OBJECT RECOGNITION WITH VIDEO-THEODOLITES AND WITHOUT TARGETING THE OBJECT

At the Depart ment of Applied Geodes y and Engineering Geodes y (TU Vienna) an new kind of theodolite measurement s ystem is under development, enableing measurements with an accuracy of 1:30.000 with and without targeting the object. The main goal is, to develop an intelligent multi-sensor s ystem. Thus an operator is onl y needed to supervise the s ystem. Results are gained on-line and can be stored in a CAD s ystem. If no artificial targets are used identification of points has to be performed b y the Master-Theodolite. The method, used in our project, is based on interest operators. The Slave-Theodolite has to track the master b y searching for homologous regions. The before described method can onl y be used, if there is some texture on the surface of the object. If that is not fulfilled, a “gridline-method” can be used, to get informations about the surface of the object. In the case of a Kartesian co-ordinate s yst m, for instance, the gridlines can be chosen b y the operator before the measurement process is started. The theodolitemeasurement s y tem is then able to detect the grid-lines and to find the positions where the gridlines intersect the surface of the object. 1. COMPUTER CONTROLLED ROTATING CCD-CAMERAS A theodolite measurement s ystem makes use of the intersections provided b y a series of pointings to the same points (targets) b y several theodolites to determine co-ordinates of the points measured. For the computation of the point co-ordinates, we use the angle readings on the scale circles of the theodolites pointing to the target. The co-ordinates of the measured points and of the instrument stations are determined b y three-dimensional network adjustment. The s ystem is oriented in a given co-ordinate s ystem by reference to control points which form part of that system. In addition these control points provide the scale of the s yst m. The measurement s y tem is a combination of different components: theodolites used as sensors, computer system, software, accessories. The sensors used to capture data are computer controlled video-theodolites. A video-theodolite has a CCD-camera in its optical path (fig.1). The horizontal and vertical axes carrying the telescope and the CCD-camera, are driven by motors. The motors are controlled by a computer. Fig.1:Optical path of a video-theodolite The images of the telescope's visual field are projected onto the camera's CCD-chip. They consist of the target object and a frame in the focus plain of the telescope replacing the standard telescope reticule. The CCD-camera is capable of capturing mosaic panoramic images through camera rotation. With appropriate calibration these images are accurately georeferenced and orientated as the horizontal and vertical angles of rotation are continuously measured by electronic angle measuring systems and fed into the computer. The oriented images can then be used directly for direction measurements with no need for object control points or photogrammetric orientation processes. The image resolution can be chosen by selecting different camera lenses and should be limited only by the angular precision of the total station. In practise, the camera system should have two camera lens systems. A wide angle system should provide overview images to support the organization of the measurements and a narrow angle system which provides images for the measurements. The focal length of the narrow angle lense system should be chosen so that the image pixel angle is compatible with the angular precision of the theodolite. In addition to the motor drives for both theodolite axes, the focusing drive is also motorized. The search for an optimal autofocus distance is controlled by an autofocus function. With the before described measurement process normally only a small part of the information, produced by the camera for the detection of targets, is used. Therefore a new type of measurement system was under investigation, which enables not only fast and accurate pointing of targets but in addition allows object reconstruction without targeting by using nearly all the information the images contain. In the following the methods enableing objectreconstruction without targeting will be described. 2. AN INTERACTIVE MEASUREMENT SYSTEM – A FIRST STEP The key element of the first system is a vision software which supports the operator to find ``natural targets'' (extracted features) on the object [Roic, M. , 1996]. The diagram of the videometric system is shown in fig.2 . The main steps are image formation, image preprocessing, iconical image processing, image feature extraction, image interpretation and image analysis. Image formation involves the light source and the object, reflecting the light. The optical image is transformed into a two-dimensional function of the object and stored as a matrix of gray values. The hardware contains amplifiers, A/D-converters, framegrabbers and other devices. The preprocessing software enables the improvement of the radiometric quality of the images and enhancement. Enhancement includes improvement of the geometric quality of the image and data reduction by applying digital filters to support the feature extraction and positioning phase. Iconical image processing is used to create new images, finally resulting in sets, only containing the desired information. Image feature extraction (segmentation) comprices searching the objects of interest from the rest of the scene with the aim of partitioning the image into various clusters. Thresholding is a special method in region segmentation assigning ``white'' to each pixel in the image with gray scale above a particular value. All pixels below this become ``black'' (fig.3). Finally the operator has to extract feature information from the enhanced and segmented images and to analyse, if they can be used as targets. There is no standard telescope reticule in the telescope's visual field, it is replaced by a reference frame. Therefore, for interactive pointing a recticule (identical with the co-ordinate axes of the telescope co-ordinate system) has to be added to the optical system by special image processing software. This recticule then can be used for the alignment of artificial and natural targets on the object. To test the measurement system, a facade was monitored. Instead of targeting the videometric system, described in fig.2, was used. Sixteen selected edges were monitored (fig.3a). To enhance the edges a Sobel-operator was used combined with histogram equalization and thresholding (fig.3b and c). Fig.4 depicts the standard deviations of the horizontal directions, we got, after we had monitored the detected edges. The facade was monitored with two video-theodolites. The distance between the object and the theodolites was about 33 m; that leads to an accuracy of the determined edges of ± 2 mm (1 σ). The accuracy was only limited by the roughness of the surface. In addition, the observations were performed only with the standard theodolite without using image processing. Then the standard deviations increased to about ± 5 mm (1 σ). Other projects showed that the videometric system can also sucessfully be used for extremely accurate measurement and shape fit of small industrial objects. Accuracies of about 0.1 mm (1 σ) are possible [Kahmen, H. , Roic, M. , 1995], [Roic, M. , 1996]. The results encouraged us, to start with the development of an automatic system. Fig.2:Diagram of videometric system of the theodolites