Italian Chapter Conference 2014 (STAG: Smart Tools & Apps for Graphics)

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    CHROMAGRAM: A Real-time Chroma Key Application for Mobile Devices
    (The Eurographics Association, 2014) Corda, Fabrizio; Sorrentino, Fabio; Scateni, Riccardo; Andrea Giachetti
    Chroma Key is a special-effects technique widely used by television and motion picture industries for image composition. This technique allows users to replace sections identified by a chosen colour in a multimedia stream (like a video or a photo) with another image or video stream. In this paper, we describe an easy-to-implement technique for the creation of an Android based application for mobile devices (like smartphones and tablets) that applies Chroma Key-based effects to the video stream coming from the device camera. We discuss the algorithm used to achieve the Chroma Key effect focusing on the computational performance and on the quality of its final result. Using a picture selected from the device gallery, this application makes possible the replacement of video stream background areas characterized by a chroma value with the chosen picture.
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    Practical Line Rasterization for Multi-resolution Textures
    (The Eurographics Association, 2014) Taibo, Javier; Jaspe, Alberto; Seoane, Antonio; Agus, Marco; Hernández, Luis; Andrea Giachetti
    Draping 2D vectorial information over a 3D terrain elevation model is usually performed by real-time rendering to texture. In the case of linear feature representation, there are several specific problems using the texturing approach, specially when using multi-resolution textures. These problems are related to visual quality, aliasing artifacts and rendering performance. In this paper, we address the problems of 2D line rasterization on a multi-resolution texturing engine from a pragmatical point of view; some alternative solutions are presented, compared and evaluated. For each solution we have analyzed the visual quality, the impact on the rendering performance and the memory consumption. The study performed in this work is based on an OpenGL implementation of a clipmap-based multi-resolution texturing system, and is oriented towards the use of inexpensive consumer graphics hardware.
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    Automatic Generation of Synthetic Retinal Fundus Images
    (The Eurographics Association, 2014) Fiorini, S.; Biasi, M. De; Ballerini, L.; Trucco, E.; Ruggeri, A.; Andrea Giachetti
    This study aims to generate synthetic and realistic retinal fundus colour images, similar in characteristics to a given dataset, as well as the values of all morphological parameters. A representative task could be, for example, the synthesis of a retinal image with the corresponding vessel tree and optic nerve head binary map, measurement of vessel width in any position, fovea localisation and so on. The presented paper describes the techniques developed for the generation of both vascular and non-vascular regions (i.e. retinal background, fovea and optic disc). To synthesise convincing retinal backgrounds and foveae, a patch-based algorithm has been developed; model-based texture synthesis techniques have also been implemented for the generation of realistic optic discs and vessel networks. The validity of our synthetic retinal images has been demonstrated by visual inspection and quantitative experiments.
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    AR Turn-by-turn Navigation in Small Urban Areas and Information Browsing
    (The Eurographics Association, 2014) Cherchi, Gianmarco; Sorrentino, Fabio; Scateni, Riccardo; Andrea Giachetti
    Navigation systems allow to discover cities and their urban areas easily and quickly, finding the shortest path to reach them and giving directions to users saving their time and energy. At present time, these systems are based over streets maps offered by the major mapping services like Tele Atlas, Navteq or OpenStreetMap. In recent years, thanks to the Google StreetView service it has been possible to discover main cities locations both indoor and outdoor. What is missing in this frame is the possibility to map small urban areas of small and medium sized cities, due to their lack of relevance for the big players. In this cities there could be very interesting areas for tourists. Example locations could be botanical gardens, archeological sites, protected natural areas among others. In this work we tried to set up a navigation system for limited extensions inside urban areas which permits to wander around and gives access to related information using augmented reality techniques. Due to the possible poor wireless coverage in these locations we designed an application that stores all required data on the user's device, splitting the information in packages according to the chosen language. A key issue was to achieve good results combining all these features in a single device with a small display, overwhelming the constraints due to the mobile environment.
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    Adaptive Clipping of Splats to Models with Sharp Features
    (The Eurographics Association, 2014) Ivo, Rafael; Ganovelli, Fabio; Vidal, Creto; Scopigno, Roberto; Andrea Giachetti
    Splat-based models are a good representation because of its absense of topology, making complex modeling operations easier, but keeping the same approximation ratio from triangular meshes. However corners cannot be properly represented by splats without clipping them. We present a new method for clipping splats in models with sharp features. Each splat is an ellipse equipped with a few parameters that allow to define how the ellipse can be clipped against a bidimensional rational Bézier curve and thus it can be used for all those surfaces that show a large number of edge features and different sampling rate around them. The simple and uniform data used to define the clipping curve makes easy the implementation in GPU. We designed and implemented an automatic computation of the clipping curves and a pipeline for sampling a generic surface with splats and render it. In this paper we show how this technique outperforms the previous clipping techniques in precision for objects such as mechanical parts and CAD- like models keeping the rendering speed.
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    Multi-modal Digitalization of Cultural Heritage Artifacts
    (The Eurographics Association, 2014) Stanco, Filippo; Gallo, Giovanni; Cannata, Giovanna; Lombardo, Marcella; Andrea Giachetti
    Objects made of different media, paintings, marbles, clay and wooden objects, textiles etc., form the historical collections of most of the middle to medium sized Museums in Italy. This variety poses difficult challenges to these institutions when they face the digitalization of part of their patrimony. This paper provides a report of an ongoing inter-disciplinary experimental program for a digitalization effort to be carried out by one of such institution. Through the discussion of the digitalization of different objects obtained with the use and integration of different techniques we illustrate some of the lessons learned in transferring to the final intended users the graphical tools and the know-how previously acquired in a research laboratory.
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    TESTIMAGES: a Large-scale Archive for Testing Visual Devices and Basic Image Processing Algorithms
    (The Eurographics Association, 2014) Asuni, Nicola; Giachetti, Andrea; Andrea Giachetti
    We present the TESTIMAGES archive, a huge and free collection of sample images designed for analysis and quality assessment of different kinds of displays (i.e. monitors, televisions and digital cinema projectors) and image processing tecnhiques. The archive includes more than 2 million images originally acquired and divided in four different categories: SAMPLING and SAMPLING_PATTERNS (aimed at testing resampling algorithms), COLOR (aimed at testing color rendering on different displays) and PATTERNS (aimed at testing the rendering of standard geometrical patterns). The archive is currently online as a SourceForge project and, even if not yet publicized in the scientific community, it has already been used in different contexts and cited in scientific publications. We plan to extend the archive including datasets for other kinds of specific analyses.
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    Stateless Level of Detail Lighting for Automotive Visualization
    (The Eurographics Association, 2014) Weber, Christoph; Stamminger, Marc; Andrea Giachetti
    Car models typically consist of highly specular surfaces including sharp angles. Renderings of such models contain very fine, sharp lighting features, that both make Level-of-Detail switches well visible and result in strong aliasing artifacts. In this paper we address both issues. As Level-of-Detail approach, we apply on-the-fly vertex clustering and introduce a texture coordinate correction to remove popping artifacts, that become visible during mesh simplification. By replacing vertex normals with texture normals, we can provide stable lighting features, even if the underlying mesh changes. To prevent aliasing effects on specular highlights, we then apply a variant of LEAN mapping (linear efficient antialiased normal mapping). We observe that LEAN mapping cannot be applied directly, because in our scenario we need to filter object space normals, as we have no presupposed tangent-space availeble. We therefore create a per-texel tangent space from an object space MIP normal, thus retaining the benefits of a 2D distribution without a preexisting tangent space. Both approaches in combination, allow us to render highly reflective, detailed models with continuous level of detail and anti-aliased lighting, at the price of moderately increased memory consumption and render time.
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    Enhanced Sphere Tracing
    (The Eurographics Association, 2014) Keinert, Benjamin; Schäfer, Henry; Korndörfer, Johann; Ganse, Urs; Stamminger, Marc; Andrea Giachetti
    In this paper we present several performance and quality enhancements to classical sphere tracing: First, we propose a safe, over-relaxation-based method for accelerating sphere tracing. Second, a method for dynamically preventing self-intersections upon converting signed distance bounds enables controlling precision and rendering performance. In addition, we present a method for significantly accelerating the sphere tracing intersection test for convex objects that are enclosed in convex bounding volumes. We also propose a screen-space metric for the retrieval of a good intersection point candidate, in case sphere tracing does not converge thus increasing rendering quality without sacrificing performance. Finally, discontinuity artifacts common in sphere tracing are reduced using a fixed-point iteration algorithm. We demonstrate complex scenes rendered in real-time with our method. The methods presented in this paper have more universal applicability beyond rendering procedurally generated scenes in real-time and can also be combined with path-tracing-based global illumination solutions.
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    Stereo-browsing from Calibrated Cameras
    (The Eurographics Association, 2014) Baldacci, Andrea; Ganovelli, Fabio; Corsini, Massimiliano; Scopigno, Roberto; Andrea Giachetti
    Modern Structure-from-Motion (SfM) methods enable the registration of a set of cameras and the reconstruction of the corresponding sparse point cloud of the object/location depicted in the input images. Despite the quality of such techniques they often fail where the input point cloud has a very low density thus decreasing the final user experience. On the other side, modern image-based rendering (IBR) techniques try to avoid a full reconstruction of the geometry by empowering the user with interfaces for the smooth navigation of the acquired images. In such methods, images viewed from viewpoints in-between the actual cameras are generated in some way, for example by using a textured proxy or by warping properly the input images. Usual navigation interfaces, however, neglect to use the inherent nature of such set of cameras which, despite having a wide-baseline, are often well spatially organized as they usually maintain a good overlap between images, varying smoothly both the position and the orientation of the camera. Given such a set of registered cameras, we present a framework for the stereoscopic exploration of the object/location depicted using any type of stereoscopic devices. In the proposed system, the users can have a full tridimensional experience without the need of a complete 3D reconstruction. Our method starts by building a graph where each node is associated to a calibrated camera that represents a virtual eye. Two virtual eyes give a stereo pair. Along each edge of this graph we can instantiate a novel virtual camera using simple linear interpolation of the extrinsic parameters and we can generate its corresponding novel view by using known IBR techniques. This, in practice, extends the domain of the possible views from the discrete set of acquired cameras to a continuous domain given by our graph. Combining any couple of cameras that we can pick on this graph we obtain the set of all possible stereo pairs, that is the codomain of our graph. We give a formal definition of this space, that we called StereoSpace. Built on this, we designed our prototype system for the stereoscopic exploration of photo collections.