A pixel is “the smallest unit of information in a digitized
image.” Every digitized image is a compilation of little, square pixels that,
together, create the image you see.
Before digital cameras, scanners were used to digitize an
image, expressing it as pixels. With digital cameras this is not necessary, as
the image is digital when it’s captured. Point-and-shoot cameras capture images
as JPEGs—a compressed file format. However, advanced level cameras allow for
images to be stored in RAW format. RAW format files have very little, if any,
compression made by the camera. This file format is exceptionally flexible, as
it allows for more control in editing images in Photoshop. However, the file
size of RAW images is much larger when compared to JPEG.
Generally, image resolution is measured in pixels per inch
(ppi). While the proper resolution for web images is clear, 72 ppi, appropriate
resolution for printing is debatable. However, in most cases, a resolution of
300 ppi for printing is sufficient. Bitmap images are sometimes referred to as
line art images. These images “contain only black and white pixels with no
shades of gray.” Bitmap images can produce high-resolution images in a small
file size.
More than likely, when using an image in a design, you will
have to crop and/or rotate it to make it fit the design properly. When cropping
an image, it’s generally a good idea to keep the original, uncropped image. If
you are certain of how you want to crop the image and its end use, it’s not
necessary to have the original. However, having the original gives you the
option of making further changes and avoids losing more details of the image.
Rotating and resizing images also causes loss of detail and damages the
appearance of an image. To minimize the loss of detail, it’s generally a good
idea to rotate images in even increments. Additionally, avoid consecutive
alterations, as they are harmful to an image. Performing transformations in
image-editing software is better than performing them after they are in the layout
software because it most accurately portrays how the finished product will
appear.
The file format you save a raster image is mainly dictated
by its intended use. The most commonly supported image file format is TIFF
(tagged image file format), it “supports multiple layers as well as RGB and
CMYK color spaces, and even allows an image to contain sport-color channels.”
The EPS (encapsulated PostScript) file format is strongly associated with
vector art. However, it can include raster images, vector images, or a
combination of both. The pixels in the EPS file format aren’t unlike those in
the TIFF file format, they are merely contained and expressed differently. Therefore,
both TIFF and EPS are appropriate file formats for printing. Photoshop PSD and
Photoshop PDF files are also appropriate for print. However, blending modes in
Photoshop files are not supported by the other software (i.e., illustrator).
Not all formats are intended for print, some are for onscreen and web use
only—PNG, BMP, GIF, JPEG.
Unlike raster images, vector graphics are pixel free. Thus,
vector graphics are not restricted to the scaling limitations like raster
images. When creating a vector graphic in illustrator the less clicking you do
in a line the better. This results in a smoother looking image. Additionally,
there are tools in Illustrator that allow for smoothing a graphic and simplify
paths. At one time, EPS was the most widely used file format for vector images.
EPS files can be placed in and understood by a broad range of applications (i.e.,
page layout software). However, these programs do not allow for editing of the
EPS file—all editing must be performed in the software in which it was
generated. There really isn’t a reason to save an illustrator created vector
image as an EPS unless it is being submitted in a workflow that doesn’t support
native Illustrator (.AI) format. Additionally, saving a vector as a PDF should
only be done if it a finished artwork—do not save it as a PDF it is to be
placed in a layout file. All of these file formats are appropriate for print
depending on the circumstance. While many file formats for vector image allow
for importing into page-layout software, not all are suitable for print. Microsoft Windows Metafile Format (WMF)
and Enhanced Metafile Format (EMF) do not support CMYK and, therefore are not
proper formats for print.
When a vector graphic includes text, it is important to
perform the necessary process that will ensure it prints as expected. It is
possible to use the process used in page-layout software. However, Illustrator
isn’t able to automatically perform this procedure. There is a plug-in
available that would enable Illustrator to do it, but embedding and outlining
are more commonly used for vector graphics with fonts. As long as the font allows,
Illustrator “can embed fonts in an EPS or AI file for placement in other
programs.” With this method, the font information is available for printing
and/or display, but not for editing. In order to edit, the necessary fonts must
be activated. Keep in mind that not all fonts can be embedded, a font creator
can prevent this. This doesn’t prevent you from using the font, but it doesn’t
allow for the font to be embedded in AI, EPS, or PDF files. Outlining text is
an option that eradicates the worries related to embedding fonts. Although
outlining text eliminate hinting, which can cause text to not print as
expected.
Source: Real World Print Production with Adobe Creative Suite Applications by Claudia McCue
