136 Graphic Communications
changes should be made with a full-featured image
manipulation program, or image editor. Some of the
most commonly used image manipulation programs
include Adobe®Photoshop®,Macromedia®Freehand®
MX, CorelDRAW®, and Adobe® Illustrator®.
Full-featured image editors allow almost any
aspect of an image to be manipulated, including
cropping, color and contrast, adding or removing
visual information, and even combining images. See
Figure 7-14. Experienced users of image manipulation
programs can sharpen, blur, and smudge edges;
mix, choose, and apply colors; paint, draw, work with
multiple layers, clone, apply fi lters, create gradients
and textures, adjust color, and print color separations,
as well as composites. Many programs allow the user
to restrict modifi cations to one area of a picture or to
make picture-wide changes.
In digital prepress, graphics can be broadly divided
into two groups, bitmapped images and vector images.
Bitmapped Images
Bitmapped images are graphics fi les that
contain a map of pixels, each of which is assigned
characteristics. See Figure 7-15. A continuous tone
image (photograph) that has been digitized using a
digital capture device is an example of a bitmapped
image. In a simple black-and-white bitmap, one bit
of information is assigned to each pixel: either it is
on (black) or off (white). This information, in turn,
determines where ink is placed on the paper when
the image is printed. Graphics programs also store
location information for each pixel, providing gray
scale and color data. Each pixel might require eight
or even 32 bits of information to describe.
Working with bitmapped images
Artwork created by a paint program is a bitmapped
image. When using a paint program, Figure 7-16,
the rows and columns of squares that compose the
image are visible by zooming in on any given area.
Color may be added or deleted by fi lling in or emptying
each square on the grid. When working with bitmap
images, the individual pixels are edited, rather than
whole objects or shapes. Through these actions, the
image size, shape, or colors may be modifi ed.
When bitmaps are enlarged or reduced, the
edges can become ragged because they are
composed of squares that do not create a smooth
line. This process is referred to as aliasing. Aliasing
is the process by which smooth curves and other
lines become jagged due to the reduced resolution of
the graphics device or fi le. Antialiasing is a software
technique for diminishing jagged lines, or jaggies.
These stairstep-like lines occur because the output
device is not equipped with high enough resolution
to represent a smooth line. Antialiasing reduces the
prominence of jaggies by surrounding them with
intermediate shades of gray or color, Figure 7-17.
Shades of gray are used for gray-scaling devices,
and color is used for color output devices. Although
this reduces the jagged appearance of the lines, it
also makes them fuzzier. Many programs provide
an antialiasing option that is extremely useful when
placing text in an image.
Another method to reduce jaggies is called
smoothing. Some printers accomplish smoothing
Figure 7-14. When cropping an image in an image editing
program, such as Adobe Photoshop, use the marquee tool
to make a selection, and then trim the image.
Figure 7-15. This piece of bitmapped art has been
greatly enlarged to show the individual pixels.