Classification and Localization of Vehicle Occupants Using 3D Range Images

University dissertation from Stockholm : KTH

Abstract: This thesis deals with the problem of classifying automotive vehicle occupants and estimating their position. This information is critical in designing future smart airbag systems providing maximum protection for passengers. According to the American National Highway Traffic Safety Administration (NHTSA), since 1990, in the USA, 227 deaths have been attributed to airbags deployed in low-speed crashes which included 119 children, and 22 infants. In these cases, intelligent deployment of the airbag, based on the type and position of occupant could have avoided these fatalities. Current commercial classification systems based on traditional sensors, such as pressure sensors are not able to detect the position of occupants. Vision-based systems are advantageous over pressure sensor based systems, as they can provide additional functionalities like dynamic occupant position analysis or child seat orientation detection. On the other hand, vision-based systems have to cope with several challenges, such as, illumination conditions, temperature, humidity, large variation of scenes, cost, and computational aspects.This thesis presents new pattern recognition techniques for classifying, localizing and tracking vehicle occupants using a low-resolution 3-D optical time-of-flight range camera. This sensor is capable of providing directly a dense range image, independent of the illumination conditions and object textures. Based on this technology, IEE S.A. is presently developing a camera system for the application of occupant classification. A prototype of this camera has been the basis for this study. The first part of the thesis presents the problem of occupant classification. Herein, we investigate geometric feature extraction methods to discriminate between different occupant types. We develop features that are invariant under rotation and translation. A method for reducing the size of the feature set is analyzed with emphasis on robustness and low computational complexity while maintaining highly discriminative information. In addition, several classification methods are studied including Bayes quadratic classifier, Gaussian Mixture Model (GMM) classifier and polynomial classifier. We propose the use of a cluster based linear regression classifier using a polynomial kernel which is particularly well suited to coping with large variations within each class. Full scale experiments have been conducted which demonstrate that a classification reliability of almost 100\% can be achieved with the reduced feature set in combination with a cluster based classifier.In this safety critical application, it is equally important to address the problem of reliability estimation for the system. State-of-the-art methods to estimate the reliability of the classification are based either on classification output or based on density estimation. The second part of the thesis treats estimation of the reliability of the pattern classification system. Herein, a novel reliability measure is proposed for classification output which takes into account the local density of training data. Experiments verify that this reliability measure outperforms state-of-the-art methods in many cases.Lastly, the problem of dynamically detecting out-of-position occupants is addressed in the third part of the thesis. This task requires detecting and localizing the position of the occupant's head. Traditional head detection methods, such as detecting head-like objects in the image by analyzing the local shapes are not robust with the current sensor. Many regions in a scene such as the shoulder or the elbow of the occupant can be incorrectly detected as the head. In order to cope with these challenges, we exploit topology information in the range image. A modified Reeb graph technique has been developed that extracts a topological skeleton of the 3D contour that is invariant under rotation and translations. Results verify that the Reeb graph detects successfully the head i.e., the head always corresponds to one of the end points of the skeleton. Subsequently, a data association algorithm to select the correct head candidate out of the Reeb graph candidates is presented. Results show that the resulting head detection algorithm based on Reeb graphs is robust under scene changes.