Using Help
|
Contents
|
Index
Back
26
Adobe Illustrator Help
Setting Up Artwork in Illustrator
Using Help
|
Contents
|
Index
Back
26
Bitmap images
Paint and image-editing software, such as Adobe Photoshop, generate bitmap images,
also called
raster images
. The images use a grid (also known as a
bitmap
or
raster
) of small
squares, known as
pixels
,
to represent graphics. Each pixel in a bitmap image has a specific
location and color value assigned to it. For example, a bicycle tire in a bitmap image is
made up of a collection of pixels in that location, with each pixel part of a mosaic that
gives the appearance of a tire. When working with bitmap images, you edit pixels rather
than objects or shapes.
Bitmap images are the most common electronic medium for continuous-tone images,
such as photographs or images created in painting programs, because they can represent
subtle gradations of shades and color. Bitmap images are resolution dependent—that is,
they represent a fixed number of pixels. As a result, they can appear jagged and lose detail
if they are scaled on-screen or if they are printed at a higher resolution than they were
created for.
Bitmap images are good for reproducing subtle gradations of color, as in photographs. They can
have jagged edges when printed at too large a size or displayed at too high a magnification.
About resolution in bitmap images
Resolution
is the number of dots or pixels per linear unit used to reproduce artwork and
images. Output devices display images as groups of pixels. The resolution of vector
graphics, such as Illustrator artwork, depends on the device used to display the artwork.
The resolution of bitmap images, such as digital photographs, depends on both the
display device and the inherent resolution of the bitmap image.
Pixel dimensions
The number of pixels along the height and width of a bitmap image
.
The display size of an image on-screen is determined by the pixel dimensions of the image
plus the size and setting of the monitor. The file size of an image is proportional to its pixel
dimensions.
A typical 21-inch monitor displays 1152 pixels horizontally and 870 vertically. An image
with pixel dimensions of 1152 by 870 would fill this small screen.
Image resolution
The number of pixels displayed per unit of printed length in an image,
usually measured in pixels per inch (ppi). An image with a high resolution contains more,
and therefore smaller, pixels than an image of the same printed dimensions with a low
resolution. For example, a 1-inch-by-1-inch image with a resolution of 72 ppi contains a
total of 5184 pixels (72 pixels wide x 72 pixels high = 5184). The same 1-inch-by-1-inch
image with a resolution of 300 ppi would contain a total of 90,000 pixels.
24:1
3:1
