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<META name=vsisbn content="1558515682"><META name=vstitle content="Java Digital Signal Processing"><META name=vsauthor content="Douglas A. Lyon"><META name=vsimprint content="M&T Books"><META name=vspublisher content="IDG Books Worldwide, Inc."><META name=vspubdate content="11/01/97"><META name=vscategory content="Web and Software Development: Programming, Scripting, and Markup Languages: Java"><TITLE>Java Digital Signal Processing:An Introduction to Image Processing</TITLE>
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<H2><A NAME="Heading1"></A><FONT COLOR="#000077">Chapter 7<BR>An Introduction to Image Processing
</FONT></H2>
<P ALIGN="RIGHT">From her whole frame-—an atmosphere which quite<BR>Arrayed her in its beams, tremulous and soft and bright.</P>
<P ALIGN="RIGHT">—<I>The Revolt of Islam</I>, A Poem in Twelve Cantos.</P>
<P ALIGN="RIGHT">Canto Eleventh.<BR>Peter Packed a Pickled Pixel<BR>Its color was that of hash<BR>for every program that displayed<BR>This pixel it would crash</P>
<P ALIGN="RIGHT"><I>—D. Lyon</I></P>
<P>Image processing is a kind of digital signal processing that occurs on a two-dimensional array of sampled data. For example, it has been shown that image processing techniques can be used to compress DEM (digital elevation map) data [Franklin]. DEM data consists of a 2-D array of ranges, sometimes called a <I>range image</I>.</P>
<P>Often, the input image is acquired by a sensor that detects energy—typically, light energy. For the purpose of this chapter, we assume that we will sample an image using a uniform grid over which we perform the spatial sampling of the energy.</P>
<P>When we speak about the resolution of the image, we talk about the size of the array used to store the <I>significant</I> image data. For example, an array with 640 columns and 480 rows can contain double-precision floating-point numbers; if only 8 bits of information is significant, then the image resolution is 640 × 480 by 8 bits per pixel.</P>
<P>Image processing has applications in the areas of art, science, industry, government, and space [Holzmann] [Pratt]. Branches of image processing (such as digital image warping) have found applications in remote sensing, computer vision, special effects, and computer graphics [Wolberg].</P>
<H3><A NAME="Heading2"></A><FONT COLOR="#000077">Video Cameras and Scanners</FONT></H3>
<P>Energy used to form an image typically starts as an analog signal (as in the one-dimensional DSP case). The primary difference is that the 2-D transducer detects the power in the incident energy and its relative direction.
</P>
<P>Several techniques are commonly used to determine the relative direction of energy. A mechanical scanning technique (such as radar) physically moves a sensor to obtain a range image. Some flatbed scanners have a mechanical arm, called a <I>platen</I>, that contains a linear solid-state array. The platen is moved over an illuminated image. Another example of a mechanical scanner is a drum scanner. Typically a high-end scanner used in prepress applications, a drum scanner spins a drum on which art-work has been mounted.</P>
<P>Electronic scanning is performed by electron-beam deflection via an electric or magnetic field. The electron beam scans the photosensitive surface, which in turn alters the beam current in proportion to the incident light. Tube cameras are an example. The semiconductor industry has produced solid-state alternatives to tube cameras that dominate, both in cost and in performance specifications, all but niche markets. As a result, tube cameras have fallen out of favor; professional and consumer users typically favor solid-state cameras.</P>
<P>The primary solid-state camera in use today is the CCD (charged coupled device) camera, which was invented by Willard S. Boyle and George E. Smith at Bell Labs in the early 1970s. For semiconductor-based sensors, such as CCD cameras, a scanning array has individual sensing elements, each of which corresponds to a pixel. The size of the elements ranges from 7 to 14 µm. Recall that 1 µm (also called a micron) is 1 × 10<SUP>-6</SUP> meters. The wavelength of a He-Ne laser (a red light) is 638 nm (nm = nanometers = 1 × 10<SUP>-9</SUP> meters).</P>
<P>Figure 7.1 shows an image with an increasing gray level and a single white line that has been resampled from its center.</P>
<P><A NAME="Fig1"></A><A HREF="javascript:displayWindow('images/07-01.jpg',232,246 )"><IMG SRC="images/07-01t.jpg"></A>
<BR><A HREF="javascript:displayWindow('images/07-01.jpg',232,246)"><FONT COLOR="#000077"><B>Figure 7.1</B></FONT></A> A stepped gray wedge and the output of a linear scanning array.</P>
<P>Flatbed scanners typically have high-resolution linear CCD elements with a mechanically deflected platen that gives them higher resolution than hand-held cameras. For this reason, people often take a picture with a film camera and then scan the image to obtain a high-resolution scan. Film ranges in quality from common to laboratory grade. Common grade film (such as 35 mm Kodak 5369) has a grain size that yields 100 lines per mm (10,000 nm). Laboratory grade film yields about 1000 lines per mm (1,000 nm) [Kodak]. The high-end 35-mm digital slide scanners yield 2,048 × 3,072 pixels with 36 bits per pixel (see <A HREF="http://www.davidmyers.com.au/rfs2035.htm">http://www.davidmyers.com.au/rfs2035.htm</A> for an example). This is about 6.2 million pixels per image. Electronic still cameras have CCDs that have 850 × 984 pixel resolution (for example, <A HREF="http://www.kodak.com/daihome/pdf/dc120.pdf">http://www.kodak.com/daiHome/pdf/dc120.pdf</A>). Video cameras (image sequence digitization cameras) typically operate with CCDs that have resolutions of 640 × 480 24-bit pixels (for example, <A HREF="http://www.kodak.com/daihome/pdf/dvc300.pdf">http://www.kodak.com/daiHome/pdf/dvc300.pdf</A>).</P>
<P>Although film is 100 to 1,000 times higher in resolution per unit of imaging area than CCD cameras, another factor is total size. The common size for film is 35-mm, but larger format negatives are available for special applications (70-mm movie film, 127-mm portrait cameras, large format X-ray film, panorama cameras, and so on). These large formats can greatly increase the number of pixels available. The other factor is cost; the price of digital still-image cameras appears to be dropping quickly. Also, it is not clear that people need the high resolution that film offers. We have found that students have no idea how much data can be produced by a high-resolution scan of a large image. For example, a 3 × 5 inch color photo scanned at 24-bits per pixel and at 1200 pixels per inch produces 3*5*1,200*1,200*3 = 64MB of data (MB = megabyte; Mb = megabit). We have found students turning up the virtual memory to 450MB when running Photoshop so that they can scan 8 × 10 photos at 1,200 pixels per inch (8*10*1,200*1,200*3 = 345MB). Some students have even said that they need such images for their home pages (no, really).</P>
<H3><A NAME="Heading3"></A><FONT COLOR="#000077">The Observer Interface</FONT></H3>
<P>Although the Observer interface may seem out of place here, it is a central class for understanding how the <I>Image</I> class works. The <I>Observer</I> interface resides in the <I>java.util</I> package and is used to require implementation of methods that are needed to maintain consistency in an object-oriented environment. The relationship between the class that implements the <I>Observer</I> interface and class that extends the <I>Observable</I> class is the relationship between a view and a model (sometimes called the model-view relationship). An example of the model-view relationship is that between the a car’s gas tank and its fuel gauge. If the amount of fuel in the tank changes, the fuel gauge readout changes. In Java, the simulation of a fuel tank notifies the fuel gauge readout so that the screen is consistent with the underlying simulation of the fuel tank.</P><P><BR></P>
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