A bitmap, or raster, image gets its detail from the number of pixels it contains, which is called resolution.
Since they depend on the number of pixels for detail, bitmap images are said to be resolution-
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.
CROSS--REF 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).
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.
Images that are made up of some combination of solid colors, simple gradations between colors,
distinct shapes, and lines can be constructed as vector images. Simple logos and cartoon-type illustrations
often have these characteristics, but some complex illustrations do, too. While eliminating
the worry about having enough resolution is an advantage, keep in mind that the vector components
often have to be created or adjusted by hand, so creating vector images can be more timeconsuming
than creating bitmap images. Bitmap images can be created by taking a photo with a
digital camera or by scanning an image. Vector images must be hand-drawn using vector tools in
software such as Photoshop or Illustrato. Note that an image that is a good candidate for the vector
format does not necessarily have to be in vector format. It can be a bitmap image.
Since numerous file formats can contain vector information, it is not immediately apparent
whether an image is a vector image or contains vector information. If an image contains vector
information, you can find it in any of four places in the image:
On a vector layer mask (right-click [Ctrl-click on a Mac] on a layer mask; if you see vector
options, it is a vector mask rather than a standard layer mask).
On a type layer in the Layers palette. As long as you can still see the T in the layer thumbnail,
the type is made up of vectors.
On a path in the Paths palette. All paths in the Paths palette are vectors.
In a Smart Object layer. Some have vector information embedded. You can double-click
the Smart Object layer to open its linked file and examine it for vector information. The
linked file may be a Photoshop or Illustrator file. (Content in Illustrator files is typically
vector.)
There can be a combination vector and bitmap image. If an image contains bitmap parts, the image
has to have enough resolution to describe the detail in the bitmap parts.
In Figure 1.3, the vector information — the path of the dog — is shown on the vector layer mask.
A vector-only image in which the active vector information is contained in a vector layer mask
The type of layer shown is a shape layer, a layer completely filled with a color (in this case, black)
and that has a vector layer mask that hides part of the color on the layer, leaving visible what looks
like a shape. White areas on a vector mask allow the color on the layer to be visible; gray on a vector
mask hides the layer’s color. The layer name is “Shape Layer-Black,” and the vector layer mask
is immediately to the left of the layer name.
The dog path on the mask allows the corresponding area of black color on the layer to be visible,
and the leftover gray part of the vector mask hides all the other black on the layer (makes it invisible,
or transparent). The vector mask in Figure 1.3 is small, so it’s hard to see that the dog path on
the vector mask is filled with white; if you examine the file on the CD, you can see that it is filled
with white.The dog path has also been saved in the Paths palette, just for safekeeping. All paths in
the Paths palette and on vector layer masks are vector components of the image.
Figure 1.4 shows a vector image in which the vector information is stored in a Smart Object.
Smart Object layers have a special icon at the lower right of the layer thumbnail and can contain
linked Photoshop or Illustrator files. Smart Objects also allow certain Photoshop functions that
regular layers may not allow. To confirm that a Smart Object contains vector information, you can
double-click the Smart Object, and check the linked file that opens for vector information.
Vector-only files can contain color, but in order to take full advantage of the vector format, separate
colors must be on separate layers; however, gradients that include more than one color can be on a
vector layer. Gradients generally don’t contain the kind of detail that requires the bitmap format.
Figure 1.5 shows a vector-only image with four colors. The layers are linked so that they resize
together.
Figure 1.6 shows a combination vector and bitmap image. The type layer contains vector information,
the Star Shape Layer contains the star shape in the vector layer mask, and the Background
A combination vector and bitmap image
While the vector parts of the image are still resolution-independent and can be resized with no
worries, the image must have enough resolution to describe the detail in the bitmap photo.
Printing an enlarged bitmap image that doesn’t have enough resolution to describe the detail in the
enlarged size often results in a pixilated look, shown in Figure 1.7.
Printing a bitmap image at a size larger than its resolution can result in a pixilated look.
Figure 1.7.
You can print a vector-only image as large as you want without regard to image resolution, and it
will still look smooth and not pixilated, as shown in Figure 1.8. A vector-only image will appear
pixilated on-screen if you zoom in far enough, but it will print smoothly.
A vector image will print smoothly without regard to resolution, even if it is enlarged.
Figure 1.8
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