Thomas Olsvik Opsahl

• Haavardsholm, Trym Vegard; Opsahl, Thomas Olsvik; Skauli, Torbjørn & Stahl, Annette (2022). Compact multimodal multispectral sensor system for tactical reconnaissance. Proceedings of SPIE, the International Society for Optical Engineering. ISSN 0277-786X. doi: 10.1117/12.2632701. Full text in Research Archive Show summary

  Multispectral imaging is an attractive sensing modality for small unmanned aerial vehicles (UAVs) in numerous military and civilian applications such as reconnaissance, target detection, and precision agriculture. Cameras based on patterned filters in the focal plane, such as conventional colour cameras, represent the most compact architecture for spectral imaging, but image reconstruction becomes challenging at higher band counts. We consider a camera configuration where six bandpass filters are arranged in a periodically repeating pattern in the focal plane. In addition, a large unfiltered region permits conventional monochromatic video imaging that can be used for situational awareness (SA), including estimating the camera motion and the 3D structure of the ground surface. By platform movement, the filters are scanned over the scene, capturing an irregular pattern of spectral samples of the ground surface. Through estimation of the camera trajectory and 3D scene structure, it is still possible to assemble a spectral image by fusing all measurements in software. The repeated sampling of bands enables spectral consistency testing, which can improve spectral integrity significantly. The result is a truly multimodal camera sensor system able to produce a range of image products. Here, we investigate its application in tactical reconnaissance by pushing towards on-board real-time spectral reconstruction based on visual odometry (VO) and full 3D reconstruction of the scene. The results are compared with offline processing based on estimates from visual simultaneous localisation and mapping (VSLAM) and indicate that the multimodal sensing concept has a clear potential for use in tactical reconnaissance scenarios.

• Shimoni, Michal; Idrissa, Mahamadou; Borghys, Dirk; Haavardsholm, Trym Vegard; Opsahl, Thomas Olsvik & Perneel, Christiaan (2017). Urban features classification using 3D hyperspectral data. Workshop on Hyperspectral Image and Signal Processing, Evolution in Remote Sensing. ISSN 2158-6276. 2013-June, p. 1–4. doi: 10.1109/WHISPERS.2013.8080665. Full text in Research Archive
• Morrison, Aiden; Sokolova, Nadezda; Haavardsholm, Trym Vegard; Hagen, Ove Kent; Opsahl, Thomas Olsvik & Ånonsen, Kjetil Bergh [Show all 7 contributors for this article] (2017). Collaborative indoor navigation for emergency services personnel. IEEE Aerospace Conference. Proceedings. ISSN 1095-323X. 2017-June. doi: 10.1109/AERO.2017.7943729. Full text in Research Archive Show summary

  First responders and other emergency services personnel must often enter buildings which prevent the use of GPS or other satellite navigation signals for positioning. Loss of navigation capability combined with the fact that the buildings are often unknown to the personnel in question makes it more difficult for individual team members to coordinate with one another, and difficult or impossible for the team leader to monitor and direct the actions of each team member. While inertial navigation or pedestrian dead reckoning provide for some degree of navigation in GPS signal denied environments, these solutions degrade with time and may require prohibitively large and expensive inertial solutions to navigate over extended periods, while also allowing each individual user to accumulate independent positioning errors and thereby appearing to ‘drift away’ from one another. This paper presents an implementation of a collaborative navigation system utilizing each of user-to-user radio links, Global Navigation Satellite Systems (GNSS) when available, inertial navigation, pedestrian dead reckoning, as well as camera based Simultaneous Location and Mapping (SLAM) to provide a team of users with absolute and relative situational awareness for themselves and their team. The application of collaborative navigation to such a team provides the triple benefits of providing improved absolute navigation accuracy, improved relative navigation accuracy, and greatly enhanced situational awareness for all cooperating team members.

• Torkildsen, Hans Erling; Haavardsholm, Trym Vegard; Opsahl, Thomas Olsvik; Datta, Urmila; Skaugen, Atle & Skauli, Torbjørn (2016). Compact multispectral multi-camera imaging system for small UAVs. Proceedings of SPIE, the International Society for Optical Engineering. ISSN 0277-786X. doi: 10.1117/12.2224495. Full text in Research Archive Show summary

  Cameras with filters in the focal plane provide the most compact solution for multispectral imaging. A small UAV can carry multiple such cameras, providing large area coverage rate at high spatial resolution. We investigate a camera concept where a patterned bandpass filter with six bands provides multiple interspersed recordings of all bands, enabling consistency checks for improved spectral integrity. A compact sensor payload has been built with multiple cameras and a data acquisition computer. Recorded imagery demonstrates the potential for large area coverage with good spectral integrity. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

• Skauli, Torbjørn; Torkildsen, Hans Erling; Nicolas, Stephane; Opsahl, Thomas Olsvik; Haavardsholm, Trym Vegard & Kåsen, Ingebjørg [Show all 7 contributors for this article] (2014). Compact camera for multispectral and conventional imaging based on patterned filters. Applied Optics. ISSN 1559-128X. doi: 10.1364/AO.53.000C64. Full text in Research Archive Show summary

  A multispectral camera concept is presented. The concept is based on using a patterned filter in the focal plane, combined with scanning of the field of view. The filter layout has stripes of different bandpass filters extending orthogonally to the scan direction. The pattern of filter stripes is such that all bands are sampled multiple times, while minimizing the total duration of the sampling of a given scene point. As a consequence, the filter needs only a small part of the area of an image sensor. The remaining area can be used for conventional 2D imaging. A demonstrator camera has been built with six bands in the visible and near infrared, as well as a panchromatic 2D imaging capability. Image recording and reconstruction is demonstrated, but the quality of image reconstruction is expected to be a main challenge for systems based on this concept. An important advantage is that the camera can potentially be made very compact, and also low cost. It is shown that under assumptions that are not unreasonable, the proposed camera concept can be much smaller than a conventional imaging spectrometer. In principle, it can be smaller in volume by a factor on the order of several hundred while collecting the same amount of light per multispectral band. This makes the proposed camera concept very interesting for small airborne platforms and other applications requiring compact spectral imagers.

• Torkildsen, Hans Erling; Hovland, Harald; Opsahl, Thomas Olsvik; Haavardsholm, Trym Vegard; Nicolas, Stephane & Skauli, Torbjørn (2014). Characterization of a compact 6-band multifunctional camera based on patterned spectral filters in the focal plane. Proceedings of SPIE, the International Society for Optical Engineering. ISSN 0277-786X. doi: 10.1117/12.2054477. Full text in Research Archive Show summary

  In some applications of multi- or hyperspectral imaging, it is important to have a compact sensor. The most compact spectral imaging sensors are based on spectral filtering in the focal plane. For hyperspectral imaging, it has been proposed to use a "linearly variable" bandpass filter in the focal plane, combined with scanning of the field of view. As the image of a given object in the scene moves across the field of view, it is observed through parts of the filter with varying center wavelength, and a complete spectrum can be assembled. However if the radiance received from the object varies with viewing angle, or with time, then the reconstructed spectrum will be distorted. We describe a camera design where this hyperspectral functionality is traded for multispectral imaging with better spectral integrity. Spectral distortion is minimized by using a patterned filter with 6 bands arranged close together, so that a scene object is seen by each spectral band in rapid succession and with minimal change in viewing angle. The set of 6 bands is repeated 4 times so that the spectral data can be checked for internal consistency. Still the total extent of the filter in the scan direction is small. Therefore the remainder of the image sensor can be used for conventional imaging with potential for using motion tracking and 3D reconstruction to support the spectral imaging function. We show detailed characterization of the point spread function of the camera, demonstrating the importance of such characterization as a basis for image reconstruction. A simplified image reconstruction based on feature-based image coregistration is shown to yield reasonable results. Elimination of spectral artifacts due to scene motion is demonstrated.

• Ringaby, Erik; Friman, Ola; Forssén, Per-Erik; Opsahl, Thomas Olsvik; Haavardsholm, Trym Vegard & Kåsen, Ingebjørg (2014). Anisotropic scattered data interpolation for pushbroom image rectification. IEEE Transactions on Image Processing. ISSN 1057-7149. doi: 10.1109/TIP.2014.2316377. Full text in Research Archive Show summary

  This paper deals with fast and accurate visualization of pushbroom image data from airborne and spaceborne platforms. A pushbroom sensor acquires images in a line-scanning fashion, and this results in scattered input data that need to be resampled onto a uniform grid for geometrically correct visualization. To this end, we model the anisotropic spatial dependence structure caused by the acquisition process. Several methods for scattered data interpolation are then adapted to handle the induced anisotropic metric and compared for the pushbroom image rectification problem. A trick that exploits the semiordered line structure of pushbroom data to improve the computational complexity several orders of magnitude is also presented.

• Opsahl, Thomas Olsvik & Haavardsholm, Trym Vegard (2013). Estimating the pixel footprint distribution for image fusion by ray tracing lines of sight in a Monte Carlo scheme. Proceedings of SPIE, the International Society for Optical Engineering. ISSN 0277-786X. doi: 10.1117/12.2015746. Full text in Research Archive Show summary

  Images from airborne cameras can be a valuable resource for data fusion, but this typically requires them to be georeferenced. This usually implies that the information of every pixel should be accompanied by a single geographical position describing where the center of the pixel is located in the scene. This geospatial information is well suited for tasks like target positioning and orthorectification. But when it comes to fusion, a detailed description of the area on the ground contributing to the pixel signal would be preferable over a single position. In this paper we present a method for estimating these regions. Simple Monte Carlo simulations are used to combine the influences of the main geometrical aspects of the imaging process, such as the point spread function, the camera’s motion and the topography in the scene. Since estimates of the camera motion are uncertain to some degree, this is incorporated in the simulations as well. For every simulation, a pixel’s sampling point in the scene is estimated by intersecting a randomly sampled line of sight with a 3D-model of the scene. Based on the results of numerous simulations, the pixel’s sampling region can be represented by a suitable probability distribution. This will be referred to as the pixel’s footprint distribution (PFD). We present results for high resolution hyperspectral pushbroom images of an urban scene.

• Opsahl, Thomas Olsvik; Haavardsholm, Trym Vegard & Winjum, Ingebrigt (2011). Real-time georeferencing for an airborne hyperspectral imaging system. Proceedings of SPIE, the International Society for Optical Engineering. ISSN 0277-786X. doi: 10.1117/12.885069. Full text in Research Archive Show summary

  The paper describes the georeferencing part of an airborne hyperspectral imaging system based on pushbroom scanning. Using ray-tracing methods from computer graphics and a highly efficient representation of the digital elevation model (DEM), georeferencing of high resolution pushbroom images runs in real time by a large margin. By adapting the georeferencing to match the DEM resolution, the camera field of view and the flight altitude, the method has potential to provide real time georeferencing, even for HD video on a high resolution DEM when a graphics processing unit (GPU) is used for processing.

• Skauli, Torbjørn; Haavardsholm, Trym Vegard; Kåsen, Ingebjørg; Arisholm, Gunnar; Kavara, Amela & Opsahl, Thomas Olsvik [Show all 7 contributors for this article] (2010). An airborne real-time hyperspectral target detection system. Proceedings of SPIE, the International Society for Optical Engineering. ISSN 0277-786X. 7695. doi: 10.1117/12.850443. Full text in Research Archive Show summary

  An airborne system for hyperspectral target detection is described. The main sensor is a HySpex pushbroom hyperspectral imager for the visible and near-infrared spectral range with 1600 pixels across track, supplemented by a panchromatic line imager. An optional third sensor can be added, either a SWIR hyperspectral camera or a thermal camera. In real time, the system performs radiometric calibration and georeferencing of the images, followed by image processing for target detection and visualization. The current version of the system implements only spectral anomaly detection, based on normal mixture models. Image processing runs on a PC with a multicore Intel processor and an Nvidia graphics processing unit (GPU). The processing runs in a software framework optimized for large sustained data rates. The platform is a Cessna 172 aircraft based close to FFI, modified with a camera port in the floor.

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• Haavardsholm, Trym Vegard; Opsahl, Thomas Olsvik; Skauli, Torbjørn & Stahl, Annette (2022). Compact multimodal multispectral sensor system for tactical reconnaissance. Full text in Research Archive
• Torkildsen, Hans Erling; Haavardsholm, Trym Vegard; Opsahl, Thomas Olsvik; Datta, Urmila; Skaugen, Atle & Skauli, Torbjørn (2018). Compact multispectral multi-camera imaging system for small UAVs. Full text in Research Archive
• Morrison, Aiden; Sokolova, Nadezda; Haavardsholm, Trym Vegard; Hagen, Ove Kent; Opsahl, Thomas Olsvik & Ånonsen, Kjetil Bergh [Show all 7 contributors for this article] (2017). Collaborative indoor navigation for emergency services personnel. Full text in Research Archive
• Morrison, Aiden; Haavardsholm, Trym Vegard; Hagen, Ove Kent; Opsahl, Thomas Olsvik; Ånonsen, Kjetil Bergh & Eriksen, Erik Holthe (2017). Collaborative Indoor Navigation for Emergency Services Personnel. Full text in Research Archive
• Skauli, Torbjørn; Torkildsen, Hans Erling; Kåsen, Ingebjørg; Nicolas, Stephane; Haavardsholm, Trym Vegard & Opsahl, Thomas Olsvik (2013). Multispectral and conventional imaging combined in a compact camera by using patterned filters in the focal plane. Full text in Research Archive
• Resta, Salvatore; Acito, Nicola; Diani, Marco; Corsini, Giovanni; Opsahl, Thomas Olsvik & Haavardsholm, Trym Vegard (2011). Detection of Small Changes in Airborne Hyperspectral Imagery: Experimental Results over Urban Areas. Full text in Research Archive
• Skauli, Torbjørn; Haavardsholm, Trym Vegard; Kåsen, Ingebjørg; Opsahl, Thomas Olsvik; Kavara, Amela & Skaugen, Atle (2011). Hyperspectral imaging technology and systems, exemplified by airborne real-time target detection. Full text in Research Archive
• Opsahl, Thomas Olsvik & Haavardsholm, Trym Vegard (2010). Real time direct georeferencing and orthorectification of images from airborne pushbroom cameras. Full text in Research Archive
• Haavardsholm, Trym Vegard; Winjum, Ingebrigt & Opsahl, Thomas Olsvik (2010). Real-time direct georeferencing of images from airborne line scan cameras. Full text in Research Archive Show summary

  The Norwegian Defense Research Establishment (FFI) is developing a technology demonstrator for airborne real-time hyperspectral target detection. The system includes two nadir-pointing line scan cameras. The line scanned images are georeferenced in real-time by intersecting rays cast from the cameras with a 3D model of the terrain underneath. The georeferenced images may then easily be ortho-rectified (e.g by using texture mapping in OpenGL) and overlaid digital maps. This poster presents the performance of a cuda implementation of the georeferencing method.

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