| 1. |
- Maria Marreiros, Filipe Miguel, 1978-, et al.
(författare)
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Calculating the Stereo Pairs of a Mirror-Based Augmented Reality System
- 2005
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Ingår i: Proceedings 13º Encontro Português de Computação Gráfica. - 9729846464 ; , s. 41-48
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Konferensbidrag (refereegranskat)abstract
- This paper describes a solution for the calculation of the correct stereo pairs of a Mirror-Based Augmented Reality System (MBARS).To achieve augmentation half-silvered mirrors are applied due to their dual reflection characteristic that allow simultaneous seeing-through of real objects and the viewing of reflected virtual ones. This way it is possible to merge virtual content reflected by the mirror with real objects that the user can see through it. The virtual con-tent can be either generated by a monitor or a projector. In this paper we will only refer to the monitor-based systems. These systems are view dependent, meaning that depending upon the viewer’s position the reflected im-ages have to be recomputed. This is why a Head-Tracking System is needed to supply the current viewer’s posi-tion. Also we need to consider the main system components and there location, to correctly compute the stereo pairs. Furthermore we present how the result of the mathematical calculations can be used in combination with com-mands of the OpenGL API to achieve the desired transformations and to allow interactivity.
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| 2. |
- Maria Marreiros, Filipe Miguel, 1978-, et al.
(författare)
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GPU-based ray-casting of non-rigid deformations : a comparison between direct and indirect approaches
- 2014
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Ingår i: Proceedings of SIGRAD 2014, Visual Computing, June 12-13, 2014, Göteborg, Sweden. - : Linköping University Electronic Press. - 9789175192123 ; , s. 67-74
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Konferensbidrag (refereegranskat)abstract
- For ray-casting of non-rigid deformations, the direct approach (as opposed to the traditional indirect approach) does not require the computation of an intermediate volume to be used for the rendering step. The aim of this study was to compare the two approaches in terms of performance (speed) and accuracy (image quality).The direct and the indirect approach were carefully implemented to benefit of the massive GPU parallel power, using CUDA. They were then tested with Computed Tomography (CT) datasets of varying sizes and with a synthetic image, the Marschner-Lobb function.The results show that the direct approach is dependent on the ray sampling steps, number of landmarks and image resolution. The indirect approach is mainly affected by the number of landmarks, if the volume is large enough.These results exclude extreme cases, i.e. if the sampling steps are much smaller than the voxel size and if the image resolution is much higher than the ones used here. For a volume of size 512×512×512, using 100 landmarks and image resolution of 1280×960, the direct method performs better if the ray sampling steps are approximately above 1 voxel. Regarding accuracy, the direct method provides better results for multiple frequencies using the Marschner-Lobb function.The conclusion is that the indirect method is superior in terms of performance, if the sampling along the rays is high, in comparison to the voxel grid, while the direct is superior otherwise. The accuracy analysis seems to point out that the direct method is superior, in particular when the implicit function is used.
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| 3. |
- Maria Marreiros, Filipe Miguel, 1978-, et al.
(författare)
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Mirror Stereoscopic Display for Direct Volume Rendering
- 2014
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Ingår i: Proceedings of SIGRAD 2014, Visual Computing, June 12-13, 2014, Göteborg, Sweden. - : Linköping University Electronic Press. ; , s. 75-82
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Konferensbidrag (refereegranskat)abstract
- A new mirror stereoscopic display for Direct Volume Rendering (DVR) is presented. The stereoscopic display system is composed of one monitor and one acrylic first surface mirror. The mirror reflects one image for one of the eyes. The geometrical transformations to compute correctly the stereo pair is presented and is the core of this paper. System considerations such as mirror placement and implications are also discussed.In contrast to other similar solutions, we do not use two monitors, but just one. Consequently one of the images needs to be skewed. Advantages of the system include absence of ghosting and of flickering.We also developed the rendering engine for DVR of volumetric datasets mostly for medical imaging visualization. The skewing process in this case is integrated into the ray casting of DVR. Using geometrical transformations, we can compute precisely the directions of the rays, producing accurate stereo pairs.
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| 4. |
- Maria Marreiros, Filipe Miguel, 1978-, et al.
(författare)
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Multi-view 3D vessel tracking using near-infrared cameras
- 2014
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Ingår i: Proceedings of the 27th International Congress and Exhibition on Computer Assisted Radiology and Surgery. - : Springer. ; , s. S165-S165
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 5. |
- Maria Marreiros, Filipe Miguel, 1978-, et al.
(författare)
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Non-rigid Deformation Pipeline for Compensation of Superficial Brain Shift
- 2013
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Ingår i: Medical Image Computing and Computer-Assisted Intervention, MICCAI 2013. - Berlin, Heidelberg : Springer Berlin/Heidelberg. - 9783642407628 - 9783642407635 ; , s. 141-148
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Konferensbidrag (refereegranskat)abstract
- The correct visualization of anatomical structures is a critical component of neurosurgical navigation systems, to guide the surgeon to the areas of interest as well as to avoid brain damage. A major challenge for neuronavigation systems is the brain shift, or deformation of the exposed brain in comparison to preoperative Magnetic Resonance (MR) image sets. In this work paper, a non-rigid deformation pipeline is proposed for brain shift compensation of preoperative imaging datasets using superficial blood vessels as landmarks. The input was preoperative and intraoperative 3D image sets of superficial vessel centerlines. The intraoperative vessels (obtained using 3 Near-Infrared cameras) were registered and aligned with preoperative Magnetic Resonance Angiography vessel centerlines using manual interaction for the rigid transformation and, for the non-rigid transformation, the non-rigid point set registration method Coherent Point Drift. The rigid registration transforms the intraoperative points from the camera coordinate system to the preoperative MR coordinate system, and the non-rigid registration deals with local transformations in the MR coordinate system. Finally, the generation of a new deformed volume is achieved with the Thin-Plate Spline (TPS) method using as control points the matches in the MR coordinate system found in the previous step. The method was tested in a rabbit brain exposed via craniotomy, where deformations were produced by a balloon inserted into the brain. There was a good correlation between the real state of the brain and the deformed volume obtained using the pipeline. Maximum displacements were approximately 4.0 mm for the exposed brain alone, and 6.7 mm after balloon inflation.
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| 6. |
- Maria Marreiros, Filipe Miguel, 1978-, et al.
(författare)
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Non-rigid point set registration of curves : registration of the superficial vessel centerlines of the brain
- 2016
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Ingår i: MEDICAL IMAGING 2016. - : SPIE - International Society for Optical Engineering. - 9781510600218
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Konferensbidrag (refereegranskat)abstract
- In this study we present a non-rigid point set registration for 3D curves (composed by 3D set of points). The method was evaluated in the task of registration of 3D superficial vessels of the brain where it was used to match vessel centerline points. It consists of a combination of the Coherent Point Drift (CPD) and the Thin-Plate Spline (TPS) semilandmarks. The CPD is used to perform the initial matching of centerline 3D points, while the semilandmark method iteratively relaxes/slides the points. For the evaluation, a Magnetic Resonance Angiography (MRA) dataset was used. Deformations were applied to the extracted vessels centerlines to simulate brain bulging and sinking, using a TPS deformation where a few control points were manipulated to obtain the desired transformation (T-1). Once the correspondences are known, the corresponding points are used to define a new TPS deformation(T-2). The errors are measured in the deformed space, by transforming the original points using T-1 and T-2 and measuring the distance between them. To simulate cases where the deformed vessel data is incomplete, parts of the reference vessels were cut and then deformed. Furthermore, anisotropic normally distributed noise was added. The results show that the error estimates (root mean square error and mean error) are below 1 mm, even in the presence of noise and incomplete data.
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| 7. |
- Maria Marreiros, Filipe Miguel, 1978-, et al.
(författare)
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Single-Monitor-Mirror Stereoscopic Display
- 2013
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Ingår i: Journal of Graphics Tools. - : Taylor & Francis. - 2165-347X. ; 17:3, s. 85-97
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Tidskriftsartikel (refereegranskat)abstract
- A new single-monitor-mirror stereoscopic display is presented. The stereoscopic display system is composed of one monitor and one acrylic first-surface mirror. The mirror reflects one image for one of the eyes. The geometrical transformations to compute correctly the stereo pair are derived and presented. System considerations such as mirror placement and implications are also discussed.In contrast to other similar solutions that use fixed configurations, we try to optimize the display area by controlling the mirror placement. Consequently, one of the images needs to be skewed. Advantages of the system include absence of ghosting and flickering.We also developed the rendering engine for direct volume rendering (DVR) of volumetric datasets mostly for medical imaging visualization and using OpenGL for polygonal datasets and stereoscopic digital photography. The skewing process in this case is integrated into the ray-casting of DVR. Using geometrical transformations, we can compute precisely the directions of the rays, producing accurate stereo pairs. A similar operation is also performed using OpenGL.
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| 8. |
- Maria Marreiros, Filipe Miguel, 1978-, et al.
(författare)
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Stereoscopic static depth perception of enclosed 3D objects
- 2013
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Ingår i: SAP '13 Proceedings of the ACM Symposium on Applied Perception. - New York, USA : Association for Computing Machinery (ACM). - 9781450322621 ; , s. 15-22
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Konferensbidrag (refereegranskat)abstract
- Depth perception of semi-transparent virtual objects and the visu-alization of their spatial layout are crucial in many applications, in particular medical applications. Depth cues for opaque objects have been extensively studied, but this is not the case for stereo-scopic semi-transparent objects, in particular in the case when one 3D object is enclosed within a larger exterior object.In this work we explored different stereoscopic rendering methodsto analyze their impact on depth perception accuracy of an enclosed3D object. Two experiments were performed: the first tested the hypotheses that depth perception is dependent on the color blending of objects (opacity - alpha) for each rendering method and that one of two rendering methods used is superior. The second experiment was performed to corroborate the results of the first experiment and to test an extra hypothesis: is depth perception improved if an auxiliary object that provides a relationship between the enclosed objectand the exterior is used?The first rendering method used is simple alpha blending with Blinn-Phong shading model, where a segmented brain (exterior object) and a synthetic tumor (enclosed object) were blended. The second rendering method also uses Blinn-Phong, but the shading was modified to preserve silhouettes and to provide an illustrative rendering. Comparing both rendering methods, the brighter regionsof the first rendering method will become more transparent in the second rendering method, thus preserving silhouette areas.The results show that depth perception accuracy of an enclosed object rendered with a stereoscopic system is dependent on opacity for some rendering methods (simple alpha blending), but this effect is less pronounced than the dependence on object position in relation to the exterior object. The illustrative rendering method is less dependent on opacity. The different rendering methods also perform slightly differently; an illustrative rendering method was superior and the use of an auxiliary object seems to facilitate depth perception.
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| 9. |
- Maria Marreiros, Filipe Miguel, 1978-, et al.
(författare)
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Superficial vessel reconstruction with a multiview camera system
- 2016
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Ingår i: Journal of Medical Imaging. - : SPIE - International Society for Optical Engineering. - 2329-4302 .- 2329-4310. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- We aim at reconstructing superficial vessels of the brain. Ultimately, they will serve to guide the deformation methods to compensate for the brain shift. A pipeline for three-dimensional (3-D) vessel reconstruction using three mono-complementary metal-oxide semiconductor cameras has been developed. Vessel centerlines are manually selected in the images. Using the properties of the Hessian matrix, the centerline points are assigned direction information. For correspondence matching, a combination of methods was used. The process starts with epipolar and spatial coherence constraints (geometrical constraints), followed by relaxation labeling and an iterative filtering where the 3-D points are compared to surfaces obtained using the thin-plate spline with decreasing relaxation parameter. Finally, the points are shifted to their local centroid position. Evaluation in virtual, phantom, and experimental images, including intraoperative data from patient experiments, shows that, with appropriate camera positions, the error estimates (root-mean square error and mean error) are similar to 1 mm.
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