AutoCAD For Dummies. Ralph Grabowski

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       An image on a computer screen is made up of pixels. When you look closely at the screen with a strong magnifying glass, you’ll see that the image is formed from a large number of small dots of light, as shown in Figure 1-4, called pixels. This has nothing to do with Tinker Bell, except that an onscreen image of her would indeed be made up of pixels.

       All programs that display a graphic image simply turn on or off suitable spots to build the picture. This is a raster image. A straight line in a raster image is just a fortuitous alignment of appropriate dots, and after it’s been created, it can’t be edited as a single object.

       A major difference between CAD programs and computer graphics programs (such as Microsoft Paint) lies in how they save the image to disk. When the image from a Paint-type program is saved to disk, it’s stored as a bitmap that simply lists the color of each pixel. It's simply a snapshot of what you see onscreen.

       All CAD programs work with and store on a vector file on disk. A vector file is a big collection of numbers and words that list the type, size, and location of every entity in the drawing. When a CAD program displays your drawing onscreen, it analyzes the vector data and calculates which pixels to turn on or off, depending on which portion of the drawing you’re viewing. CAD programs understand that a circle is a closed curve with a center point and a constant radius. If you change its radius, the CAD program redraws the image onscreen to show the new size.FIGURE 1-4: Pixels.

       AutoCAD doesn’t limit you to working only with what you can see onscreen. You can include as much detail in a drawing as needed. You can zoom in to see more detail and zoom out to see the big picture. At any time, the screen shows only those entities and their detail that the screen is capable of showing.Some screens can show more pixels than others can. The number ranges from the 320 per row by 200 rows (320 x 200) of the very old Color Graphics Adapter (CGA) of the 1980s to 3840 x 2160 and beyond for today’s 4K monitors. However, the drawing file always contains the same information. If it were moved to a computer with a higher resolution graphics adapter and monitor, then greater detail would show without you having to zoom in as far. Conversely, a drawing file moved to a computer with a lower screen resolution does not lose any detail, but you'll need to zoom in more closely to see details clearly. How big is “the big picture”? AutoCAD can draw a circle with a radius of 1099 (a 1 followed by 99 zeros) units, but the observable part of the universe is “only” about 5 x 1023 miles in diameter, depending on how you measure and whose numbers you use (subject to change without notice). Check out en.wikipedia.org/wiki/Observable_universe for the latest number.Conversely, AutoCAD can draw a circle with a with a radius as small as 10-99 (which equals 0.00000[plus 90 more zeros plus]0001) units in diameter, as opposed to the classical radius of an electron, which is positively huge at 2.8179403267 x 10-13 cm.

       It’s possible for a drawing file to contain much more than you can see at any one time. The computer screen is not really the drawing; it is just a viewer that lets you look at all or part of the drawing file.