We present a line-scan stereo system and descriptor-based dense stereo matching for high-performance vision applications. The stochastic binary local descriptor (STABLE) descriptor is a local binary descriptor that builds upon the principles of compressed sensing theory. The most important properties of STABLE are the independence of the descriptor length from the matching window size and the possibility that more than one pair of pixels contributes to a single-descriptor bit. Individual descriptor bits are computed by comparing image intensities over pairs of balanced random subsets of pixels chosen from the whole described area. On a synthetic as well as real-world examples, we demonstrate that STABLE provides competitive or superior performance than other state-of-the-art local binary descriptors in the task of dense stereo matching. The real-world example is derived from line-scan binocular stereo imaging, i.e., two line-scan cameras are observing the same object line and 2-D images are generated due to relative motion. We show that STABLE performs significantly better than the census transform and local binary patterns (LBP) in all considered geometric and radiometric distortion categories to be expected in practical applications of stereo vision. Moreover, we show as well that STABLE provides comparable or better matching quality than the binary robust-independent elementary features descriptor. The low computational complexity and flexible memory footprint make STABLE well suited for most hardware architectures. We present quantitative results based on the Middlebury stereo dataset as well as illustrative results for road surface reconstruction.
Authentication and optical verification of travel documents upon crossing borders is of utmost importance for national security. Understanding the workflow and different approaches to ICAO 9303 travel document scanning in passport readers, as well as highlighting normalization issues and designing new methods to achieve better harmonization across inspection devices are key steps for the development of more effective and efficient next- generation passport inspection. This paper presents a survey of state-of-the-art document inspection systems, showcasing results of a document reader challenge investigating 9 devices with regards to optical characteristics.
Diffractive Optically Variable Image Devices (DOVIDs), sometimes loosely referred to as holograms, are popular security features for protecting banknotes, ID cards, or other security documents. Inspection, authentication, as well as forensic analysis of these security features are still demanding tasks requiring special hardware tools and expert knowledge. Existing equipment for such analyses is based either on a microscopic analysis of the grating structure or a point-wise projection and recording of the diffraction patterns. We investigated approaches for an examination of DOVID security features based on sampling the Bidirectional Reflectance Distribution Function (BRDF) of DOVIDs using photometric stereo- and light-field-based methods. Our approach is demonstrated on the practical task of automated discrimination between genuine and counterfeited DOVIDs on banknotes. For this purpose, we propose a tailored feature descriptor which is robust against several expected sources of inaccuracy but still specific enough for the given task. The suggested approach is analyzed from both theoretical as well as practical viewpoints and w.r.t. analysis based on photometric stereo and light fields. We show that especially the photometric method provides a reliable and robust tool for revealing DOVID behavior and authenticity.
KEYWORDS: Cameras, Sensors, Imaging systems, Line scan image sensors, 3D acquisition, Inspection, 3D modeling, Computed tomography, Machine vision, Signal to noise ratio
We present a multi-line-scan light-field image acquisition and processing system designed for 2.5/3-D inspection of fine surface structures in industrial environments. The acquired three-dimensional light field is composed of multiple observations of an object viewed from different angles. The acquisition system consists of an area-scan camera that allows for a small number of sensor lines to be extracted at high frame rates, and a mechanism for transporting an inspected object at a constant speed and direction. During acquisition, an object is moved orthogonally to the camera’s optical axis as well as the orientation of the sensor lines and a predefined subset of lines is read out from the sensor at each time step. This allows for the construction of so-called epipolar plane images (EPIs) and subsequent EPI-based depth estimation. We compare several approaches based on testing a set of slope hypotheses in the EPI domain. Hypotheses are derived from block matching, namely the sum of absolute differences, modified sum of absolute differences, normalized cross correlation, census transform, and modified census transform. Results for depth estimation and all-in-focus image generation are presented for synthetic and real data.
KEYWORDS: Cameras, Sensors, Line scan image sensors, Inspection, Spatial resolution, Imaging systems, 3D acquisition, 3D image processing, Signal to noise ratio, Visualization
We present a light-field multi-line-scan image acquisition and processing system intended for the 2.5/3-D inspection
of fine surface structures, such as small parts, security print, etc. in an industrial environment. The
system consists of an area-scan camera, that allows for a small number of sensor lines to be extracted at high
frame rates, and a mechanism for transporting the inspected object at a constant speed. During the acquisition,
the object is moved orthogonally to the camera’s optical axis as well as the orientation of the sensor lines. In
each time step, a predefined subset of lines is read out from the sensor and stored. Afterward, by collecting all
corresponding lines acquired over time, a 3-D light field is generated, which consists of multiple views of the
object observed from different viewing angles while transported w.r.t. the acquisition device. This structure
allows for the construction of so-called epipolar plane images (EPIs) and subsequent EPI-based analysis in order
to achieve two main goals: (i) the reliable estimation of a dense depth model and (ii) the construction of an
all-in-focus intensity image. Beside specifics of our hardware setup, we also provide a detailed description of
algorithmic solutions for the mentioned tasks. Two alternative methods for EPI-based analysis are compared
based on artificial and real-world data.
We demonstrate the design, setup, and results for a line-scan stereo image acquisition system using a single area- scan sensor, single lens and two planar mirrors attached to the acquisition device. The acquired object is moving relatively to the acquisition device and is observed under three different angles at the same time. Depending on the specific configuration it is possible to observe the object under a straight view (i.e., looking along the optical axis) and two skewed views. The relative motion between an object and the acquisition device automatically fulfills the epipolar constraint in stereo vision. The choice of lines to be extracted from the CMOS sensor depends on various factors such as the number, position and size of the mirrors, the optical and sensor configuration, or other application-specific parameters like desired depth resolution. The acquisition setup presented in this paper is suitable for the inspection of a printed matter, small parts or security features such as optical variable devices and holograms. The image processing pipeline applied to the extracted sensor lines is explained in detail. The effective depth resolution achieved by the presented system, assembled from only off-the-shelf components, is approximately equal to the spatial resolution and can be smoothly controlled by changing positions and angles of the mirrors. Actual performance of the device is demonstrated on a 3D-printed ground-truth object as well as two real-world examples: (i) the EUR-100 banknote - a high-quality printed matter and (ii) the hologram at the EUR-50 banknote { an optical variable device.
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