Rafael P. Torchelsen

Technical Section: A survey of raster-based transparency techniques

Transparency is an important effect for several graphics applications. Correct transparency rendering requires fragment-sorting, which can be more expensive than sorting geometry primitives, and can handle situations that might not be solved in geometry space, such as object interpenetrations. In this paper we survey different transparency techniques and analyze them in terms of processing time, memory consumption, and accuracy. Ideally, the perfect method computes correct transparency in real-time with low memory usage. However, achieving these goals simultaneously is still a challenging task. We describe features and trade-offs adopted by each technique, pointing out pros and cons that can be used to help with the decision of which method to use in a given situation.

Hybrid transparency

Hybrid transparency is an approach for real-time approximation of order-independent transparency. Our hybrid approach combines an accurate compositing, of a few core transparent layers, with a quick approximation, for the remaining layers. Its main advantage, the ability to operate in bounded memory without noticeable artifacts, enables its usage with high scene complexity and image resolution, which other approaches fail to handle. Hybrid transparency is suitable for highly-parallel execution, can be implemented in current GPUs and further improved, with minimal architecture changes. We present quality, memory, and performance analysis and comparisons which demonstrate that hybrid transparency is able to generate high-quality images at competitive frames rates and with the lowest memory consumption among comparable OIT techniques.

Memory-Efficient Order-Independent Transparency with Dynamic Fragment Buffer

Order-independent transparency (OIT) rendering is computationally intensive due to required sorting and sufficient memory to store fragments before sorting. We present Dynamic Fragment Buffer, a revamped two-pass OIT rendering technique, which performs correct blending of a large number of transparent layers at interactive frame rates. Our approach self-adjusts memory allocation to handle a variable number of fragments per pixel without wasting memory. In this paper we perform a detailed analysis of several design decisions which lead to this technique. We present a collection of experiments that illustrate the advantages of our technique with respect to others OIT algorithms in the literature.

Real-time multi-agent path planning on arbitrary surfaces

Path planning is an active topic in the literature, and efficient navigation over non-planar surfaces is an open research question. In this work we present a novel technique for navigation of multiple agents over arbitrary triangular domains. The proposed solution uses a fast hierarchical computation of geodesic distances over triangular meshes to allow interactive frame rates, and a GPU-based collision avoidance technique to guide individual agents. Unlike most previous work, the method imposes no limitations on the surface over which the agents are moving, and can naturally deal with non-planar meshes of arbitrary genus and curvature. Moreover, the implementation is a hybrid CPU/GPU algorithm that explores the current trend of increasing the number of CPU cores and GPU programmability. This approach exploits the best qualities in each processor, thus achieving very high performance.

Visual analysis of bike-sharing systems

Abstract Bike-sharing systems are a popular mode of public transportation, increasing in number and size around the world. Public bike-sharing systems attend to the needs of a large number of commuters while synchronizing to the rhythm of big cities. To better understand the usage of such systems, we introduce an interactive visualization system to explore the dynamics of public bike-sharing systems by profiling its historical dataset. By coordinating a pixel-oriented timeline with a map, and introducing a scheme of partial reordering of time series, our design supports the identification of several patterns in temporal and spatial domains. We take New York City׳s bike-sharing program, Citi Bike, as a use case and implement a prototype to show changes in the system over a period of ten months, ranking stations by different properties, using any time interval in daily and monthly timelines. Different analyses are presented to validate the visualization system as a useful operational tool that can support the staff of bike-sharing programs of big cities in the exploration of such large datasets, in order to understand the commuting dynamics to overcome management problems and provide a better service to commuters.

Visualizing Running Races through the Multivariate Time-Series of Multiple Runners

The recent widespread of heart rate (HR) monitors is allowing people to measure body response during and after exercise, which produces a collection of time-series on multivariate aspects, such as heartbeat, speed, geolocation, etc. Such monitoring can be extremely important for people with low fitness levels, since they are susceptible to cardiovascular diseases or other physical injuries when exercising at high heartbeat frequencies. Even though most monitors provide tools to export and display this information for each individual, the ability to visualize the collection of multiple runners in a given running race is mostly unexplored. In this work, we present a design study that aims to support analysis and answer several questions raised by an expert on exercise physiology about a given running race. We describe each visualization design and how they individually, or in collaboration, can be used to reveal interesting aspects of the data. We illustrate our results with use cases that provide evaluation and feedback about the visualization designs proposed.

Transparency and Anti-Aliasing Techniques for Real-Time Rendering

Transparency and anti-aliasing are crucial to enhance realism in computer-generated images, which have a high demand for such effects. Transparency is largely used to denote relationships among objects in a scene, and to render several structures, such as particles and foliage. Anti-aliasing (AA) is also important, since jagged edges can be easily spotted and create disruptive distractions during a scene walkthrough, which are unacceptable in real-time applications. Figure 1 illustrates both effects. In common, they have the fact that they rely on processing discrete samples from a given function, but using the samples for different purposes. In this tutorial we review state-of the-art techniques for transparency and anti-aliasing effects, their initial ideas and subsequent GPU accelerations. We support our presentation with a discussion on their strengths and limitations.

Special Section on Computer Graphics in Brazil: A selection of papers from SIBGRAPI 2012: Memory-optimized order-independent transparency with Dynamic Fragment Buffer

Exact order-independent transparency (OIT) rendering is memory demanding because it requires per-pixel blending of an unknown number of fragments that need to be stored and sorted before compositing. In this paper, we describe the Dynamic Fragment Buffer (DFB) algorithm, which efficiently manages memory to perform correct compositing for pixels with varying numbers of fragments. We present a collection of experiments that illustrate the advantages of the DFB algorithm with respect to other OIT algorithms, and analyze the impact of the proposed variations.

Approximate on‐Surface Distance Computation using Quasi‐Developable Charts

There is a vast number of applications that require distance field computation over triangular meshes. State‐of‐the‐art algorithms have quadratic or sub‐quadratic worst‐case complexity, making them impractical for interactive applications. While most of the research on this subject has been focused on reducing the computation complexity of the algorithms, in this work we propose an approximate algorithm that achieves similar results working in lower resolutions of the input meshes. The creation of lower resolution meshes is the essence of our proposal. The idea is to identify regions on the input mesh that can be unfolded into planar regions with minimal area distortion (i.e. quasi‐developable charts). Once charts are computed, their interior is re‐triangulated to reduce the number of triangles, which results in a collection of simplified charts that we call a base mesh. Due to the properties of quasi‐developable regions, we are able to compute distance fields over the base mesh instead of over the input mesh. This reduces the memory footprint and data processed for distance computations, which is the bottleneck of these algorithms. We present results that are one order of magnitude faster than current exact solutions, with low approximation errors.

Efficient Surgical Cutting with Position-Based Dynamics

Simulations of cuts on deformable bodies have been an active research subject for more than two decades. However, previous works based on finite element methods and mass spring meshes cannot scale to complex surgical scenarios. This article presents a novel method that uses position-based dynamics (PBD) for mesh-free cutting simulation. The proposed solutions include a method to efficiently render force feedback while cutting, an efficient heat diffusion model to simulate electrocautery, and a novel adaptive skinning scheme based on oriented particles.https://extras.computer.org/extra/mcg2017030024s1.mp4