Handbook of Machine and Computer Vision (eBook)

The Guide for Developers and Users

Alexander Hornberg (Herausgeber)

eBook Download: EPUB

2017 | 2. Auflage
860 Seiten
Wiley (Verlag)
978-3-527-41341-6 (ISBN)

Infos

The second edition of this accepted reference work has been updated to reflect the rapid developments in the field and now covers both 2D and 3D imaging.
Written by expert practitioners from leading companies operating in machine vision, this one-stop handbook guides readers through all aspects of image acquisition and image processing, including optics, electronics and software. The authors approach the subject in terms of industrial applications, elucidating such topics as illumination and camera calibration. Initial chapters concentrate on the latest hardware aspects, ranging from lenses and camera systems to camera-computer interfaces, with the software necessary discussed to an equal depth in later sections. These include digital image basics as well as image analysis and image processing. The book concludes with extended coverage of industrial applications in optics and electronics, backed by case studies and design strategies for the conception of complete machine vision systems. As a result, readers are not only able to understand the latest systems, but also to plan and evaluate this technology.
With more than 500 images and tables to illustrate relevant principles and steps.

The editor, Alexander Hornberg, worked as development and software engineer in industry. Since 1997 he has been working in the field of machine vision in an academic environment. He is Professor for Image Processing and Applied Optics at the University of Applied Sciences Esslingen, Germany.
All contributors to this work are written by practitioners from leading companies which operate in the field of computer vision.

Textauszug

Chapter 1
Processing of Information in the Human Visual System

Frank Schaeffel

Sektion für Neurobiologie des Auges, Forschungsinstitut für Augenheilkunde, Universitätsklinikum Tübingen, Calwerstrasse 7/1, 72076 Tübingen, Germany

1.1 Preface

To gather as much necessary information as possible of the visual world, and neglect as much unnecessary information as possible, the visual system has undergone an impressive optimization in the course of evolution, which is fascinating in each detail that is examined. A few aspects will be described in this chapter. Similar limitations may exist in machine vision, and comparisons to the solutions developed in the visual system in the course of 5 billion years of evolution might provide some insights.

1.2 Design and Structure of the Eye

As in any camera, the first step in vision is the projection of the visual scene on an array of photodetectors. In the vertebrate camera eye, this is achieved by the cornea and lens in the eye (Figure 1.1) which transmit the light in the visible part of the spectrum, 400–, by 60–. Another 20– is lost as a result of scattering in the ocular media. Only about is finally absorbed by the photoreceptor pigment [1]. Because of the content of proteins, both cornea and the lens absorb in the ultraviolet, and because of the water content, the transmission is blocked in the far infrared. The cornea consists of a thick central layer – the stroma – which is sandwiched between two semipermeable membranes (total thickness ). It is composed of collagen fibrils, with mucopolysaccharides filling the space between the fibrils. Water content is tightly regulated to 75–, and clouding occurs if it changes beyond these limits. The crystalline lens is built up from proteins, called crystallines, which are characterized by their water solubility. The proteins in the periphery have high solubility, but those in the center are largely insoluble. The vertebrate lens is characterized by its continuous growth throughout life, with the older cells residing in the central core, the nucleus. With age, the lens becomes increasingly rigid and immobile and the ability to change its shape and focal length to accommodate for close viewing distances disappears – a disturbing experience for people around 45 who now need reading glasses (presbyopia). Accommodation is an active neuromuscular deformation of the crystalline lens that changes focal length from to in young adults.

Figure 1.1 Dimensions and schematic optics of the left human eye, seen from above. The anterior corneal surface is traditionally set to coordinate zero. All positions are given in millimeters, relative to the anterior corneal surface (drawing not to scale). The refracting surfaces are approximated by spheres so that their radii of curvatures can be defined. The cardinal points of the optical system, shown on the top, are valid only for rays close to the optical axis (Gaussian approximation). The focal length of the eye in the vitreous (the posterior focal length) is 24.0 mm H = 22.65 mm. The nodal points K and K permit us to calculate the retinal image magnification. In the first approximation, the posterior nodal distance (PND, which is the distance K to the focal point at the retina) determines the linear distance on the retina for a given visual angle. In the human eye, this distance is about 24.0 mm 7.3 mm = 16.7 mm. One degree in the visual field maps on the retina to 16.7 tan(1) = 290 m. Given that the foveal photoreceptors are 2 m thick, 140 receptors are sampling in the visual field, which leads to a maximum resolution of 70 cycles per degree. The schematic eye by Gullstrand represents an average eye. The variability in natural eyes is so large that it does not make sense to provide average numbers on dimensions with several digits. Refractive indices, however, are surprisingly similar among different eyes. The index of the lens (here homogenous model, n = 1.41) is not real but calculated to produce a lens power that makes the eye emmetropic. In a real eye, the lens has a gradient index (see text).

Both media have higher refractive index than the water-like solutions in which they are embedded (tear film – on the corneal surface, aqueous – the liquid in the anterior chamber between the cornea and the lens, and vitreous humor – the gel-like material filling the vitreous chamber between the lens and the retina, Figure 1.1). Because of their almost spherical surfaces, the anterior cornea and both surfaces of the lens have positive refractive power with a combined optical focal length of . This matches almost perfectly (with a tolerance of a millimeter) the distance from the first principal plane (Figure 1.1, ) to the photoreceptor layer in the retina. Accordingly, the projected image from a distant object is in focus when accommodation is relaxed. This optimal optical condition is called emmetropia but, in of the population in industrialized countries, the eye has grown too long so that the image is in front of the retina even with accommodation completely relaxed (myopia).

The projected image is first analyzed by the retina in the back of the eye. For developmental reasons, the retina in all vertebrate eyes is inverted. This means that the photoreceptor cells, located at the backside of the retina, are pointing away from the incoming light. Therefore, the light has to pass through the retina (about a fifth of a millimeter thick) before it can be detected. To reduce scatter, the retina is highly translucent, and the nerve fibers that cross on the vitreal side, from where the light comes in, to the optic nerve head are not surrounded by myelin, a fat-containing sheet that normally insulates spiking axons (see below). Scattering in retinal tissue still seems to be a problem because, in the region of the highest spatial resolution, namely the fovea, the cells are pushed to the side. Accordingly, the fovea in the vertebrate eye can be recognized as a pit. However, many vertebrates do not have a fovea [2]; they have then lower visual acuity but their acuity can remain similar over large regions of the visual field, which is then usually either combined with high motion sensitivity (i.e., rabbit) or high light sensitivity at dusk (crepuscular mammals). It is striking that the retina in all vertebrates has a similar three-layered structure (Figure 1.9), with similar thickness. This makes it likely that the functional constraints were similar. The function of the retina will be described in the following.

The optical axis of the eye is not perfectly defined because the cornea and lens are not perfectly rotationally symmetrical and also are not centered on one axis. Nevertheless, even though one could imagine that the image quality is best close to the optical axis, it turns out that the human fovea is not centered in the globe (Figure 1.2). In fact, it is displaced to the temporal retina by an angle , ranging in different subjects from to but highly correlated in both eyes. Apparently, a few degrees away from the optical axis, the optical image quality is still good enough not to limit visual acuity in the fovea.

Figure 1.2 Binocular geometry of human eyes, seen from above. Since the fovea is temporally displaced with regard to the optical axis by the angle , the optical axes of the eyes do not reflect the direction of fixation. is highly variable among eyes, ranging from 0 to 11, with an average of . In the illustrated case, the fixation target is at a distance for which the optical axes happen to be parallel and straight. The distance of the fixation target for which this is true can be easily calculated: for an angle of 4, and a pupil distance of 64 mm, this condition would be met if the fixation target is at 32 mm/tan(4, or 457.6 mm. The optic nerve head (also called the optic disk, or blind spot, the position at which the axons of the retinal ganglion cells leave the eye) is nasally displaced relative to the optical axis. The respective angle is in a similar range as . Under natural viewing conditions, the fixation angles must be extremely precise since double vision will be experienced if the fixation lines do not exactly cross on the fixation target – the tolerance is only a few minutes of arc).

1.3 Optical Aberrations and Consequences for Visual Performance

One would imagine that the optical quality of the cornea and lens must limit the visual acuity since the biological material is mechanically not as stable and the surfaces are much more variable than in technical glass lenses. However, this is not true. In daylight, pupil sizes <2.5 constitute the optics of the human eye close to the diffraction limit (further improvement is physically not possible because of the wave properties of light). An eye is said to be diffraction-limited when the ratio of the area under its modulation transfer function (MTF; Figure 1.3) and the area under the diffraction-limited MTF (Strehl ratio) is higher than 0.8 (Marcos [3], Rayleigh criterion). With a pupil, diffraction cuts off all spatial frequencies (SFs) higher than cycles per degree – a limit that is very close to the maximum behavioral resolution achieved by human subjects. By the way, diffraction-limited optics is achieved only in some birds and primates, although it has been recently claimed that also an alert cat has diffraction-limited optics...

Produkt-Details

• Erscheint lt. Verlag: 6.3.2017
• Sprache: englisch
• Themenwelt: Informatik ► Theorie / Studium ► Künstliche Intelligenz / Robotik
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Themenwelt: Technik
• Schlagworte: Aberrations • Adaptation • Algorithmen u. Datenstrukturen • Algorithms & Data Structures • Bildgebende Systeme u. Verfahren • Bildverarbeitung • Cell • Component Manufacturing • Computer Peripherals (printers,modems,scanners,etc.) • Computer-Ratgeber • Computer Science • computer vision • contributors • Different • Digital Cameras & Photography • Digitalkameras u. Digitalfotographie • Electrical & Electronics Engineering • Elektrotechnik u. Elektronik • End-User Computing • First • human visual • Imaging Systems & Technology • Informatik • Information • Komponentenfertigung • levels • light • Maschinelles Sehen • Monochromatic • Neural • Numbers • Optics & Photonics • Optik u. Photonik • Performance • Peripheriegeräte (Drucker, Scanner, Modems usw.) • Physics • Physik • Processing • Retinal processing • Robotics • Robotik • System • Visual • Visualisierung u. Computergraphik • Visualization & Computer Graphics • whole • xedition • xxv list
• ISBN-10: 3-527-41341-3 / 3527413413
• ISBN-13: 978-3-527-41341-6 / 9783527413416