project results as seen from the media. In order to manage the quantity as well as the complexity of the information available, the description of these road maps is divided in four time periods: 1975–1984, 1985–1994, 1995–2004 and 2004–2015. The paper also gives the view of the authors on how they see the future of Computer Graphics and Visual Computing. At the end, the paper includes an extensive list of references for the reported content. [ABSTRACT FROM AUTHOR]

• Propose an immersive method of sketch-based tree modeling in virtual reality. • Present a series of algorithms to deal with the problems of generating branch geometry for real-time 3D sketching. • Provide a bidirectional ray-hit algorithm to determine the branch radius in real-time. • Present a twigs generation algorithm to randomly generate twigs based on Perlin noise and parent branch direction. Tree modeling has been a widely-discussed topic in computer graphics. However, with existing methods, the modeling process is occupied with complex data collection and tedious parameter adjustment, lacking a rich sensory modeling experience. In this paper, we propose an approach to sketch-based tree modeling in an immersive virtual reality environment, aiming to lower the difficulty of modeling and enhance the immersion in the designing process. We first present a sketch sampling and points optimization algorithm to obtain the skeleton of the branch in 3D space. As generating geometry along the skeleton, we apply a vector-projection method to fix the branch polygon twisting. Then, we introduce a bidirectional ray-hit algorithm to determine the branch radius as real-time sketching. To generate random twigs on branches, we introduce a twigs generation algorithm based on Perlin noise and the parent branch direction. Finally, we design a series of interactive methods for users to create tree models in a 3D virtual scene with the VR HMD and controller. Experimental results indicate that our approach can accomplish creating realistic tree models in real-time. The interactive and immersive modeling experience enables users to readily convey their ideas on tree structures in a simple and direct way of sketching. Visualizing and modifying the real-time generated branch results can contribute to provoking the inspiration for creation. [ABSTRACT FROM AUTHOR]

• Introduction of Riemannian global illumination. • A path tracer algorithm to approximate the solution of the Riemannian global illumination. • A map of the algorithm to the GPU, using the RTX platform. • We apply the Non-Euclidean path tracer to render "Photorealistic" inside views of the 3D flat torus, Poincaré sphere, and the hyperbolic mirrored dodecahedron. These are examples of the Euclidean, spherical, and hyperbolic spaces: the Thurston classical geometries. This paper presents a novel path tracer algorithm for immersive visualization of Riemannian manifolds. To do this, we introduce Riemannian illumination, a generalization of classical Computer Graphics illumination models. In this context, global light transport is expressed by extending the rendering equation to Riemannian manifolds. Using Monte Carlo integration to solve this equation results in the novel path tracer for Non-Euclidean spaces. We discuss its basic principles, as well as the general CPU algorithm. Additionally, we discuss in detail how to implement a GPU version, using the RTX pipeline. Finally, we apply the algorithm to render "photorealistic" inside views of the flat torus, Poincaré sphere, and the hyperbolic mirrored dodecahedron. These are examples of Euclidean, spherical, and hyperbolic spaces: the Thurston classical geometries. [ABSTRACT FROM AUTHOR]

• Appearance and essence-preserving simplification of real cities. • Accurate non-uniform layout shrinking of real cities. • Removal of unimportant areas from city layouts. One important use of realistic city environments is in the video game industry. When a company works on a game whose action occurs in a real-world environment, a team of designers usually creates a simplified model of the real city. In particular, the resulting city is desired to be smaller in extent to increase playability and fun, avoiding long walks and "boring" neighborhoods. This is manual work, usually started from scratch, where the first step is to take the original city map as input, and from it create the street network of the final city, removing insignificant streets and bringing important places closer together in the process. This first draft of the city street network is like a kind of skeleton with the most important places connected, from which the artist can (and should) start working until the desired result is obtained. In this paper, we propose a solution to automatically generate such a first simplified street network draft. This is achieved by using the well-established seam-carving technique applied to a skeleton of the city layout, built with the important landmarks and streets of the city. The output that our process provides is a street network that reduces the city area as much as the designer wants, preserving landmarks and key streets, while keeping the relative positions between them. For this, we run a shrinking process that reduces the area in an irregular way, prioritizing the removal of areas of less importance. This way, we achieve a smaller city but retain the essence of the real-world one. To further help the designer, we also present an automatic filling algorithm that adds unimportant streets to the shrunken skeleton. [ABSTRACT FROM AUTHOR]

• Real-time artist friendly procedural texturing method to model high quality surface details. • An anisotropic filtering scheme to achieve high quality results during rendering. • An analytic normal map maintaining user control and preventing aliasing. • Noise applications to stylized rendering and non-stationary procedural texturing. To deal with the increasing demand for complex visual details in virtual worlds, procedural methods for content authoring are an expanding field in Computer Graphics. Focusing on-the-fly texture generation, we present in this paper a content authoring process based on Locally Controlled Spot Noise. Through the control of both the impulses distribution and the spatially-defined kernel, this process can cover a wide range of appearances. In this context, we introduce a new kernel formulation that provides an efficient anisotropic filtering of the generated texture. Furthermore, our method allows users to interactively create the desired appearance by controlling both albedo and meso-geometry of the underlying surface, tackling on-the-fly normal map generation. Our method can be used as an artist friendly tool to model high-quality surface details with direct control over the final appearance in real-time. [ABSTRACT FROM AUTHOR]

Abstract: Morphological shape design is interpreted in this paper as a search for new shapes from a particular application domain represented by a set of selected shape instances. This paper proposes a new foundation for morphological shape design and generation. In contrast to existing generative procedures, an approach based on a user-controlled metamorphosis between functionally based shape models is presented. A formulation of the pairwise metamorphosis is proposed with a variety of functions described for the stages of deformation, morphing and offsetting. This formulation is then extended to the metamorphosis between groups of shapes with user-defined, dynamically correlated and weighted feature elements. A practical system was implemented in the form of plugin to Maya and tested by an industrial designer on a group of representative shapes from a particular domain. [Copyright &y& Elsevier]

Abstract: Afterimages comprise a common, recurring perceptual phenomenon experienced on a daily basis. Afterimages are best realized when staring at some high-intensity light source (i.e., a light bulb) and then changing the fixation to other less luminous portions of the scene: a temporary “ghost” image of that strong intensity remains noticeable for a period of time. During the time the afterimage stays active, several color gradations appear and fade, with little resemblance to the color that originally stimulated them. Although research on the topic has been moderately active in the ophthalmology and vision domains, no definitive model has been devised. In computer graphics, research on afterimages remains young. In this paper we propose a technique that addresses the duration as well as the luminance and color transitions of the effect, all inspired by psychophysical and physiological evidence. The main contribution of this paper is a model for color transitions in afterimages (also known as flight-of-colors). To our knowledge, no record exist on the subject in the computer graphics literature. The proposed method is also fast and suitable for real-time applications. Our stance towards afterimages is more than just curiosity on this peculiar effect and we believe that its understanding and proper simulation can assist on relevant tasks such as urban and road engineering for safer pedestrian and vehicle mobility at adverse lighting conditions. [Copyright &y& Elsevier]

Abstract: This paper presents a new method to control scene sampling in complex ray-based rendering environments. It proposes to constrain image sampling density with a combination of object features, which are known to be well perceived by the human visual system, and image space saliency, which captures effects that are not based on the object''s geometry. The presented method uses Non-Photorealistic Rendering techniques for the object space feature evaluation and combines the image space saliency calculations with image warping to infer quality hints from previously generated frames. In order to map different feature types to sampling densities, we also present an evaluation of the object space and image space features'' impact on the resulting image quality. In addition, we present an efficient, adaptively aligned fractal pattern that is used to reconstruct the image from sparse sampling data. Furthermore, this paper presents an algorithm which uses our method in order to guarantee a desired minimal frame rate. Our scheduling algorithm maximizes the utilization of each given time slice by rendering features in the order of visual importance values until a time constraint is reached. We demonstrate how our method can be used to boost or stabilize the rendering time in complex ray-based image generation consisting of geometric as well as volumetric data. [Copyright &y& Elsevier]

Abstract: In a recent paper (Ma and Wang, 2009), it was found that the limit curve corresponding to a regular edge path of a Loop subdivision surface reduces to a uniform cubic B-spline curve (CBSC) under a degeneration condition. One can thus define a Loop subdivision surface interpolating a set of input CBSCs with various topological structures that can be mapped to regular edge paths of the underlying surface. This paper presents a new solution for defining a Loop subdivision surface interpolating an arbitrary number of CBSCs meeting at an extraordinary point. The solution is based on the concept of a polygonal complex method previously used for Catmull–Clark surface interpolation and is built upon an extended set of constraints of the control vertices under which local edge paths meeting at an extraordinary point reduces to a set of endpoint interpolating CBSCs. As a result, the local subdivision rules near an extraordinary point can be modified such that the resulting Loop subdivision surface exactly interpolates a set of input endpoint interpolating CBSCs meeting at the extraordinary point. If the given endpoint interpolating CBSCs have a common tangent plane at the meeting point, the resulting Loop surface will be G1 continuous. The proposed method of curve interpolation provides an important alternative solution in curve-based subdivision surface design. [Copyright &y& Elsevier]

• We propose the Binary Sparse Frame a frame for the sparse synthesis and analysis of the step functions. • We exploit the sparse representation directly provided by the BSF. • We compare the proposed frame with the state of the art methods for functional space representation on manifolds. • We prove and evaluate how our spectral definition is well suited for the transfer of step functions between shapes. • We start an analysis of the parameters involved in the BSF definition. Step functions are non-smooth and piecewise constant functions with a finite number of pieces. Each of these pieces indicates a local region contained in the entire domain. Several geometry processing applications involve step functions defined on non-Euclidean domains, such as shape segmentation, partial matching and self-similarity detection. Standard signal processing cannot handle this class of functions. The classical Fourier series, for instance, does not give a good representation of these non-smooth functions. In this paper, we define a new frame for the sparse approximation and transfer of the step functions defined on manifolds. The definition of our frame is completely spectral and provides a concise representation through an efficient computation. We exploit the sparse representation that takes full advantage of the proposed frame. Furthermore, our frame is built specifically to enhance its use in combination with the functional maps, a powerful tool for transferring signals between manifolds. This functional approach makes the proposed framework stable to isometric and non-isometric deformations. A large set of experiments confirms that the proposed frame improves the sparse approximation and transfer of step functions. [ABSTRACT FROM AUTHOR]

Highlights • A grammar for procedurally generate motion graphics. • The formalization of motion graphics with timeslice grammars. • A grammar to manipulate both the temporal and spatial components of an animation. • A graphics user interface to improve the usability of the grammar. • Stochastic variations during grammar expansion controlled using high priority rules. Graphical abstract Abstact Motion graphics are a form of animation where several sets of shapes are choreographed together using a wide range of effects to produce compelling footage. We present a method to procedurally generate motion graphics. We formally model motion graphics with timeslice grammars , an extension of split and group grammars, that are able to control and manipulate both the temporal and spatial components of an animation. We are able to combine both these aspects by representing animations as sets of affine transformations sampled uniformly in both space and time. These transformations are controlled by the grammar. Shapes in our system are represented as highly tessellated polygons, and are animated by applying the affine transforms to each shape vertex given the vertex position and the animation time. We introduce a small set of operators showing how we can produce convincing animations, combining the expressive power of the grammar model, the composability of the operators with themselves, and the capabilities that derive from our shape representation. Throughout the paper, we show how timeslice grammars can, in few tens of seconds, produce a wide variety of animations that would take artists hours of tedious and time-consuming work. [ABSTRACT FROM AUTHOR]

Highlights • Our survey investigated the introductory Computer Graphics course. • 28 universities from all over the world were surveyed. • On average, Rendering together with Geometric Modeling account for 89% of content. • Animation, Fundamentals and Visualization account for the other 11%. • Online contents confirmed by 71.14% of the instructors. Graphical abstract Abstract Computer Graphics is a very active field, with new knowledge being published every day at a high rate. There is, therefore, the pressure to regularly review our teaching contents and adjust accordingly. Among the courses on a standard curriculum, the introductory Computer Graphics course is very often the door for students into the exciting area of Computer Graphics. It is also the opportunity to attract and engage the best talent for the field. In this paper, we address the question of content in the introductory Computer Graphics course as a community. Our main motivation was to find out what our peers are teaching in this first course and use this knowledge to ease the redesign of our introductory course. We have surveyed 28 introductory Computer Graphics undergraduate courses from higher level educational institutions from around the world. We have asked the instructors of these courses to send us data on their courses, such as the weekly list of topics, and others such as textbooks. We gathered and processed this data using a bottom-up approach. The final top-level list of subjects and percentages for the introductory Computer Graphics courses, following the knowledge units defined in the 2013 ACM/IEEE recommendation, is as follows: Rendering (71.3%), Geometric Modeling (17.4%), Animation (7.8%), Fundamentals (3.0%), and Visualization (0.5%). We believe this survey will be helpful for institutions considering designing a new introductory course from scratch or redesigning an existing one, by providing the current state-of-practice of top Computer Graphics institutions around the world. [ABSTRACT FROM AUTHOR]

Highlights • A set of implicit shapes representing character muscles deforming at constant volume. • The driving of the muscles with PBD, producing jiggling and handling collisions. • Their integration within the Implicit Skinning framework Together, these contributions enable controllable muscle deformations in character animations at interactive frame rates. Graphical abstract Abstract Most current methods for character skinning can be categorized into 1) geometric techniques, which are fast and easy to use but often lack physical realism, 2) data-driven approaches, which require a large set of examples that are tedious to edit, and 3) physics-based methods, which are highly realistic but slow and difficult to use. Recently introduced geometric Implicit Skinning methods can solve contact interactions and skin elasticity with results comparable to physics-based simulation in real-time. In this paper we introduce an animation method that adds anatomical plausibility while benefiting from the advantages of Implicit Skinning. We propose an efficient way to model muscle primitives with implicit surfaces. Volumetric extrusions of individual muscles are attached to muscle center lines simulated with a fast, low-dimensional physics-based approach (Position Based Dynamics of one-dimensional line segments). This combination of physics-based simulation with implicit modeling allows us to elegantly resolve muscle-muscle and muscle-bone collisions and add dynamic effects such as flesh jiggling while guaranteeing volume preservation (which is a property of real biological muscles) and producing visually plausible skin-skin contact behavior. Our method runs at interactive frame-rates and features intuitive modeling parameters which allow animators to quickly explore a variety of designs and physics-based effects. [ABSTRACT FROM AUTHOR]

Abstract: Cosine functions have been widely used for representing Bidirectional Reflection Distribution Functions (BRDF) such as Lambert, Phong and Lafortune models. They are well suited to represent both high and low frequency signals. However, they are difficult to use with visibility and incident radiance. In most systems, the rendering equation terms are thus estimated using various methods. Several interactive rendering systems rather rely on the projection of each term onto orthonormal basis functions such as spherical harmonics or wavelets. These methods are easier to handle since the integration becomes a dot product. However, these functions are also subject to several drawbacks. For instance the number of coefficients is high for the representation of high frequency phenomena; the pre-computation time required for projecting each term of the rendering equation cannot be neglected. This paper demonstrates that cosine lobes can be generalized to visibility and incoming radiance with several advantages. First, cosine lobes do not form an orthonormal basis of functions and the number of parameters remains naturally adapted to the signal. This is very interesting for complex and high frequency functions: glossy BRDF or small light sources for instance. We also use this property for reducing the number of parameters as the computation goes along. Second, Lambert, Phong and Lafortune BRDF models are already used in many rendering systems. Since they already rely on this representation, no transformation into other types of model is necessary. This paper shows how it is possible to rapidly integrate the product of cosine lobes. As a demonstration of our methodology, we propose an interactive rendering system for direct lighting, including soft shadows and spatially varying materials. [ABSTRACT FROM AUTHOR]

Abstract: The creation of realistic virtual human heads is a challenging problem in computer graphics, due to the diversity and individuality of the human being. In this paper, an easy-to-use geometry-optimized virtual human head is presented. By using a uniform two-step refinement operator, the virtual model has a natural multiresolution structure. In the coarsest level, the model is represented by a control mesh which serves as the anatomical structure of the head. To achieve fine geometric details over , a multi-level detail set is assigned to the corresponding control vertices in , with the aid of a uniquely defined local frame field. We show in this paper that with a carefully designed control mesh, the presented virtual model not only captures the physical characteristics of the head, but also is optimal in the geometric sense. The contributions of this paper also include that diverse applications of the presented virtual model are presented, showing that (1) the model deformation is smooth and natural, and (2) the presented virtual model is easy to use. [Copyright &y& Elsevier]

This paper presents a novel gradient-based image completion algorithm for removing significant objects from natural images or photographs. Our method reconstructs the region of removal in two phases. Firstly, the gradient maps in the removed area are completed through a patch-based filling algorithm. After that, the image is reconstructed from the gradient maps by solving a Poisson equation. A new patch-matching criterion is developed in our approach to govern the completion of gradient maps. Both the gradient and the color information are incorporated in this new criterion, so a better image completion result is obtained. Several examples and comparisons are given at the end of the paper to demonstrate the performance of our gradient-based image completion approach. [Copyright &y& Elsevier]

Guest Editors It is our pleasure to present this special section of Computers & Graphics (C&G), featuring the best paper presented at the 25th ACM Symposium on Virtual Reality Software and Technology (VRST 2019), held November 12-15, 2019 in Sydney, Australia. ACM VRST is a premier international forum for the exchange of experiences and knowledge among researchers and developers concerned with virtual reality software and technology. This paper, "Realistic haptic rendering of hyper-elastic material via measurement-based FEM model identification and real-time simulation" by the authors Arsen Abdulali, Ibragim Atadjanov, Seungkyu Lee and Seokhee Jeon [1], presents a haptic rendering framework that is capable of enhancing haptic feedback realism by means of an innovative, measurement-based modeling and rendering paradigm. [Extracted from the article]

We are pleased to present this special section of Computers & Graphics (C&G), featuring the selected peer reviewed extended versions of the full papers presented at the 12th Eurographics Workshop on 3D Object Retrieval - 3DOR 2019 (http://3dor2019.ge.imati.cnr.it/) which was held on May 5-6, 2019 in Genova, Italy. All papers were evaluated in a blind process by at least three reviewers from our international program committee, consisting of 32 experts in the area. In the paper "ConvPoint: Continuous Convolutions for Point Cloud Processing" [3], Alexandre Boulch investigates the use of a generalization of discrete convolutional neural networks (CNNs) for processing unstructured point clouds. [Extracted from the article]

Abstract: Real-time rendering of virtual weather conditions has been investigated in many papers. Inserting fog or snow in a scene is rather straightforward. Rain is one of the most encountered natural phenomena, but its rendering often lacks realism. In this paper, we propose a realistic real-time rain rendering method using programmable graphics hardware. In order to simulate the refraction of the scene inside a raindrop, the scene is captured to a texture which is distorted according to optical properties of raindrops. This texture is mapped onto each raindrop. Our method also takes into account retinal persistence, and interaction with light sources. [Copyright &y& Elsevier]

Abstract: In this paper, we present some of the problems we have faced in visualizing patterns coming from Chua''s circuit, with the main aim of sharing these investigations with researchers in Computer Graphics. Furthermore, the paper deals about advanced Computers Graphics techniques for managing complexity in representing chaotic and hyperchaotic systems. The problems we have faced deal about the nature of these configurations, their shapes and dimensions, the granularity of these objects, since the output could be made of different elements such as cubes, cylinders and so on. Instead, cognitive issues related to perception of forms in 3D spaces and their recognition are still unexplored. [Copyright &y& Elsevier]

You are reading the December 2019 Issue of the Computers and Graphics Journal. The paper by Ang et al [1] proposes 3D printing to visualize blood flow data from 4D Flow MRI, a tangible alternative to digital 3D flow visualizations. A Survey Paper by Cardoso and Perrotta [4] looks at real locomotion for HMD-based virtual reality settings, focusing on interaction techniques combining devices, user actions and system feedback. [Extracted from the article]

Abstract: In this paper, we summarize some of the techniques used for modelling certain elements of nature such as trees. We describe some of the current methods in their order of appearance in the classification spectrum given in this survey. We discuss the geometrical and biological aspects that help us to identify the position of the work in our spectrum. The paper also identifies the interaction capability of the current techniques with the environmental factors and external forces. [Copyright &y& Elsevier]