Datasheet

10
Welcome to Adobe Photoshop CS4
Part I
Bitmap (raster) and vector
A bitmap, or raster, image gets its detail from the number of pixels it contains, which is called reso-
lution. Since they depend on the number of pixels for detail, bitmap images are said to be resolu-
tion-dependent. Resolution is commonly expressed in Photoshop as the number of pixels per inch
(ppi). Choose Image ➪ Image Size to see the resolution of an image.
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Appendix A includes a chart of required resolutions for different kinds of images.
The more pixels a bitmap image has, the more detail the image can contain. Larger bitmap images
need to have more pixels than smaller ones in order to preserve detail. Photos are bitmap images,
and paintings or illustrations that donít have well-defined shapes are usually bitmap images.
Very simple images can also be bitmap format (see Figure 1.2). It is necessary for bitmap images
to have many pixels if they need to show a lot of detail, and in the case of photos or scanned
images, the detail must be captured in the initial creation of the image. It is possible for an image
to look deceptively good on-screen but bad in its printed form. If it doesnít have enough pixels,
the printed version may lack detail and look blurry or pixilated (the undesirable appearance of
individual square pixels or their jagged edges).
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Photoshop images can have multiple layers that contain different parts of an image.
Having parts on separate layers can make it easier to manipulate the individual
parts. You can also create layer masks that can be used to hide and show parts of a layer. Layers
are explained in more detail in Chapter 8.
FIGURE 1.2
A bitmap image
A vector image gets its detail from shapes and lines that are mathematically calculated. The shapes
and lines in vector images retain their smoothness no matter how much they are resized, regardless
of how many pixels are in the image. Although there are pixels in a vector image that display color,
a vector imageís detail does not depend on the number of pixels in the image. Therefore, vector
images are resolution-independent.