To successfully segment a transmission x-ray image the inherent blurring or structural unsharpness present in such an image must be taken into account. Initial work with an established wavelet-based segmentation algorithm resulted in an oversensitive performance with regard to the number of segmented regions detected. To improve on this performance, two segmentation algorithms are developed and tested. The approaches utilize a wavelet transform together with region merging and, alternatively, multilevel thresholding with region merging. These techniques outperform the ordinary wavelet method. Manually segmented images representing ground truth are used for a comparative evaluation of the segmentation techniques. Our work is part of an ongoing collaborative research program with the U.K. Home Office to analyze information extracted from dual-energy x-ray images for aviation security screening applications.
The X-ray screening of luggage by aviation security personnel may be badly hindered by the lack of visual cues to depth in an image that has been produced by transmitted radiation. Two-dimensional "shadowgraphs" with "organic" and "metallic" objects encoded using two different colors (usually orange and blue) are still in common use. In the context of luggage screening there are no reliable cues to depth present in individual shadowgraph X-ray images. Therefore, the screener is required to convert the 'zero depth resolution' shadowgraph into a three-dimensional mental picture to be able to interpret the relative spatial relationship of the objects under inspection. Consequently, additional cognitive processing is required e.g. integration, inference and memory. However, these processes can lead to serious misinterpretations of the actual physical structure being examined. This paper describes the development of a stereoscopic imaging technique enabling the screener to utilise binocular stereopsis and kinetic depth to enhance their interpretation of the actual nature of the objects under examination. Further work has led to the development of a technique to combine parallax data (to calculate the thickness of a target material) with the results of a basis material subtraction technique to approximate the target's effective atomic number and density. This has been achieved in preliminary experiments with a novel spatially interleaved dual-energy sensor which reduces the number of scintillation elements required by 50% in comparison to conventional sensor configurations.
A novel 3D X-ray imaging technique to enhance the visual interpretation of complex X-ray images routinely encountered in aviation security applications has been developed. The 3D information is visualised as a smooth object rotation on a video display monitor. Further work enabled motion parallax to be combined with binocular parallax to produce a dynamic stereoscopic display. This imaging technique is equally applicable to both standard monochrome X-ray imaging and dual-energy X-ray imaging. The latter exploits the difference in magnitude between a high energy X-ray signal and a low energy X-ray signal to compute materials discrimination information made available to the human observer by colour encoding the resultant images. To produce the required image data an integrated dual-energy X-ray camera incorporating a novel castellated dual-energy scintillator arrangement has been developed.
A novel multiple view line-scan imaging technique that can be applied to transmission x-ray imaging as well as reflected light cameras is presented. In either case an area array image sensor is treated as a contiguous set of precisely arranged line-scan devices utilizing a single perspective center. IN the case of reflected light the perspective center is the nodal point of a lens whilst in the x-ray case it is the focal spot of an x-ray source. The line-scan images are accumulated in digital memory whilst the object under inspection is linearly translated through the field of view of the camera. In this way a number of perspective images, typically 6 to 16 are produced. The 3D information inherent in the perspective views can be visualized as a smooth object rotation or as a dynamic binocular stereoscopic sequence of views.
Sensor choice is critical in all object inspection imaging systems and the suitability of different types of sensor must be thoroughly assessed before a decision is made. Despite its wide use, a television type camera may not represent the best choice for certain object inspection applications. Specifically, when inspection of an object that has a degree of cylindrical symmetry is required, a line-scan camera is a viable alternative to the television type and, in general, any matrix camera, offering a number of unique advantages. By applying rotational motion to the object of interest and using a line-scan device, an 360 degree(s) view of the object is obtained. The cylindrical surface of the object is effectively unfolded into a planar 2D one, allowing for the efficient inspection of the entire surface of the object from a single, continuous image. To allow accurate object space co-ordinate measurement, a line-scan camera calibration technique has been developed, catering for both interior and exterior parameter calibration. The former accounts for the lens effective focal length, the pierce-pixel value and the timing of the line-scan camera, while the latter yields the relative position and orientation of the camera with respect to a reference object space co-ordinate system.
This paper describes on-going research into the development of a 21/2D image modeling technique based on the extraction of relative depth information from stereoscopic x-ray images. This research was initiated in order to aid operators of security x-ray screening equipment in the interpretation of complex radiographic images. It can be shown that a stereoscopic x-ray image can be thought of as a series of depth planes or slice images which are similar in some respects to tomograms produced by computed tomography systems. Thus, if the slice images can be isolated the resulting 3D data set can be used for image reconstruction. Conceptually, the production of a 21/2D image from a stereoscopic image can be thought of as the process of replacing the physiological depth cue of binocular parallax, inherent in a stereoscopic image, with the psychological depth cues such as occlusion and rotation. Once the data is represented in this form it is envisaged that, for instance in the case of a security imaging scenario a suspicious object could be electronically unpacked. The work presented in this paper is based on images obtained from a stereoscopic folded array dual energy x-ray screening system, designed and developed by the Nottingham Trent University group.
The interpretation of standard 2D X-ray images by humans is often very difficult due to the lack of visual cues to depth in an image produced by transmitted radiation. The 3D Imaging Group has previously developed stereoscopic X-ray systems providing binocular parallax as a depth cue to aid images interpretation. The stereoscopic images produced have proven suitable for human viewing and allow the observer to determine the relative position of objects within the scene under consideration. Such additional information is useful for scene interpretation and understanding. The binocular parallax introduced into X-ray images can be utilized in a similar way to television type stereoscopic systems where the disparity is used to determine the range of objects within the scene. This range information can be used in a number of ways, for instance co-ordinate measurement. Current research at Nottingham has concentrated on grouping object points of similar depth and producing a series of contiguous slices through the scene of interest. The purpose of producing this new data base is to combine this and existing reconstruction software used in CAT scanning techniques to provide a 21/2D visualization of the observed scene or object. This representation of the scene is intended to introduce an alternative view to the observer, further enhancing their interpretation ability.
This paper describes continuing work with three-dimensional (3-D) rotating line-scan vision systems in robotics and measurement. Mathematical algorithms have been developed for use with the line-scan arrangement allowing the extraction of three-dimensional co-ordinate information from an observed object workspace. To determine the measurement capability of the stereoscopic system, comparison is made between the system output data (calculated from image space values) and a calibrated volume in object space containing a distribution of target points. This paper describes the mathematical model and results pertaining to the current research demonstrate the use of the rotating line-scan scenario to the solution of specific applications where conventional sensor modalities may not be appropriate.
We describe research carried out to investigate a stereoscopic line-scan system for the extraction of 3-D coordinate information from a scene of interest. Initial work involved analyzing the operating characteristics of a line-scan device for the production of 2-D images. Following this, a theoretical appraisal was undertaken of a sensor in a stereoscopic arrangement, and a mathematical model was derived for the calibration of a novel camera system. Algorithms to determine the 3-D relationship of points in the object space were developed using this model. To test the suitability of the model, a complete stereoscopic line-scan system was constructed. Experiments were conducted with the stereo camera to establish the accuracy that is achieved with such a system using the developed algorithms. The results indicate that the relative position of points in the object space could be determined to an accuracy of less than 1 mm at a range of 1.5 m.
This paper describes the operating characteristics of a line-scan system in both two- dimensional and stereoscopic arrangements. Details are given on algorithms that have been developed to extract range information and results are presented indicating the accuracies obtained in all three coordinate axes. The application of this stereoscopic system to both semi- autonomous and autonomous mobile platform arrangements is discussed and the advantages and disadvantages of incorporating such a system are identified.
This paper describes continuing work with both 2-D and 3-D line-scan vision systems using rotation of both the camera and of an object to produce 2-D images. A rotating line-scan camera arrangement has been used to produce images of an area surrounding the camera stage. Due to the nature of the movement parameter these images can be arranged to cover a variable field of view in the movement axis, i.e.: from as small as 5 degree(s) up to 360 degree(s). This same arrangement has been used to produce images of a rotating object allowing a single 2-D image to contain information from the front, back and both sides of the object of interest. This 2-D system is to be extended into a full stereoscopic line-scan vision system. This will enable an investigation into extracting 3-D coordinate information from both the rotating camera and the rotating object.
This paper describes on-going research into 3-D x-ray imaging techniques conducted by the 3- D Imaging Group at The Nottingham Trent University. This work was initiated to enhance the visual interpretation of complex x-ray images, specifically in response to problems encountered in the routine screening of freight and hand luggage at airports. The interpretation of standard 2-D x-ray images by humans is difficult due to the lack of visual cues to depth in an image produced by transmitted radiation. The solution proposed is to introduce binocular parallax, a powerful physiological depth cue, into the resultant shadowgraph x-ray images. This is accomplished by implementing a stereoscopic imaging technique specifically developed by the Nottingham group for use with linear x-ray detector arrays and has culminated in the development of two experimental machines. Current research involves the investigation of techniques that allow the extraction of the 3-D data contained in the stereoscopic images such that `slice' images can be obtained. This data set may then be used with existing reconstruction software utilized by CAT scanning techniques.
It is the intention of this paper to give details of the continuing research into obtaining 3-D coordinate information from an object space using non-standard video sources. Details are given on producing images with the line-scan sensor by rotating the device relative to an object space. Theoretically, this could provide picture information from a potential 360 degree panoramic view. However, initial results have demonstrated that such images are difficult for humans to interpret. Details are given in this paper on the limitations of the line-scan images produced from camera rotation for presentation to human observers.
A program of work has been carried out over a number of years to develop X-ray equipment having a full three-dimensional (i.e., binocular stereoscopic) capability. Early equipment produced for H.M. Customs and Excise was based on basic line-scan technology. More recently a system has been developed in collaboration with the PSDB, Home Office, which uses folded array line-scan sensors and has a materials identification capability. Since the information contained in these images can be assumed to exist on a number of identifiable depth planes it can be manipulated in the same way as the slice data available from computed tomography (CT) type equipment. A considerable amount of software is available for use with such CT data which enables image models to be built. The current program of work involves interfacing the new 3-D X-ray technology to the existing software routines in an attempt to automatically produce 2 1/2-D image models from the full stereoscopic (3-D) information.
This paper describes research into 3D line-scan systems for applications covering inspection and both autonomous and semi-autonomous control. Image generation using line-scan devices requires relative motion between camera and object. The type of motion investigated has concentrated on lateral displacement and the rotation of the line-scan device in relation to a static object or scene. Results are presented on the accuracy of the lateral displacement arrangement in resolving the position of points in the object space in all three co-ordinate axes.
X-ray images by their very nature are difficult to interpret. For most security applications these images have been generated by using Linear Array type imaging sensors, and presented on standard video monitors. A major problem for the observer is the lack of three dimensional information present. It must be remembered that the images are essentially shadows which have been projected onto a plane by transmitted radiation. Therefore, the cues to depth which we say associate with a normal photograph (i.e. produced by reflected light), such as, occlusion and to some extent linear perspective, are missing. The loss of depth cues can, and does, cause serious ambiguities to arise in the interpretation of complex x-ray images.
This paper describes on-going research into machine vision systems based on the line-scan or linear array type cameras. Such devices have been used successfully in the production line environment, as the inherent movement within the manufacturing process can be utilized for image production. However, applications such as these have traditionally involved using the line-scan device in a purely two-dimensional role. Initial research was carried out to extend such 2-D arrangements into a 3-D system, retaining the lateral motion of the object with respect to the camera. The resulting stereoscopic camera allowed three-dimensional coordinate data to be extracted from a moving object volume (workspace). The most recent work has involved rotating line-scan systems in relation to a static scene. This allows images to be produced with fields of view varying in both size and position in the rotation. Due to the nature of the movement the images can be complex dependent on the size of the field of view selected. Benefits of obtaining images in this fashion include `all-round' observation, variable resolution in the movement axis, and a calibrated volume that can be moved to observe any point in a 360 degree arc.
The Fourier Transform Raman and Infrared spectra of softwoods and temperate and tropical hardwoods have been studied. The differences observed between the spectra are discussed in terms of the composition of the lignins contained in the woods.
Recent results obtained from a stereoscopic-vision system incorporating line-scan sensors are described. The research forms a part of the continuing program of work in both human- and machine-vision systems carried out a Nottingham Polytechnic. Line-scan sensors have been used extensively in shop-floor environments, for applications as diverse as pattern matching in the textile industry to quality inspection of printed circuit boards. However, all of this work has involved the use of a single line-scan device in a two-dimensional role. It is the author's intention to show that a logical progression of this previous research is to construct a stereoscopic sensor using line-scan devices and so enable three-dimensional coordinate information to be obtained from a moving object volume.