Photogrammetry
Department for Photogrammetry
Nussallee 15
53115 Bonn
Phone:
Email:
Function:
+49.228.73-2904
(em.)
Room Number: 11

Fields of interest

  • Image Understanding
  • Pattern Recognition
  • Spatial Reasoning
  • Statistics
  • Geomatics

Curriculum Vitae (pdf)

to appear 4/2015: Buch: Photogrammetric Computer Vision (W. Förstner, B. P. Wrobel

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Private

Music

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Recent Publications

2014

Katja Herzog and Ribana Roscher and Markus Wieland and Anna Kicherer and Thomas Läbe and Wolfgang Förstner and Heiner Kuhlmann and Reinhard Töpfer, "Initial steps for high-throughput phenotyping in vineyards", VITIS - Journal of Grapevine Research., January, 2014. Vol. 53(1), pp. 1-8. 2014.

The evaluation of phenotypic characters of grape- vines is required directly in the vineyard and is strongly limited by time, costs and the subjectivity of person in charge. Sensor-based techniques are prerequisite to al- low non-invasive phenotyping of individual plant traits, to increase the quantity of object records and to reduce error variation. Thus, a Prototype-Image-Acquisition- System (PIAS) was developed for semi-automated cap- ture of geo-referenced RGB images in an experimental vineyard. Different strategies were tested for image in- terpretation using Matlab. The interpretation of imag- es from the vineyard with the real background is more practice-oriented but requires the calculation of depth maps. Images were utilised to verify the phenotyping results of two semi-automated and one automated pro- totype image interpretation framework. The semi-auto- mated procedures enable contactless and non-invasive detection of bud burst and quantification of shoots at an early developmental stage (BBCH 10) and enable fast and accurate determination of the grapevine berry size at BBCH 89. Depending on the time of image ac- quisition at BBCH 10 up to 94 % of green shoots were visible in images. The mean berry size (BBCH 89) was recorded non-invasively with a precision of 1 mm.

@article{Herzog2014Initial,
  author = {Herzog, Katja and Roscher, Ribana and Wieland, Markus and Kicherer,Anna and L\"abe, Thomas and F\"orstner, Wolfgang and Kuhlmann, Heiner and T\"opfer, Reinhard},
  title = {Initial steps for high-throughput phenotyping in vineyards},
  journal = {VITIS - Journal of Grapevine Research},
  year = {2014},
  volume = {53},
  number = {1},
  pages = {1--8}
}

Lasse Klingbeil and Matthias Nieuwenhuisen and Johannes Schneider and Christian Eling and David Droeschel and Dirk Holz and Thomas Läbe and Wolfgang Förstner and Sven Behnke and Heiner Kuhlmann, "Towards Autonomous Navigation of an UAV-based Mobile Mapping System", In 4th International Conference on Machine Control & Guidance., March, 2014., pp. 136-147. 2014.

For situations, where mapping is neither possible from high altitudes nor from the ground, we are developing an autonomous micro aerial vehicle able to fly at low altitudes in close vicinity of obstacles. This vehicle is based on a MikroKopterTM octocopter platform (maximum total weight: 5kg), and contains a dual frequency GPS board, an IMU, a compass, two stereo camera pairs with fisheye lenses, a rotating 3D laser scanner, 8 ultrasound sensors, a real-time processing unit, and a compact PC for on-board ego-motion estimation and obstacle detection for autonomous navigation. A high-resolution camera is used for the actual mapping task, where the environment is reconstructed in three dimensions from images, using a highly accurate bundle adjustment. In this contribution, we describe the sensor system setup and present results from the evaluation of several aspects of the different subsystems as well as initial results from flight tests.

@inproceedings{Klingbeil2014Towards,
  author = {Klingbeil, Lasse and Nieuwenhuisen, Matthias and Schneider, Johannes and Eling, Christian and Droeschel, David and Holz, Dirk and L\"abe, Thomas and F\"orstner, Wolfgang and Behnke, Sven and Kuhlmann, Heiner},
  title = {Towards Autonomous Navigation of an UAV-based Mobile Mapping System},
  booktitle = {4th International Conference on Machine Control \& Guidance},
  year = {2014},
  pages = {136--147},
  url = {http://www.digibib.tu-bs.de/?docid=00056119}
}

Johannes Schneider and Wolfgang Förstner, "Real-time Accurate Geo-localization of a MAV with Omnidirectional Visual Odometry and GPS", In Proceedings of the 5th International Workshop on Computer Vision in Vehicle Technology (CVVT) with Special Session in Micro Aerial Vehicles (in conj. with ECCV), to appear. 2014.

This paper presents a system for direct geo-localization of a MAV in an unknown environment using visual odometry and precise real time kinematic (RTK) GPS information. Visual odometry is performed with a multi-camera system with four fisheye cameras that cover a wide field of view which leads to better constraints for localization due to long tracks and a better intersection geometry. Visual observations from the acquired image sequences are refined with a high accuracy on selected keyframes by an incremental bundle adjustment using the iSAM2 algorithm. The optional integration of GPS information yields long-time stability and provides a direct geo-referenced solution. Experiments show the high accuracy which is below 3 cm standard deviation in position.

@inproceedings{Schneider2014Accurate,
  author = {Schneider, Johannes and F\"orstner, Wolfgang},
  title = {Real-time Accurate Geo-localization of a MAV with Omnidirectional Visual Odometry and GPS},
  booktitle = {Proceedings of the 5th International Workshop on Computer Vision in Vehicle Technology (CVVT) with Special Session in Micro Aerial Vehicles (in conj. with ECCV), to appear},
  year = {2014},
  note = {to appear}
}

Johannes Schneider and Thomas Läbe and Wolfgang Förstner, "Real-Time Bundle Adjustment with an Omnidirectional Multi-Camera System and GPS", In Proceedings of the 4th International Conference on Machine Control & Guidance., pp. 98-103. 2014.

In this paper we present our system for visual odometry that performs a fast incremental bundle adjustment for real-time structure and motion estimation in an unknown scene. It is applicable to image streams of a calibrated multi-camera system with omnidirectional cameras. In this paper we use an autonomously flying octocopter that is equipped for visual odometry and obstacle detection with four fisheye cameras, which provide a large field of view. For real-time ego-motion estimation the platform is equipped, besides the cameras, with a dual frequency GPS board, an IMU and a compass. In this paper we show how we apply our system for visual odometry using the synchronized video streams of the four fisheye cameras. The position and orientation information from the GPS-unit and the inertial sensors can optionally be integrated into our system. We will show the obtained accuracy of pure odometry and compare it with the solution from GPS/INS.

@inproceedings{Schneider2014Realtime,
  author = {Schneider, Johannes and L\"abe, Thomas and F\"orstner, Wolfgang},
  title = {Real-Time Bundle Adjustment with an Omnidirectional Multi-Camera System and GPS},
  booktitle = {Proceedings of the 4th International Conference on Machine Control \& Guidance},
  year = {2014},
  pages = {98--103},
  url = {http://www.digibib.tu-bs.de/?docid=00056119}
}

2013

Timo Dickscheid and Wolfgang Förstner, "A Trainable Markov Random Field for Low-Level Image Feature Matching with Spatial Relationships", Photogrammetrie, Fernerkundung, Geoinformation (PFG). Vol. 4, pp. 269-284. 2013.

Many vision applications rely on local features for image analysis, notably in the areas of object recognition, image registration and camera calibration. One important example in photogrammetry are fully automatic algorithms for relative image orientation. Such applications rely on a matching algorithm to extract a sufficient number of correct feature correspondences at acceptable outlier rates, which is most often based on the similarity of feature descriptions. When the number of detected features is low, it is advisable to use multiple feature detectors with complementary properties. When feature similarity is not sufficient for matching, spatial feature relationships provide valuable information. In this work, a highly generic matching algorithm is proposed which is based on a trainable Markov random field (MRF). It is able to incorporate almost arbitrary combinations of features, similarity measures and pairwise spatial relationships, and has a clear statistical interpretation. A major novelty is its ability to compensate for weaknesses in one information cue by implicitely exploiting the strengths of others.

@article{Dickscheid2013Trainable,
  author = {Dickscheid, Timo and F\"orstner, Wolfgang},
  title = {A Trainable Markov Random Field for Low-Level Image Feature Matching with Spatial Relationships},
  journal = {Photogrammetrie, Fernerkundung, Geoinformation (PFG)},
  year = {2013},
  volume = {4},
  pages = {269--284},
  doi = {10.1127/1432-8364/2013/0176}
}

Wolfgang Förstner, "Graphical Models in Geodesy and Photogrammetry", Photogrammetrie, Fernerkundung, Geoinformation (PFG). Vol. 4, pp. 255-268. 2013.

The paper gives an introduction into graphical models and their use in specifying stochastic models in geodesy and photogrammetry. Basic task in adjustment theory can intuitively be described and analysed using graphical models. The paper shows that geodetic networks and bundle adjustments can be interpreted as graphical models, both as Bayesian networks or as conditional random fields. Especially hidden Markov random fields and conditional random fields are demonstrated to be versatile models for parameter estimation and classification.

@article{Foerstner2013Graphical,
  author = {F\"orstner, Wolfgang},
  title = {Graphical Models in Geodesy and Photogrammetry},
  journal = {Photogrammetrie, Fernerkundung, Geoinformation (PFG)},
  year = {2013},
  volume = {4},
  pages = {255--268},
  doi = {10.1127/1432-8364/2013/0175}
}

Wolfgang Förstner, "Photogrammetrische Forschung - Eine Zwischenbilanz aus Bonner Sicht", Photogrammetrie, Fernerkundung, Geoinformation (PFG). Vol. 4, pp. 251-254. 2013.

Photogrammetrische Forschung - Eine Zwischenbilanz aus Bonner Sicht

@article{Foerstner2013Photogrammetrische,
  author = {F\"orstner, Wolfgang},
  title = {Photogrammetrische Forschung - Eine Zwischenbilanz aus Bonner Sicht},
  journal = {Photogrammetrie, Fernerkundung, Geoinformation (PFG)},
  year = {2013},
  volume = {4},
  pages = {251--254},
  doi = {10.1127/1432-8364/2013/0186}
}

A. Kicherer and R. Roscher and K. Herzog and S. Šimon and W. Förstner and R. Töpfer, "BAT (Berry Analysis Tool): A high-throughput image interpretation tool to acquire the number, diameter, and volume of grapevine berries", Vitis. Vol. 52(3), pp. 129-135. 2013.

QTL-analysis (quantitative trait loci) and marker development rely on efficient phenotyping techniques. Objectivity and precision of a phenotypic data evaluation is crucial but time consuming. In the present study a high-throughput image interpretation tool was developed to acquire automatically number, size, and volume of grape berries from RGB (red-green-blue) images. Individual berries of one cluster were placed on a defined construction to take a RGB image from the top. The image interpretation of one dataset with an arbitrary number of images occurs automatically by starting the BAT (Berry-Analysis-Tool) developed in MATLAB. For validation of results, the number of berries was counted and their size was measured using a digital calliper. A measuring cylinder was used to determine reliably the berry volume by displacement of water. All placed berries could be counted by BAT 100% correctly. Manual ratings compared with BAT ratings showed strong correlation of r=0,964 for mean berry diameter/image and r=0.984 for berry volume.

@article{Kicherer2013,
  author = {Kicherer, A. and Roscher, R. and Herzog, K. and {\vS}imon, S. and F\"orstner, W. and T\"opfer, R.},
  title = {BAT (Berry Analysis Tool): A high-throughput image interpretation tool to acquire the number, diameter, and volume of grapevine berries},
  journal = {Vitis},
  year = {2013},
  volume = {52},
  number = {3},
  pages = {129-135}
}

Falko Schindler and Wolfgang Förstner, "DijkstraFPS: Graph Partitioning in Geometry and Image Processing", Photogrammetrie, Fernerkundung, Geoinformation (PFG). Vol. 4, pp. 285-296. 2013.

Data partitioning is a common problem in the field of point cloud and image processing applicable to segmentation and clustering. The general principle is to have high similarity of two data points, e.g.pixels or 3D points, within one region and low similarity among regions. This pair-wise similarity between data points can be represented in an attributed graph. In this article we propose a novel graph partitioning algorithm. It integrates a sampling strategy known as farthest point sampling with Dijkstra's algorithm for deriving a distance transform on a general graph, which does not need to be embedded in some space. According to the pair-wise attributes a Voronoi diagram on the graph is generated yielding the desired segmentation. We demonstrate our approach on various applications such as surface triangulation, surface segmentation, clustering and image segmentation.

@article{Schindler2013DijkstraFPS,
  author = {Schindler, Falko and F\"orstner, Wolfgang},
  title = {DijkstraFPS: Graph Partitioning in Geometry and Image Processing},
  journal = {Photogrammetrie, Fernerkundung, Geoinformation (PFG)},
  year = {2013},
  volume = {4},
  pages = {285--296},
  doi = {10.1127/1432-8364/2013/0177}
}

Johannes Schneider and Wolfgang Förstner, "Bundle Adjustment and System Calibration with Points at Infinity for Omnidirectional Camera Systems", Z. f. Photogrammetrie, Fernerkundung und Geoinformation. Vol. 4, pp. 309-321. 2013.

We present a calibration method for multi-view cameras that provides a rigorous maximum likelihood estimation of the mutual orientation of the cameras within a rigid multi-camera system. No calibration targets are needed, just a movement of the multi-camera system taking synchronized images of a highly textured and static scene. Multi-camera systems with non-overlapping views have to be rotated within the scene so that corresponding points are visible in different cameras at different times of exposure. By using an extended version of the projective collinearity equation all estimates can be optimized in one bundle adjustment where we constrain the relative poses of the cameras to be fixed. For stabilizing camera orientations - especially rotations - one should generally use points at the horizon within the bundle adjustment, which classical bundle adjustment programs are not capable of. We use a minimal representation of homogeneous coordinates for image and scene points which allows us to use images of omnidirectional cameras with single viewpoint like fisheye cameras and scene points at a large distance from the camera or even at infinity. We show results of our calibration method on (1) the omnidirectional multi-camera system Ladybug 3 from Point Grey, (2) a camera-rig with five cameras used for the acquisition of complex 3D structures and (3) a camera-rig mounted on a UAV consisting of four fisheye cameras which provide a large field of view and which is used for visual odometry and obstacle detection in the project MoD (DFG-Project FOR 1505 "Mapping on Demand").

@article{Schneider2013Bundle,
  author = {Schneider, Johannes and F\"orstner, Wolfgang},
  title = {Bundle Adjustment and System Calibration with Points at Infinity for Omnidirectional Camera Systems},
  journal = {Z. f. Photogrammetrie, Fernerkundung und Geoinformation},
  year = {2013},
  volume = {4},
  pages = {309--321},
  doi = {10.1127/1432-8364/2013/0179}
}

Susanne Wenzel and Wolfgang Förstner, "Finding Poly-Curves of Straight Line and Ellipse Segments in Images", Photogrammetrie, Fernerkundung, Geoinformation (PFG). Vol. 4, pp. 297-308. 2013.

Simplification of given polygons has attracted many researchers. Especially, finding circular and elliptical structures in images is relevant in many applications. Given pixel chains from edge detection, this paper proposes a method to segment them into straight line and ellipse segments. We propose an adaption of Douglas-Peucker's polygon simplification algorithm using circle segments instead of straight line segments and partition the sequence of points instead the sequence of edges. It is robust and decreases the complexity of given polygons better than the original algorithm. In a second step, we further simplify the poly-curve by merging neighbouring segments to straight line and ellipse segments. Merging is based on the evaluation of variation of entropy for proposed geometric models, which turns out as a combination of hypothesis testing and model selection. We demonstrate the results of tt circlePeucker as well as merging on several images of scenes with significant circular structures and compare them with the method of sc Patraucean et al. (2012).

@article{Wenzel2013Finding,
  author = {Wenzel, Susanne and F\"orstner, Wolfgang},
  title = {Finding Poly-Curves of Straight Line and Ellipse Segments in Images},
  journal = {Photogrammetrie, Fernerkundung, Geoinformation (PFG)},
  year = {2013},
  volume = {4},
  pages = {297--308},
  doi = {10.1127/1432-8364/2013/0178}
}

Johannes Schneider and Thomas Läbe and Wolfgang Förstner, "Incremental Real-time Bundle Adjustment for Multi-camera Systems with Points at Infinity", In ISPRS Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XL-1/W2, pp. 355-360. 2013.

This paper presents a concept and first experiments on a keyframe-based incremental bundle adjustment for real-time structure and motion estimation in an unknown scene. In order to avoid periodic batch steps, we use the software iSAM2 for sparse nonlinear incremental optimization, which is highly efficient through incremental variable reordering and fluid relinearization. We adapted the software to allow for (1) multi-view cameras by taking the rigid transformation between the cameras into account, (2) omni-directional cameras as it can handle arbitrary bundles of rays and (3) scene points at infinity, which improve the estimation of the camera orientation as points at the horizon can be observed over long periods of time. The real-time bundle adjustment refers to sets of keyframes, consisting of frames, one per camera, taken in a synchronized way, that are initiated if a minimal geometric distance to the last keyframe set is exceeded. It uses interest points in the keyframes as observations, which are tracked in the synchronized video streams of the individual cameras and matched across the cameras, if possible. First experiments show the potential of the incremental bundle adjustment wrt time requirements. Our experiments are based on a multi-camera system with four fisheye cameras, which are mounted on a UAV as two stereo pairs, one looking ahead and one looking backwards, providing a large field of view.

@inproceedings{Schneider2013Incremental,
  author = {Schneider, Johannes and L\"abe, Thomas and F\"orstner, Wolfgang},
  title = {Incremental Real-time Bundle Adjustment for Multi-camera Systems with Points at Infinity},
  booktitle = {ISPRS Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences},
  year = {2013},
  volume = {XL-1/W2},
  pages = {355-360},
  url = {http://www.int-arch-photogramm-remote-sens-spatial-inf-sci.net/XL-1-W2/355/2013/isprsarchives-XL-1-W2-355-2013.pdf},
  doi = {10.5194/isprsarchives-XL-1-W2-355-2013}
}

Susanne Wenzel and Wolfgang Förstner, "Finding Poly-Curves of Straight Line and Ellipse Segments in Images", July, 2013.(TR-IGG-P-2013-02) 2013.

Simplification of given polygons has attracted many researchers. Especially, finding circular and elliptical structures in images is relevant in many applications. Given pixel chains from edge detection, this paper proposes a method to segment them into straight line and ellipse segments. We propose an adaption of Douglas-Peucker's polygon simplification algorithm using circle segments instead of straight line segments and partition the sequence of points instead the sequence of edges. It is robust and decreases the complexity of given polygons better than the original algorithm. In a second step, we further simplify the poly-curve by merging neighbouring segments to straight line and ellipse segments. Merging is based on the evaluation of variation of entropy for proposed geometric models, which turns out as a combination of hypothesis testing and model selection. We demonstrate the results of tt circlePeucker as well as merging on several images of scenes with significant circular structures and compare them with the method of sc Patraucean et al. (2012).

@techreport{Wenzel2013FindingTR,
  author = {Wenzel, Susanne and F\"orstner, Wolfgang},
  title = {Finding Poly-Curves of Straight Line and Ellipse Segments in Images},
  year = {2013},
  number = {TR-IGG-P-2013-02}
}
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