Bitmap Vs. Vector

4197c01.qxd 5/30/03 8:22 AM Page 1 CHAPTER 1 The Basics of a Scanned Image Welcome to a new world of digital imaging! This book will serve as your guide to both the how and the why of digital imaging with SilverFast, with information on pixels, color correction, image restoration, and much more. The first two chapters are not specifi- cally about the use of SilverFast, but the background information you’ll obtain about the digital imaging process is absolutely essential if you’re to make informed, appropriate choices during the scanning process. In this first chapter, the following topics are covered: ■ Bitmap vs. vector ■ Parts of an image ■ Capture bit depth, channels, and shades of gray ■ The resolution concept ■ Color theory and color modes ■ File formats for print or the Internet COPYRIGHTED MATERIAL 4197c01.qxd 5/30/03 8:22 AM Page 2 2 ■ CHAPTER 1: THE B ASICS OF A S CANNED I MAGE Bitmap vs. Vector In today’s information-packed world, we are bombarded with graphic images from television sets, computer screens, billboards, and magazines. As you float in this stream of digital con- tent, contemplate this fundamentally important bit of information: Despite the wide variety of visual communication methods used to disseminate these images, every graphic brought to you through digital processes can be placed into one of two categories; it is either vector or bitmap. Vector graphics are composed of a series of points and lines connecting the points (see Figure 1.1). Although some of these lines may be perfectly straight, it is more likely that any two points are joined by a line featuring one or more curves. The specific shape of these straight or curving lines and the coordinates of all the points these lines connect can be expressed using mathematical equations. When the entire series of equations needed to describe a particular illustration is saved as a single file, other programs can access the illustration and make it a part of a new illustration or page layout document. Don’t be put off or confused by the words pixel and vector. They are just new terms for things you are already familiar with. Before the digital age, we had painting and drawing. And we still do. We paint with pixels, and we draw with vectors. Drawing Applications Software packages used to create vector graphics are often called drawing programs, because the concept of building a digital illustration through a series of connected lines is very similar to the process of drawing a graphic with pen and paper. Vector graphics are also known as object-oriented graphics because individual objects drawn in this way can be manipulated independently (i.e., they can be resized, skewed, repositioned, etc.). It’s typical that the equations describing these graphics are expressed in an object-oriented programming language, such as Adobe’s PostScript. Common examples of vector graphics include Encapsulated PostScript (EPS) files created by drawing programs such as Macromedia FreeHand or Adobe Illustrator, as well as the out- line component of digital fonts (such as TrueType or printer fonts from the Type 1 format). Figure 1.1 Vector images consist of points and lines mathematically defined. 4197c01.qxd 5/30/03 8:22 AM Page 3 BITMAP VS. VECTOR ■ 3 Figure 1.2 Bitmap images consist of thousands or millions of individually defined points. The power of vector graphics is that they are not limited by size or resolution; they can be output as large or small as desired, and on devices of any resolution—the only restrictions on the quality of vector reproduction are the limitations of the output device itself. With that in mind, you might think that every graphic should be created and stored in vector format; unfor- tunately, that is simply not possible. Although vectors excel at reproduction of clearly defined shapes, they are not able to capture the incredible variety of shades and ambiguous details found in everyday life. For the storage of real-life scenes scanned from photographs or captured on digital cameras, we rely on graphics stored in the bitmap format. Painting Programs Painting programs such as Photoshop work with bitmapped or pixel-based images. The term bitmap refers to a collection of tiny squares called pixels, stored in an orderly rectangular grid akin to the sectors that make up a city map (see Figure 1.2). Each square pixel in this map can vary in hue (color), saturation (intensity), and brightness, but the entire square must be of a single value (for example, a single pixel cannot fade from dark at the top to a lighter shade at the bottom). Figure 1.3 The appearance of each pixel can be expressed and stored as a specific set of numbers, and Pixel values are stored as with all numbers stored in computers, these values are stored as binary values. In the binary with bytes of data. counting system, numerical values are represented as collections of 1s (ones) and 0s (zeros), which are called bits. Every bit must be either a 1 or a 0, and when bits are clustered together into groups of eight, they are known as bytes. A single byte (made up of 8 bits) expresses 1 of 256 possible values. The byte in Figure 1.3 shows the binary representation of the number 127, which could be used to designate a medium gray pixel in a grayscale image. 4197c01.qxd 5/30/03 8:22 AM Page 4 4 ■ CHAPTER 1: THE B ASICS OF A S CANNED I MAGE Simple bitmap images are composed of pixels arranged in a maplike grid where the value of each pixel is described with a single bit (1 or 0), so these images can only contain a single color, or a single shade of gray, and are often known as line-art images. More complex bitmaps can use 8 bits (1 byte), 24 bits (3 bytes), or an even greater numbers of bits to describe shades of gray or specific colors within a complex rainbow of possibilities. Parts of an Image We often refer to tonal parts or areas of an image, in terms of both viewing it and measuring and adjusting it. It is the tonal ranges that we view and adjust when we capture an image. Tonal Areas There are five basic tonal areas: highlight, quarter tone, midtone, three-quarter tone, and shadow. Figure 1.4 shows representative locales for these five tonal regions in an image. The five basic tonal areas represent the entire tonal range of grayscale values found in an image. Figure 1.4 Tonal areas of an image Shadow Three-quarter tone Midtone Quarter tone Diffuse highlight (with detail) Specular highlight (no detail) 4197c01.qxd 5/30/03 8:22 AM Page 5 CAPTURE BIT DEPTH, CHANNELS, AND SHADES OF GRAY ■ 5 It is important to note that there are two kinds of highlights, specular and diffuse. Not all images contain both kinds of highlights, but this image does. Specular highlight, seen here as twinkles on the bulbs on the tree, have no detail in them, whereas a diffuse highlight has some image detail. We typically focus on finding and setting the diffuse highlight. Histograms Figure 1.4 also includes a histogram. A histogram shows the amount and distribution of tonal data from the highlight to the shadow end of the image. The histogram is one of the primary tools we use for viewing and adjusting the tonal data in an image. Capture Bit Depth, Channels, and Shades of Gray One of the most important characteristics of a bitmapped or pixel-based image is its bit depth. And understanding bit depth is the key to understanding the Scan Type choices in the SilverFast Scan Overview dialog (Scan Control window). Bit depth refers to the amount of information (represented by bits) that is stored in an image. Bit Depth As discussed previously, all digital devices work with only two values: 0 and 1. So all image data must be broken down into those two values. If we only worked with black-and-white images, all we would need to have would be 1 bit of information—a 0 for black and 1 for white or vice versa—stored in each pixel. All we would need would be images with a bit depth of 1. But this is not the case. We need to capture, store, edit, and output images with from hundreds to millions of values. So we need more bit depth, that is, more bits per pixel to represent more shades of gray and more colors. The number of shades of gray or colors represented increases with the number of bits per pixel we have stored in our images in a geometric fashion. Every time we add another bit of information, we double the number of shades of gray or colors we can capture, store, edit, and output. The capture bit depth describes the number BIT DEPTH COLORS OR SHADES OF GRAY Table 1.1 of bits of image data that can be captured by a scanner Bit Depths or digital camera. The minimum number of shades of 1 bit 2 2 bits 4 gray we prefer to work with in printing is 256, which is 3 bits 8 × × × × × × × 8 2 2 2 2 2 2 2 2 = 256, or 2 . For the Web, we 4 bits 16 can often get by with fewer colors or shades of gray, such 5 bits 32 as 32 (25) or 64 (26). When we capture image data with 6 bits 64 a scanner or digital camera, we want to capture more 7 bits 128 than we might use on final output to allow us some data 8 bits 256 10 bits 1024 to work with in editing our images.

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