Papers in PDF format

specifications like the Initial Graphics Exchange Specification (IGES) or the successor Standard
for the Exchange of Product model data (STEP). Description languages like postscript
fall also in this category. Images stored in these formats are very useful as many operations can
be performed like scaling, and moving or deleting an object.
In many cases, however, an image can hardly be described with objects. Photographic images
for example are usually described with pixel graphics. The word pixel comes from PICture
ELement, sometimes pel is used instead. A pixel graphic could be viewed as if a grid was
placed over the image. The cells of the grid are the pixels, the finer the grid the better is the
representation of the image. Color values of the cells are calculated as the weighted averages
of the colors present in the cell.
Note: A pixel graphic is inherently „lossy“ due to the analog/digital conversion.
Consider this when thinking of lossless and lossy compression.
Each cell is represented by a value that characterizes the color. Pixel images tend to allocate a
lot of memory and need much computing power. For example an A4 page (21x29,7 cm)
scanned at 158 dots per centimeter (400 dots per inch (dpi)) with true color (3x8bit) occupies
about 44 MB of memory. High end scanners and digital cameras may produce images with a
size of several hundreds of MB.
Mixed types, that combine pixel graphics with objects, are very useful, for example think of
textual comments or marking regions of interest. Unfortunately only few image types support
the storage of object and pixel graphics in one file.
Pixel graphics may be converted to object graphics and vice versa of course with strong limitations.
In this section we will concentrate on pixel graphics, however the statements of the other
sections hold for object graphics as well.
2.1.2 Image Acquisition
There are many ways to get digital images. We have selected some of the most important:
• Kodak Photo Compact Disc (PCD)
• Scanner
• Video + Framegrabber
• Digital camera
• Imaging devices
The Kodak Photo Compact Disc (PCD) [1] is a "write once, read many times" (WORM) medium.
It is cheap in production and durable. For a Photo CD Master Disc 24x36 mm films are
scanned with highly calibrated devices at 2170 dpi and 3x12 bit color resolution (RGB). The
images are stored in the YCC color space, which is compatible to CIE-Standards; this implies
that colors can be reproduced correctly.
Available are different formats:
- Professional CD Master Disc: 25 images, 6 different resolutions
- Photo CD Master Disc: 100 images, 5 different resolutions
- Photo CD Portfolio Disc: 700 images or 60 minutes of sound or a combination
- Photo CD Catalog Disc: Up to 4500 images in low resolution
- A Photo CD Diagnostic Disc with up to 8192 * 12.288 Pixels was announced, however, the
plans have been dropped
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