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This book provides the fundamentals of image processing specifically for the practicing engineer or scientist. A large variety of example images is included to give the reader a better understanding of how particular image processing algorithms work. This book bridges the gap between existing high level texts and the need for a more practical and fundamental approach.
This chapter gives a historical background of image storage and retrieval and the development of electronic image processing. The applications and use of modern electronic image processing are also presented, followed by a short discussion of human visual perception and how it relates to electronic image processing. A model for image formation is presented, followed by the spatial sampling and quantization of images. Finally, two types of electronic image acquisition systems are presented.
1.1 Historical Background
Throughout the history of mankind, there has been the desire of the human race to record an instance of time for future generations via the use of pictures (images). Pictures have also been used in various early languages, providing an easy method of communicating information from one human to the next. The earliest documented use of images to communicate an idea is seen in the drawings created by the early cavemen. Using primitive tools, images were recorded into stone describing the details of everyday life. Important events such as the winning or losing of a battle were often recorded with these man-made images. Even though these images were used for communication purposes, today they give a historical record of early human civilization, providing the details of everyday life. The importance of pictures used as a language to represent ideas is also well illustrated by Egyptian hieroglyphics. The prolific preservation of this language engraved in the stone of various Egyptian remains has provided historians with a detailed story of the Egyptian civilization
Another early form of image generation and storage was the use of various color paints/inks to record scenes observed by the human eye. This required the talent and expertise of humans who could create an image using various colors of paints/inks from a scene that was visualized and perceived by them. For many centuries, this was the only means of recording images. It was not uncommon that an artist would accompany soldiers into battle to record the historic event. During the Middle Ages and the Renaissance, the creation of man-made visual images played an important part in conveying important religious concepts to the average person. Many artists such as Leonardo daVinci and Michelangelo were commissioned to produce paintings of religious scenes within various churches. Artists were also commissioned to produce portrait paintings of royalty. During this time, this was the only means by which the image of a person could be passed from generation to generation. These man-made images had their limitations in that all were created after the visualization and interpretation by another human. The resemblance of the final image to that of the actual scene depended on the artist producing the image. Very seldom would several artists commissioned to produce a painting of the same scene produce exactly the same painting. Each would visualize and perceive something different in the scene. In fact, the differences between the paintings were attributed to an artist's style and perception of the scene being painted.
Since the beginning of time, it has been desired by human civilization to record images of scenes as accurately as possible, removing any human interpretation. During the second half of the sixteenth century an Italian philosopher, J. B. Porta, made an important discovery by accident. He discovered that light rays penetrating a small hole in a door enclosing a dark room produced an upside down image of the exterior scene on a white screen placed behind the door in the dark room. His discovery was the forerunner to the modern day pinhole camera. Amazed at his discovery, he shortly modified his experiment, replacing the small hole with a convex lens, and with the additional use of a mirror, Porta was able to produce a right side up image of an exterior scene. Porta realized the importance of his discovery and immediately recommended his device (camera obscura) to artists wishing to record exact images of scenes onto oil paintings. It was an artist, Canaletto, who first used Porta's discovery to produce paintings of Venice.
The discovery of the camera by Porta was the first step toward modern day photography. At about the same time as Porta's camera, it was discovered by a chemist in France that silver chloride changes its characteristics from clear to black when exposed to light. It was not until two centuries later that Jacques Alexandre Charles, the inventor of the hydrogen balloon, produced simple photographs of silhouettes of his students' heads using writing paper immersed in silver chloride. Unfortunately, Jacques Alexandre Charles, like many before him, did not have a way to stop the development process. Once a sheet of silver chloride paper was exposed to a light source to produce an image, it could be viewed only in very low light levels such as candle light. Eventually, the long term exposure to light would turn the whole silver chloride paper to black. The last piece of the puzzle remained to be discovered, that of stopping (fixing) the development action.
It was not until 1835 that Henry Fox Talbot was able stop the development process. His first images were of leaves and flowers, formed by pressing them against silver nitrate immersed paper and then exposing the paper and the objects to sunlight. Once the paper was exposed to light, producing the image of a leaf or flower, Talbot's final process was to "fix" the paper to prevent any further degradation of the image. Later, in the summer of 1835, Talbot used the camera obscura to produce images of his house. On this day, the age of modern photography was born. It was not until 1839, when Talbot perfected his approach, that his work was presented to the Royal Society. It should be pointed out that even though Talbot was able to produce a stored image without it first being interpreted by a human, his images were negatives of the original objects. It was not until years later that the two step process of producing a negative and repeating the exposure and development processing one more time to produce a positive image was used. Even though the earliest photographs were on paper, in the middle part of the nineteenth century glass plates were used as a means of recording an image using chemical photography. The stability of these plates are evident in the number that remain over one hundred years later. Today they are collector items among historians and photographers.
The invention of modern photography brought with it the desire of the human civilization to record images for observation by future generations and to provide the means of freezing the "images" of history. For example, the importance of early photography is evident in the still photographs that remain and clearly document the U.S. Civil War. This war was one of the first major conflicts in history to be completely documented by modern photography. Over one hundred years later, historians are still referring to these photographs to describe key battles. For most of the nineteenth century, photography was limited to a specialized few who were trained to use the various complex cameras and were familiar with the use of the complex chemical process used in the development of photographs. Photography was used only for special events or by the wealthy as a mean of capturing images to record key events or people. It was the invention of the simple roll-camera ("Kodak" camera) in 1884 by George Eastman that made modern photography available to the average individual. What this camera accomplished was the replacement of large photo sensitive glass plates with a flexible roll of film that was easy to load into the camera. George Eastman also provided a network of laboratories that would develop and process these film-rolls making the use of modern photography as easy as possible for the general public. The technical knowledge of the chemical development process was no longer required to produce photographs using modern photography.
As the field of black and white photography was maturing in the last part of the nineteenth century, researchers were pursuing the generation of color images. It was the work of James Clerk Maxwell and James D. Forbes that showed that the infinite number of colors available in the visible spectrum could be reproduced using a three primary color system. Using a spinning top, they placed circles of different color papers on top of the top. When the top was spinning at a high rate, the different color circles blurred together, producing a new observable color. Maxwell and Forbes were able to produce the color yellow from a red and a green circle. In fact, they found that they could produce any color from a combination of three colors, red, blue, and green. Maxwell suggested a method of proving his three color theory, by photographing a color image using three (black and white) photographic plates taken of exactly the same scene but with a red, blue, or green color filter placed in front of each plate. The three plates, which represented the three primary color images of red, green and blue, were then used to produce three images that were overlaid on top of each other on a white screen. In the generation of the images, a red light source was used to illuminate the red filtered plate, a green light was used to illuminate the green, and a blue light was used to illuminate the blue. Essentially, overlaying the three images produced a composite image composed of a red, a blue, and a green image of the original color scene. On May 17, 1861, Maxwell gave a lecture to the Royal Institute, where he stunned the audience with the presentation of a color image of a plaid ribbon generated from three red, green and blue photographic plates. Maxwell's work not only laid the groundwork for modern color photography, but as a result of his research, every color television/computer system today uses the three primary colors of red, green, and blue to generate color images.
It was also during this period that research began on combining still photographic images to produce moving images. The concept of motion pictures is based on the visual phenomenon known as persistence of vision. The images received by the brain from the eye are stored for about 60 milliseconds after viewing by the eye. In this way, objects moving across the field of view of the eye in faster than 60 milliseconds are ignored by the brain. For example, consider a set of cards containing a set of images of a person walking. Each adjacent card contains the image of the person walking as he or she moves just slightly to the left. Placing these cards in front of an observer and fanning them at a rate faster than about 15 cards per second produces the illusion that the person is walking smoothly to the left. This is the fundamental concept of motion pictures.
The goal of Thomas A. Edison and his assistant William Kennedy Laurie Dickson was to produce a set of time elapsed photographs to give the illusion of a moving image. It was the invention of the roll-film by George Eastman (modified for a positive image) that made the invention of motion pictures possible. In 1889, Edison designed a camera (kinescope) that automatically advanced a roll of film past a shutter and a lens. At a periodic rate of 48 images per second, the film was held still and the shutter was opened, imaging a scene onto the roll of film. The film was then advanced and the process was repeated. When finished, the roll of film contained a time sequence of images, presenting the evolution of a scene as a function of time. After successful use of their camera, Edison and Dickson designed a projector that rapidly projected the time sequence of images saved on the roll of film onto a screen. What they observed was a smoothly moving image. Within 60 years of the invention of still photography, the recording of real scenes as a function of time was accomplished. With the advent of combining sound with moving pictures, a complete record of an important historical event and prominent people was now possible. An example is the millions of feet of newsreel film that was generated in the early part of the twentieth century. These films clearly document the important historical events of time for future generations.
Before moving onto the discussion of still images, which is the emphasis of this text, a brief history of television is in order. This invention, and the efforts to transmit images across the Atlantic Ocean began the history of transmitting images via electronic methods. These efforts laid the groundwork for the electronic generation, manipulation, transmission, and storage of still images. One of the earliest accounts of an electronic television system was given in a letter to Nature in April, 1880, by John Perry. He proposed an electronic camera that used an array of selenium detectors as a means of converting an image intensity into an array of electrical signals. The interesting thing about this camera was than it did not use any type of scanning mechanism that is typically found in many early electronic cameras. In fact, the camera proposed by Perry's paper would be referred to as a focal-plane camera and is very similar to the modern charge couple device, CCD, camera.
There were also two types of receivers described. The first used an array of magnetic needles, one for each selenium detector, to open and close a set of apertures. The size of the aperture was directly proportional to the light incident on its corresponding selenium detector. The second system used the Kerr effect to rotate the polarization of linearly polarized light passing through a small crystal. As an electric field is placed on a Kerr cell, the angle in which the polarization of light is rotated is varied. In this system, an array of small crystals equal to the number of selenium detectors was proposed. In front of each Kerr cell, a polarizer was placed so that the combination of the linearly polarized light intensity emerging from the Kerr cell and the polarizer produced an intensity variation proportional to the electric field on the Kerr cell. The goal of this receiver was to have the electric field induced on the Kerr cell be proportional to the light intensity incident on its respective selenium detector. It is interesting to note, this is the same concept that is used for flat screen liquid crystal displays, except the angle in which the polarization light is rotated is controlled by the liquid crystal elements. It took approximately one hundred years after the original concept paper by Perry before this type of receiver system was implemented. Unlike modern television systems, Perry's proposed system requires no point-by-point scanning of a scene. The major limitation of his system was that only a limited number of array elements could be used because of the complexity of the wiring and the number of parts required.
The importance of Perry's paper is that it proposed to spatially sample a continuously varying spatial image, using an array of detectors to produce an array of electric signals that contained all the information necessary to regenerate an image of the original scene. Many of the other early proposed systems were based on electro-mechanical systems that used the concept of scanning an image element by element. One of the first systems, designed by Paul Nipkow, used two rotating discs (one located at the transmitter and one located at the receiver) containing 24 holes oriented in helical fashion on each disc. The discs were then synchronized and rotated at a rate faster than the persistence rate of the eye. Located at the transmitter was a selenium detector that produced an electrical signal that varied as the scene was sequentially scanned. This electrical signal then modulated a light source at the receiver to produce a modulated intensity of light that was synchronized to the sampling of the original scene. The net effect was a perceived image of the original scene at the receiver.
Excerpted from Fundamentals of Electronic Image Processing by Arthur R. Weeks Jr. Excerpted by permission.
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Introduction to Electronic Image Processing.
Transforms Used in Electronic Image Processing.
Image Enhancement by Point Operations.
Spatial Filtering and Fourier Frequency Methods.
Nonlinear Image Processing Techniques.
Color Image Processing.
Image Geometry and Morphological Filters.
Image Segmentation and Representation.