Volume visualization software usually has to deal with datasets that are larger than the GPUs may... more Volume visualization software usually has to deal with datasets that are larger than the GPUs may hold. This is especially true in one of the most popular application scenarios: medical visualization. In this paper we explore the quality of different downsampling methods and present a new approach that produces smooth lower-resolution representations, yet still preserves small features that are prone to disappear with other approaches.
The study of the colonic volume is a procedure with strong relevance to gastroenterologists. Depe... more The study of the colonic volume is a procedure with strong relevance to gastroenterologists. Depending on the clinical protocols, the volume analysis has to be performed on MRI of the unprepared colon without contrast administration. In such circumstances, existing measurement procedures are cumbersome and time-consuming for the specialists. The algorithm presented in this paper permits a quasi-automatic segmentation of the unprepared colon on T2-weighted MRI scans. The segmentation algorithm is organized as a three-stage pipeline. In the first stage, a custom tubularity filter is run to detect colon candidate areas. The specialists provide a list of points along the colon trajectory, which are combined with tubularity information to calculate an estimation of the colon medial path. In the second stage, we delimit the region of interest by applying custom segmentation algorithms to detect colon neighboring regions and the fat capsule containing abdominal organs. Finally, within the reduced search space, segmentation is performed via 3D graph-cuts in a three-stage multigrid approach. Our algorithm was tested on MRI abdominal scans, including different acquisition resolutions, and its results were compared to the colon ground truth segmentations provided by the specialists. The experiments proved the accuracy, efficiency, and usability of the algorithm, while the variability of the scan resolutions contributed to demonstrate the computational scalability of the multigrid architecture. The system is fully applicable to the colon measurement clinical routine, being a substantial step towards a fully automated segmentation.
Throughout the years, many shading techniques have been developed to improve the conveying of inf... more Throughout the years, many shading techniques have been developed to improve the conveying of information in Volume Visualization. Some of these methods, usually referred to as realistic, are supposed to provide better cues for the understanding of volume data sets. While shading approaches are heavily exploited in traditional monoscopic setups, no previous study has analyzed the effect of these techniques in Virtual Reality. To further explore the influence of shading on the understanding of volume data in such environments, we carried out a user study in a desktop-based stereoscopic setup. The goals of the study were to investigate the impact of well-known shading approaches and the influence of real illumination on depth perception. Participants had to perform three different perceptual tasks when exposed to static visual stimuli. 45 participants took part in the study, giving us 1152 trials for each task. Results show that advanced shading techniques improve depth perception in stereoscopic volume visualization. As well, external lighting does not affect depth perception when these shading methods are applied. As a result, we derive some guidelines that may help the researchers when selecting illumination models for stereoscopic rendering.
Most visibility culling algorithms require convexity of occluders. Occluder synthesis algorithms ... more Most visibility culling algorithms require convexity of occluders. Occluder synthesis algorithms attempt to construct large convex occluders inside bulky non-convex sets. Occluder fusion algorithms generate convex occluders that are contained in the umbra cast by a group of objects given an area light. In this paper we prove that convexity requirements can be shifted from the occluders to their umbra with no loss of efficiency, and use this property to show how some special non-planar, non-convex closed polylines that we call "hoops" can be used to compute occlusion efficiently for objects that have no large interior convex sets and were thus rejected by previous approaches.
Modern adquisition techniques provide 3D datasets with unprecedented detail. Both triangle-based ... more Modern adquisition techniques provide 3D datasets with unprecedented detail. Both triangle-based and point-based techniques have been proposed for efficient rendering of these objects. We present a new technique for interactive visualization of volumetric data which combines polygonal faces for flat areas and oriented points for curved regions. The paper focuses on extracting large polygonal faces from voxel data. The key ingredient is a region-growing algorithm which maintains a discrete representation of the set of planes that satisfy a separability problem. A robust representation of this set of planes based on a discretized 4D convex polytope is also discussed. The proposed face extraction algorithm is able to recover information at sub-voxel precision and automatically detects sharp edges.
Existent algorithms to perform geometric transformations on octrees can be classi ed in two famil... more Existent algorithms to perform geometric transformations on octrees can be classi ed in two families: inverse transformation and address computation ones. Those in the inverse transformation family essentially resample the target octree from the source one, and are able to cope with all the a ne transformations. Those in the address computation family only deal with translations, but are commonly accepted as faster than the former ones for they do no intersection tests, but directly calculate the transformed address of each black node in the source tree. This work introduces a new translation algorithm that shows to perform better than previous one when very small displacements are involved. This property is particularly useful in applications such as simulation, robotics or computer animation.
Isosurface construction and rendering based on tetrahedral grids has been adequately implemented ... more Isosurface construction and rendering based on tetrahedral grids has been adequately implemented on programmable graphics hardware. In this paper we present MTCut: a volume cutting algorithm that is able to cut isosurfaces obtained by a Marching Tetrahedra algorithm on volume data. It does not require a tetrahedal representation and runs in real time for complex meshes of up to 1.8M triangles. Our algorithm takes as input the isosurface to be cut, and produces the cut geometry in response to the user interaction with a haptic device. The result is a watertight manifold model that can be interactively recovered back to CPU upon user request.
Proceedings of the Vision Modeling and Visualization Conference 2001, 2001
The Surface Octree (SO) is an extension of the Classical Octree used to maintain a decimated codi... more The Surface Octree (SO) is an extension of the Classical Octree used to maintain a decimated codification of a surface while preserving volume data information. Constrained by a region of maximal interest, we present an algorithm to extract a multiresolution surface from a SO data structure. This algorithm reconstructs the surface with maximal precision in the focus of interest and at progressively lower resolution away from it.
We describe a new approach to modelling pearlescent paints based on decomposing paint layers into... more We describe a new approach to modelling pearlescent paints based on decomposing paint layers into stacks of imaginary thin sublayers. The sublayers are chosen so thin that multiple scattering can be considered across different sublayers, while it can be neglected within each of the sublayers. Based on this assumption, an efficient recursive procedure of assembling the layers is developed, which enables to compute the paint BRDF at interactive speeds. Since the proposed paint model connects fundamental optical properties of multi-layer pearlescent and metallic paints with their microscopic structure, interactive prediction of the paint appearance based on its composition becomes possible.
The development of eective user interfaces for virtual reality environments must face dierent cha... more The development of eective user interfaces for virtual reality environments must face dierent challenges. Not only must one evolve new paradigms that are adequate in those environments, but they must also be minimally invasive (not to spoil the immersion il- lusion), and they must rely on an adequate support from the underlying models and model-handling functions, so that the maximum complexity of usable scenes is not seri- ously limited. This project aims at pushing further the state of the art, developing new techniques at the modelling level (to allow handling models like the most complex currently available, which are too large to fit in core memory for a reasonably sized workstation). It also aims at perfecting the state of the art concerning the interfaces themselves, developing specific techniques adapted to the interaction in those media. Moreover, the results will be tested developing specific prototypes to verify the usability of the proposed techniques from the point of v...
Given a client-server communication network, with workstations equipped with 3D texture hardware,... more Given a client-server communication network, with workstations equipped with 3D texture hardware, we propose a technique that guarantees the optimal use of the client texture hardware. We consider the best representation of a data model that has to be rendered on the client side as the one that requires the minimal texture space while preserving image quality. Taking into account this consideration the basis of the proposed technique is the selection of the best multiresolution representation from a volume hierarchy, maintained by the server. The key points of our proposal are: (i) the hierarchical data structure used, either by the server and the client, to maintain the data; (ii) the data management process applied by the server to satisfy client requirements.(iii) the possibility of the client to predict part of one transmission by analyzing the previous one. Such a capability allows the client to perform some computations in advance and, therefore, reduces frame rates.
The Surface Octree (SO) is an extension of the Classical Octree used to maintain a decimated codi... more The Surface Octree (SO) is an extension of the Classical Octree used to maintain a decimated codification of a surface while preserving volume data information. Constrained by a region of maximal interest, we present an algorithm to extract a multiresolution surface from a SO data structure. This algorithm reconstructs the surface with maximal precision in the focus of interest and at progressively lower resolution away from it.
Volume visualization software usually has to deal with datasets that are larger than the GPUs may... more Volume visualization software usually has to deal with datasets that are larger than the GPUs may hold. This is especially true in one of the most popular application scenarios: medical visualization. In this paper we explore the quality of different downsampling methods and present a new approach that produces smooth lower-resolution representations, yet still preserves small features that are prone to disappear with other approaches.
The study of the colonic volume is a procedure with strong relevance to gastroenterologists. Depe... more The study of the colonic volume is a procedure with strong relevance to gastroenterologists. Depending on the clinical protocols, the volume analysis has to be performed on MRI of the unprepared colon without contrast administration. In such circumstances, existing measurement procedures are cumbersome and time-consuming for the specialists. The algorithm presented in this paper permits a quasi-automatic segmentation of the unprepared colon on T2-weighted MRI scans. The segmentation algorithm is organized as a three-stage pipeline. In the first stage, a custom tubularity filter is run to detect colon candidate areas. The specialists provide a list of points along the colon trajectory, which are combined with tubularity information to calculate an estimation of the colon medial path. In the second stage, we delimit the region of interest by applying custom segmentation algorithms to detect colon neighboring regions and the fat capsule containing abdominal organs. Finally, within the reduced search space, segmentation is performed via 3D graph-cuts in a three-stage multigrid approach. Our algorithm was tested on MRI abdominal scans, including different acquisition resolutions, and its results were compared to the colon ground truth segmentations provided by the specialists. The experiments proved the accuracy, efficiency, and usability of the algorithm, while the variability of the scan resolutions contributed to demonstrate the computational scalability of the multigrid architecture. The system is fully applicable to the colon measurement clinical routine, being a substantial step towards a fully automated segmentation.
Throughout the years, many shading techniques have been developed to improve the conveying of inf... more Throughout the years, many shading techniques have been developed to improve the conveying of information in Volume Visualization. Some of these methods, usually referred to as realistic, are supposed to provide better cues for the understanding of volume data sets. While shading approaches are heavily exploited in traditional monoscopic setups, no previous study has analyzed the effect of these techniques in Virtual Reality. To further explore the influence of shading on the understanding of volume data in such environments, we carried out a user study in a desktop-based stereoscopic setup. The goals of the study were to investigate the impact of well-known shading approaches and the influence of real illumination on depth perception. Participants had to perform three different perceptual tasks when exposed to static visual stimuli. 45 participants took part in the study, giving us 1152 trials for each task. Results show that advanced shading techniques improve depth perception in stereoscopic volume visualization. As well, external lighting does not affect depth perception when these shading methods are applied. As a result, we derive some guidelines that may help the researchers when selecting illumination models for stereoscopic rendering.
Most visibility culling algorithms require convexity of occluders. Occluder synthesis algorithms ... more Most visibility culling algorithms require convexity of occluders. Occluder synthesis algorithms attempt to construct large convex occluders inside bulky non-convex sets. Occluder fusion algorithms generate convex occluders that are contained in the umbra cast by a group of objects given an area light. In this paper we prove that convexity requirements can be shifted from the occluders to their umbra with no loss of efficiency, and use this property to show how some special non-planar, non-convex closed polylines that we call "hoops" can be used to compute occlusion efficiently for objects that have no large interior convex sets and were thus rejected by previous approaches.
Modern adquisition techniques provide 3D datasets with unprecedented detail. Both triangle-based ... more Modern adquisition techniques provide 3D datasets with unprecedented detail. Both triangle-based and point-based techniques have been proposed for efficient rendering of these objects. We present a new technique for interactive visualization of volumetric data which combines polygonal faces for flat areas and oriented points for curved regions. The paper focuses on extracting large polygonal faces from voxel data. The key ingredient is a region-growing algorithm which maintains a discrete representation of the set of planes that satisfy a separability problem. A robust representation of this set of planes based on a discretized 4D convex polytope is also discussed. The proposed face extraction algorithm is able to recover information at sub-voxel precision and automatically detects sharp edges.
Existent algorithms to perform geometric transformations on octrees can be classi ed in two famil... more Existent algorithms to perform geometric transformations on octrees can be classi ed in two families: inverse transformation and address computation ones. Those in the inverse transformation family essentially resample the target octree from the source one, and are able to cope with all the a ne transformations. Those in the address computation family only deal with translations, but are commonly accepted as faster than the former ones for they do no intersection tests, but directly calculate the transformed address of each black node in the source tree. This work introduces a new translation algorithm that shows to perform better than previous one when very small displacements are involved. This property is particularly useful in applications such as simulation, robotics or computer animation.
Isosurface construction and rendering based on tetrahedral grids has been adequately implemented ... more Isosurface construction and rendering based on tetrahedral grids has been adequately implemented on programmable graphics hardware. In this paper we present MTCut: a volume cutting algorithm that is able to cut isosurfaces obtained by a Marching Tetrahedra algorithm on volume data. It does not require a tetrahedal representation and runs in real time for complex meshes of up to 1.8M triangles. Our algorithm takes as input the isosurface to be cut, and produces the cut geometry in response to the user interaction with a haptic device. The result is a watertight manifold model that can be interactively recovered back to CPU upon user request.
Proceedings of the Vision Modeling and Visualization Conference 2001, 2001
The Surface Octree (SO) is an extension of the Classical Octree used to maintain a decimated codi... more The Surface Octree (SO) is an extension of the Classical Octree used to maintain a decimated codification of a surface while preserving volume data information. Constrained by a region of maximal interest, we present an algorithm to extract a multiresolution surface from a SO data structure. This algorithm reconstructs the surface with maximal precision in the focus of interest and at progressively lower resolution away from it.
We describe a new approach to modelling pearlescent paints based on decomposing paint layers into... more We describe a new approach to modelling pearlescent paints based on decomposing paint layers into stacks of imaginary thin sublayers. The sublayers are chosen so thin that multiple scattering can be considered across different sublayers, while it can be neglected within each of the sublayers. Based on this assumption, an efficient recursive procedure of assembling the layers is developed, which enables to compute the paint BRDF at interactive speeds. Since the proposed paint model connects fundamental optical properties of multi-layer pearlescent and metallic paints with their microscopic structure, interactive prediction of the paint appearance based on its composition becomes possible.
The development of eective user interfaces for virtual reality environments must face dierent cha... more The development of eective user interfaces for virtual reality environments must face dierent challenges. Not only must one evolve new paradigms that are adequate in those environments, but they must also be minimally invasive (not to spoil the immersion il- lusion), and they must rely on an adequate support from the underlying models and model-handling functions, so that the maximum complexity of usable scenes is not seri- ously limited. This project aims at pushing further the state of the art, developing new techniques at the modelling level (to allow handling models like the most complex currently available, which are too large to fit in core memory for a reasonably sized workstation). It also aims at perfecting the state of the art concerning the interfaces themselves, developing specific techniques adapted to the interaction in those media. Moreover, the results will be tested developing specific prototypes to verify the usability of the proposed techniques from the point of v...
Given a client-server communication network, with workstations equipped with 3D texture hardware,... more Given a client-server communication network, with workstations equipped with 3D texture hardware, we propose a technique that guarantees the optimal use of the client texture hardware. We consider the best representation of a data model that has to be rendered on the client side as the one that requires the minimal texture space while preserving image quality. Taking into account this consideration the basis of the proposed technique is the selection of the best multiresolution representation from a volume hierarchy, maintained by the server. The key points of our proposal are: (i) the hierarchical data structure used, either by the server and the client, to maintain the data; (ii) the data management process applied by the server to satisfy client requirements.(iii) the possibility of the client to predict part of one transmission by analyzing the previous one. Such a capability allows the client to perform some computations in advance and, therefore, reduces frame rates.
The Surface Octree (SO) is an extension of the Classical Octree used to maintain a decimated codi... more The Surface Octree (SO) is an extension of the Classical Octree used to maintain a decimated codification of a surface while preserving volume data information. Constrained by a region of maximal interest, we present an algorithm to extract a multiresolution surface from a SO data structure. This algorithm reconstructs the surface with maximal precision in the focus of interest and at progressively lower resolution away from it.
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Papers by Isabel Navazo