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Unformatted text preview: onto the image and sensing the intensity of the reflection at every
point of the image. This conversion process can be best understood with the help of an
example. Suppose there is a document with the word "BIT" written on it. This document
has to be scanned and stored in the computer. The scanner treats the document as if it is
drawn on a graph sheet as shown in Figure 9.7(a). An intersection of a horizontal and a
vertical line of the graph is called a grid point. If there is a dark spot at a grid point, it is
represented by a 1, otherwise it is represented by a 0. Figure 9.7(b) shows the
representation of the image of Figure 9.7(a). This representation is called the bit map
representation of an image. Each bit in the representation of the image is called & pixel (a
The finer is the granularity of grid points, the better the scanner is because it can convert
a given image with greater accuracy. Scanners having 300 to 600 grid points per inch are
common. Moreover, we saw in Figure 9.7 that each grid point is encoded as just 1 bit.
More bits are used to represent each grid point in high quality black-and-white scanners
and color scanners. For example, a good quality black-and-white scanner may use 4 bits
per grid point to encode different shades from black to gray to white. Similarly, a color
scanner may use 24 bits per grid point to represent a maximum of 256 different shades of
Image scanners come in various shapes and sizes. The two commonly used types based
on this classification are flat-bed and hand-held scanners. They are briefly described
1. Flat-bed scanner. As shown in Figure 9.8, a flat-bed scanner is like a copy machine
consisting of a box having a glass plate off its top and a lid that covers the glass plate.
The document to be scanned is placed upside down on the glass plate. The light source is
situated below the glass plate and moves horizontally from left to right when activated.
After scanning one line, the light beam moves up a little and scans the next line. The
process is repeated for all the lines. It...
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This document was uploaded on 04/07/2014.
- Spring '14