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Authors: Nekouei Shahraki, Mehrdad
Title: Design and development of a calibration solution feasible for series production of cameras for video-based driver-assistant systems
Issue Date: 2022 Dissertation 149
Abstract: In this study, we reviewed the current techniques and methods in photogrammetry - especially close-range photogrammetry - and focused on camera calibration. We reviewed the new evolving field of video-based driver-assistant systems, their requirements and their applications. Exclusively of fisheye cameras and a general omnidirectional projection, we extended an existing camera calibration model to address our needs and functionality requirements. These extensions enable us to use the camera calibration model in real-time embedded mobile systems with low processing power. We also introduced the free-function model as a flexible and advantageous model for camera distortion modelling. This is a new approach for modelling the overall image distortion together with the local lens distortions that are estimated using a standard model during the calibration process. Using free-function model on different lens designs, one can achieve good calibration accuracies by modelling the very local lens distortion taking benefit from the flexibility of this model. We introduced optimization strategies for recalculation and image rectification. These optimizations are also used to minimize the amount of required processing power and device memory. This brings many advantages to variety of computational platforms such as FPGAs, x86 and ARM processors, and makes it possible to benefit from variety of parallel-processing techniques. This model is capable of being used in runtime and is an ideal calibration model for using in variety of machine vision solutions. We also discussed several important requirements for accurate camera calibration that we later used in hardware test stand design phase. We designed and developed two different test stands in order to realize the specifications and geometrical features of multiple-view test-field-based camera calibration referred to as bundle-block calibration. One of their special geometrical characteristics is the uniform point distribution, which corresponds to the uniform motion. Such a point distribution is beneficial when using calibration models such as free-function model that enable us to model of local lens distortion with good accuracy and quality all over the image. A very important feature of this test stand is having the capability of performing camera/sensor alignment testing, a feature which is very important for testing the geometrical alignment of the internal mechanical elements of each camera. Using automated machines and algorithms in test stand calibration increased the stability and accuracy of the calibration and thus ensured the quality and speed of the calibration for cameras. These test stands are capable of performing automatic camera calibration, suitable for applications such as series-production of cameras. As an accuracy -and flexibility evaluation step for the free-function model, we tested the free-function calibration model on real-world data using a stereo camera with added large local distortions taking images from a front vehicle similar to the conditions where real-world use-cases are defined. By performing the camera calibration, we compared the calibration results and accuracy parameters of the free-function model to a conventional calibration model. Using these calibration results, we generated a set of disparity maps and compared their density and availability, especially on the areas where the local distortion was present. We used this test to compare the capabilities of the proposed model to conventional ones in real-wold situations where large optical distortions could be present that cannot be easily modelled with conventional calibration models. The higher modelling capability and accuracy of the free-function model will generally influence those functions that are using the information of the disparity map or the derived 3D information as part of their input data and potentially leads to the better functionality or even their availability if local distortions are present in the image. There are many more use-cases in photogrammetry and computer-vision where a higher calibration accuracy is beneficial on hardware such as low-cost optics where sometimes optical distortion are available that cannot easily be modelled with classical models. These use-cases could all benefit from the flexibility and modelling accuracy of the free-function model.
Appears in Collections:06 Fakultät Luft- und Raumfahrttechnik und Geodäsie

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