Page 26 - Enhancing CAD Drawings with Photoshop

4386.book Page 9 Monday, November 15, 2004 3:27 PM
UNDERSTANDING MODES, BITS, AND CHANNELS 9

Bitmaps and Grayscale Images
The fundamental unit of information on a computer is called a bit. A bit can either be 1 or 0 and is the
basis of binary computing. Figure 1.9 shows the simplest kind of computer image, a bitmap.

NOTE There are 8 bits in a byte. Bytes are a common measure of information in a computer. Most files
are measured in kilobytes (KB) or megabytes (MB). Hard drives currently store many gigabytes (GB),
or billions of bytes of data.
You will open a bitmap and examine it in detail.
1. Open the file StreetBitmap.psd from the CD.
2. Zoom in to the image a few times until you clearly perceive the individual black and white pixels
that make it up.
The reason digital imaging works at all is that the human visual system undergoes a figure/
ground shift in perception when presented with a pixel image of sufficient resolution. When
you zoom in the image on the right of Figure 1.9, you might not understand what all the pixels
represent. It is only when the pixels become small enough and dense enough, that your mind
perceives the illusion of the continuous picture.
Each of the pixels in Figure 1.9 is either black or white. This visual information is stored in the
computer as either a 1 (white) or a 0 (black). In other words, each pixel is represented by a bit.
As such, the bits in memory map directly to the visual image that we perceive, and thus the
name bitmap is apt.

NOTE Fax machines transmit bitmap images.
3. Zoom back out to 100% magnification (as shown in Figure 1.9). You can double-click the Zoom
tool in the toolbox to do this quickly.

4. Open the file StreetGrayscale.psd from the CD. This image (shown in Figure 1.10) contains
far more information than the bitmap in Figure 1.9.

Figure 1.9
Bitmap image: viewed at
(left) 100% and (right)
500% magnification