Digital Images for the Information Professional

Melissa Terras, Melissa M. Terras

• Publisher: Routledge
• Year: 2016
• Print ISBN: 9780754648604
• eBook ISBN: 9781351944083
• Topic: Languages & Linguistics
• Subtopic: World History
• Index: Languages & Linguistics

In the past decade, the way image based media is created, disseminated, and shared has changed exponentially, as digital imaging technology has replaced traditional film based media. Digital images have become the pervasive photographic medium of choice for the general public. Most libraries, archives, museums, and galleries have undertaken some type of digitisation program: converting their holdings into two dimensional digital images which are available for the general user via the Internet. This raises issues for those aiming to facilitate the creation and preservation of digital images whilst supplying and improving user access to image based material. Digital Images for the Information Professional provides an overview of the place of images in the changing information environment, and the use, function, and appropriation of digital images in both institutional and personal settings. Covering the history, technical underpinnings, sustainability, application, and management of digital images, the text is an accessible guide to both established and developing imaging technologies, providing those within the information sector with essential background knowledge of this increasingly ubiquitous medium.

Chapter 1
Introduction

… the soul never thinks without an image (Aristotle, De Anima Book III, Part 7).

The illiterates of the future will be the people who know nothing of photography rather than those who are ignorant of the art of writing (László Moholy-Nagy 1932, quoted in Jeffrey 2006, 111).

Images have been part of human society, psychology and imagination since the earliest times, before the dawn of written language. In the closing stages of the twentieth century, digital images emerged as the dominant medium to capture, create, store, share and disseminate visual information. For those working in the information profession, such as librarians, archivists, curators, knowledge managers and information specialists, understanding our changing information environment and changing approach to image-based material can facilitate the creation of, access to, and long-term sustainability of digital image material.

This chapter explores some basic underpinnings which inform the rest of the book: What do we mean by ‘image’, and why are they so important to humans? What are digital images, how are they created, and for that matter, what is meant by the term ‘digital’? This text focuses on issues surrounding bitmaps, rather than vector graphics, and the reasons for this approach are explained, as is why such a text (with a focus on issues pertinent to information professionals) is necessary. Finally, an overview of the remaining content of the book is presented, detailing its scope and aims.

Images

The creation of images has been a human activity since the beginning of human society: our early ancestors painted or carved rocks with depictions of their lifestyles and beliefs. Rocks are not very portable, however, and humans like to share their images, and the knowledge contained within them. Much investment in technology has focused on the ability to create, replicate and disseminate visual information, from early print materials, to later attempts at photography and the ability to capture ‘an artificial imitation or representation of the external form of any object’ (OED 1989a) via means of recording patterns of light. Indeed, the OED definition of ‘image’ includes light as a particular source of images: an image can be:

An optical appearance or counterpart of an object, such as is produced by rays of light either reflected as from a mirror, refracted as through a lens, or falling on a surface after passing through a small aperture (ibid.).

The complete definition of an ‘image’ is wide, however, and there is no room in this book to address the complex philosophical, historical and linguistic debate regarding what images actually are: as well as the physical manifestation of light falling on photosensitive paper or light-sensitive arrays:

We speak of pictures, statues, optical illusions, maps, diagrams, dreams, hallucinations, spectacles, projections, poems, patterns, memories, and even ideas as images, and the sheer diversity of this list would seem to make any systematic unified understanding impossible (Mitchell 1984, 504).

The focus of this book is the image at its most literal: ‘a graphic, pictorial representation: a concrete material object’ (ibid. 521), a ‘non-moving representation of visual information’ (Anderson et al. 2006, 7) which has a physical manifestation, even if it is just a file which resides on a computer’s hard drive or on a web server.

Two-dimensional images are an important means of communication, source of information, and focus of both social interaction and scholarly activity in modern society. Images play an important role in cultural and social history, can contain valuable historical information, and are used more and more in academic research, which aims to study culture in its widest sense: focusing on artefacts and cultural produce. It is becoming popular for interpretations of visual material to be incorporated into academic reasoning: this is partly to do with the more exotic or interdisciplinary themes emerging in modern research ‘like the history of smells, or witchcraft. Studying themes like this, all sorts of sources, previously not often used by historians – images, literature, plays – are consulted’ (van den Berg 1992, 8).

Digital images are now the media of choice for creating, sharing, accessing and manipulating image material. Digitization projects in libraries, archives, museums and other ‘memory institutions’ are merrily creating image representations of their holdings for display, sharing and distribution via the Internet. Individuals are creating, manipulating, utilizing and sharing vast collections of digital images in ways which would not have been possible with traditional photographic media. The increasing prevalence and pervasive nature of digital imaging technologies requires engagement and understanding by those working within the information sector.

Images and Humans

The word ‘image’ stems from the Latin imaginem or the later French imagene, expressing ideas of imitation, copy and likeness, but also of thought, conception and imagination. This reflects the fact that the human perception and understanding of images is both a physiological and conceptual process. It is not entirely understood how the human eye-brain system functions.

Physically, the vertebrate eye is complex. Light passes through the opening in the front of the eye, the pupil, and the lens focuses the image on the retina, a sheet of layers of neural tissues that line the back of the eyeball. Photoreceptors generate a neural signal when they detect light, and this signal is sent via the optic nerve to the brain (Gregory 1998; Livingstone 2002). What happens next is not clear:

The eye is a simple optical instrument. With internal images projected from objects in the outside world, it is Plato’s cave with a lens. The brain is the engine of understanding. There is nothing closer to our intimate experiences, yet the brain is less understood and more mysterious than a distant star (Gregory 1998, 1). (Gregory 1998, 1).

Human perceptual and cognitive systems have limited capacities for processing information, but much of it is devoted to dealing with visual input. Being biological systems, and not identical, the cortex area devoted to processing images varies greatly from human to human (Andrews et al. 1997), but a relatively large proportion of brain activity, estimated at above 50 per cent, is devoted to vision. Research into how we see, perceive and interpret the world around us, and our reaction to images, is wide, varied and spans an interdisciplinary reach encompassing psychology, biology, physiology, chemistry, physics, philosophy and beyond (Bruce et al., 1996, introduce many of the theories and debates postulated so far). What is certainly true is that the human brain can process complex visual information quickly, it does so by various mechanisms which are not fully understood, and that humans can be stimulated in various emotional, intellectual, physical and behavioural ways by imagery and image content (Lang et al. 1993, 1998). Our enjoyment of visual art is an extension of this complex physical and perceptual activity (Molnar 1997; Livingstone 2002). ‘We humans like images: our brains are specially adapted to interpret them’ (Trefethen and Embree 2005, 8): it is little wonder, then, that the ease of producing, manipulating and disseminating them afforded by digital media and networks has resulted in an exponential increase in image material for personal and cultural consumption.

It is important to note that the human visual system suffers from many limitations. It can detect light and interpret the consequences of light stimulus – within a specific range of wavelengths – but has limited ability to perceive certain colours, being more sensitive to green and yellow, then red, rather than blue. Small variations in colour are virtually impossible for the human eye to detect: our visual system merges small details into a unified whole, being set up to view the global information of a scene or a smooth, continuous tone image, rather than individual constituent properties. The human visual system is much more sensitive to changes in luminance (light) values than changes in colour. Additionally, individuals perceive colour differently. Colour blindness affects 8 per cent of men and less than 1 per cent of women (Birch 2001, 33), where some individuals see certain pairs of colours as being identical, those with normal vision would see them as different, but aside from this, the perception of colour is subjective:

the similarities and differences between your experience of red and mine lie somewhere between the classes of cells activated in our retinas, thalamuses, and visual cortices, and the similarities between the memories activated in our frontal lobes (Livingstone 2002, 33).

Computer digital imaging and multimedia technologies often exploit these limitations in the human visual system, or try and recreate human perceptual processes, in order to present material to us effectively. For example, computer monitors generate colours by mixing three additive primaries of red, green and blue light onto small, individual points on the screen. The refresh rate with which the screen is repainted varies from 1/25th to 1/80th of a second, depending on the device, which is enough to fool an eye into seeing a still image, or continuous motion if the image is changing frame by frame. The size of the individual points is set to be below our perception threshold, so we see a continuous tone image. Our visual systems blend the image chromatically, spatially and temporally:

Chromatically in that we do not see only various shades of red, blue, and green, but all the colours in between. Our visual systems blend colours … Spatially, because the individual pixels are too small to be resolved by the photoreceptor spacing in our retinas, unless we are looking very closely at a very large monitor … [and] Temporally in that we see neither the scan pattern nor the fact that the image is presented at a rate of thirty frames per second … a rate our visual systems are unable to resolve (Livingstone 2002, 192).

In addition to this, certain approaches to compressing images, which make file sizes smaller (see pp. 50–58) exploit the way the human eye system deals with variation in colour and replicates this mathematically. JPEG compression, in particular, was designed to discard colour information that the human eye just does not readily perceive (see pp. 76–79), efficiently creating a smaller file which looks much like the original. The way that digital imaging technologies work is closely related to our own physiology and physical limitations.

Defining 'Digital'

The ordinary, or ‘real’ world of our senses, exists in a continuous flowing stream of signals across time and often space. A sound, a movement, a photographic print or a line drawn from a pencil all exist in analogue, where a varying signal represents a continuous range of values. In order to record, copy, transmit or analyse such a complex signal using computational power, it is necessary to translate this into a form which is more simple, predictable and processable. All telecommunication systems have one thing in common: the information to be sent is converted into signals, which can be transmitted and reassembled on reception, to be converted into something we can perceive as a fair copy of the original.

Digital systems are those which rely on a sequence of discreet numeric values, rather than the unconstrained and continually varying qualities of analogue signals. Numeric values are used in digital systems for processing, display, transmission and input: often sampling values from analogue sources. The term ‘digital’ comes from the Latin for finger digitus, as these systems use counting and discreet, or separate and distinct, numeric values for calculating, reflecting the way we often use our own fingers for counting or, as was common in the past, as a unit of measurement.

The most common digital systems are those used in computing and electronics which rely on the binary numeric system. This is a system which represents all numbers using only two symbols, typically 0 and 1. This is represented in a fairly straightforward manner in electrical or optical systems: typically 1 is denoted by an electronic pulse present, 0 by its absence. A string of zeroes and ones can represent numbers (1 is ‘one’; 10 represents ‘two’; 11 represents ‘three’; 100 represents ‘four’; etc.). These zeroes and ones are known as binary digits, or more commonly as the shortened derivation: ‘bits’. Any number of bits can be combined to represent a set of values. A set of 8 bits is called a ‘byte’ (a unit settled on in the 1960s due to the popularity of 8-bit microprocessors). A long string of bytes, such as an entire file, is often referred to as a ‘data stream’.

Strings of bits can represent text: text encoding formats assign a set of binary numbers to represent letters of a language alphabet. The ASCII (American Standard Code for Information Interchange) is a standardized 7-bit character code for the transmission of data, which describes the codes of the 128 different characters which can be described in 7 bits: from 0000000 to 1111111. The character D, for example, is number 68 in the specification, so is represented in the binary string 01000100. As long as the computer can recognize this binary string as ASCII it can process and display large strings of bits as ‘plain text’ (the operating system will usually examine the file and determine what format it is and how it should be displayed).

Strings of bits can also represent images. The overall purpose of this book is to describe in detail how this functions; but for the moment, imagine a black and white chessboard. If we take white to be 1 and black to be 0, the chessboard could be represented by a string of bits, from the top left white square: 1010101001010101 … etc. This creates a ‘bitmap’: the bits of the data stream are mapped, one by one, to colours in the image. As long as the computer had some way of knowing that 8 bits represented one line (the ‘scan line’ length, which is the type of information provided within a file header in an image format, see p. 65), then a representation of the chessboard could be stored in bits and easily copied and transmitted to other computers and painted onto computer screens. An alternative way to store graphical data would be to use ASCII text to create a list of instructions to a computer ‘One white square. Then one black square. Repeated four times. Next row. One black square …’ etc. A program could read this set of instructions and paint the resulting image onto the screen in a series of shapes called vectors (see pp. 7–8). However, describing photographic content in this manner is complex, and the sampling method of describing an image as solidly toned squares is most useful for complex image data.

Strings of bits can also represent sound and moving images: as the information to be represented grows more complex, more bits are required to represent it, and more complex mechanisms are used to store, display and process the information contained within the data stream.

Providing numeric, textual, image, sound and video-based data in digital format, whether they have been translated from an analogue signal into bits or ‘born digital’ by being created with computational technologies in the first place, has various advantages. These strings of bits can be easily replicated, transmitted, accessed and processed. Saving the data in a structured, predetermined format means it may be device independent and can be transferred from system to system with minimal problems. Data can be manipulated by dedicated computer programs, allowing new versions of the information to be generated. Data can also be processed: mathematically sorted through to show hidden relationships, new arrangements, different views and expanded, contracted or concatenated knowledge.

It should be noted that digital is not always best, and that there is still a place for analogue technologies in this world. Analogue devices pre-date digital and are still effective in representing information: wristwatches, gas meters, odometers in cars, etc. Human eyes and ears can sometimes distinguish between continuous analogue signals and bit by bit digital approximations. Digital media are at their most effective when their constituent parts – samples – are not detectable by human senses.

The Digital Image

A digital image is a representation of an image stored in numerical form, for potential display, manipulation or dissemination via computer technologies. Digital images store graphical information in small, discontinuous, non-homogenous, numerical elements.

On one level, a digital image is like any other computer data file: just a long code of ones and zeroes, using information as its raw material. Digital images are captured through a process of creating, storing and manipulating numbers in a computer. Once created, this sequence of numbers becomes infinitely malleable, but is usually rendered into visual form which the human eye can see. Output devices generate a representation of the image data from the mathematical detail contained within a file – although the majority of output devices (screens, prints) are actually analogue.

There are more complex three-dimensional imaging technologies – but we shall concentrate on two-dimensional images. In particular, this text concentrates on bitmap, or raster, images: those which consist of discreet numerical units called ‘pixels’ (see pp. 35–37) which combine to make a continuous tone image, rather than vector graphics which consist of a set of instructions to the computer regarding spatial primitives (squares, circles, etc.) and how these relate to one another. Bitmap graphics are by far the most popular type of image used to create representations of objects for use in memory institutions, they are the type of image output by digital cameras, and are the most popular types of image on the Internet.

Types of Digital Images: Vector and Bitmapped Graphics

There are two distinct classes of digital images: vector graphics and bitmap images. Although digitization projects and cultural and heritage institutions will be most likely to produce and deal with bitmap images (and the focus of this book is on their growing prevalence and use) it is useful to contrast and understand the different function of the two types of images and to see why bitmaps are the preferred format for storing photographic and detailed pictorial information.

Vector graphics were the first type of computer graphic to be implemented in the 1950s. They use a series of geometric drawing commands, such as lines, curves or shapes and solid or graduated colour...

Table of contents

• Cover
• Half Title
• Dedication
• Title
• Copyright
• Contents
• Preface
• Acknowledgements
• 1 Introduction
• 2 The History of Digital Images
• 3 Digital Image Fundamentals
• 4 Image File Formats
• 5 Digital Images and Memory Institutions
• 6 Personal Digital Image Collections
• 7 Image Metadata
• 8 Current Issues in Digital Imaging
• Bibliography
• Index