 Graphics > Notes
About Graphic Formats
There are two basic classes of graphic file formats:
Bitmap graphics (raster graphics) are collections of dots (pixels)
that fit together to make up an image. Bitmap file formats include:
-
PNG
-
TIFF
-
GIF
-
JPEG/JPG
-
BMP, MacPaint, and XBM
Vector
graphics use lines--not dots--to describe images. Some example
of popular vector graphic formats include:
- AI
- EPS
- WMF (Windows Metafile)
1)
Bitmaps
The bitmap file formats are good choices for photographs
and screenshots, as well as images for online uses where portability
and flexibility are key. In general, a file format is chosen based
on the following:
- Color
depth: Bitmap file formats offer a range of color depth
support. Choose a bitmap format that supports the depth of color
you need.
(That is, you don't need a format that supports millions
of colors for a black and white line drawing.)
- Compression: Uncompressed
bitmap files can easily be huge, so you will likely
want to choose a format that supports compression.
Compression can be lossless, in which all of the information
in the original bitmap is retained, or lossy, in which
some information
(subtle color shifts, for example) is discarded to obtain
a smaller file size.
- Transparency: Some graphic formats--notably PNG and GIF--support transparency,
which allows you to replace the background color
of an image and thereby get rid of distracting details
and bring focus to the main part of the graphic.
- Portability: Portability refers to how easily you can open, modify, and view
the files using different operating systems and software.
Not all bitmap file formats are supported well on all
platforms, so steer clear of proprietary formats if you'll be
using the images
on a variety of computer platforms. Similarly, not all
bitmap formats work well on the Web.
Following are descriptions of some of the most common or popular bitmap
graphic formats:
- PNG (pronounced "ping") graphics are a
relatively new standard from the World Wide Web Consortium (w3.org,
the people who
bring you HTML and XML and many other standards). This
format is recognized by the newer Web browsers, provides good,
lossless compression,
supports transparency on the Web, and supports unlimited
colors. PNG is a good choice for any Web images (if your readers
will be
using browsers that support this format), as well as
a good choice for archiving bitmap images (because of its compression
capabilities).
PNG graphics are less common because the format is so
new, but you can expect to see more and more of them as the format
gains support
and popularity.
- TIFF graphics are an old standard with dozens (if not hundreds)
of only marginally interchangeable sub-formats. TIFFs are as close
to a standard file format for bitmap files as exists, but differences
across platforms and software applications can wreak havoc on
portability. If you're trying to maintain portability, be sure
and test thoroughly and note all of the myriad options available
before using this format. TIFF files are not Web-friendly, can
be compressed in several ways (but need not be), support any color
depth you choose, and are widely recognized.
- GIF graphics are the oldest Web-friendly graphic format. GIFs
are recognized by all graphical Web browsers, provide good compression,
but support only up to 256 colors. GIF is a safe choice for any
Web images but is better for drawings or illustrations. Photographs
suffer in GIF format. Additionally, GIF support of rudimentary
animation and transparency makes GIFs quite popular for special
effects on the Web.
- JPEG/JPG graphics are widely supported on the Web and a good
choice for photographs. JPEG/JPG images support millions of colors
and can be compressed to be quite small. However, the lossy compression
makes JPG files a poor choice for archiving or any other applications
in which you might later need the full image quality. If you need
a JPG image (likely for the Web or for email), maintain an archival
copy in a format like PNG or TIFF and save a copy as JPG when
you need it.
- BMP,
MacPaint, and XBM are mostly proprietary or operating system-specific
file formats. They offer few advantages and worlds of potential
problems.
2)
Vector
Vector graphics are good choices for any original drawings or illustrations
that you create on the computer because they scale more easily and
smoothly than bitmaps do. Your main consideration in choosing a
vector file format is portability among software applications and
operating systems. Quality-wise, little differentiates one vector
format from another:
- AI (Adobe Illustrator) files are actually EPS files in disguise.
If you use Illustrator (even on multiple platforms) this is a safe
choice, but the portability to other software packages is iffy to
poor.
- EPS (Encapsulated Postscript) files are similar to, but somewhat
more portable than AI files. EPS files are great for Macintosh
or UNIX, but often problematic on Windows machines or on computers
that do not have a PostScript printer installed (although workarounds
do exist). The list of gotchas is fairly long, but EPS files are
still the best bet for portable vector graphics.
- WMF (Windows MetaFile) If you're using only Windows systems
and have no need to maintain portability with other platforms,
the WMF format is a good choice. They are simply not portable
to other platforms, though, so do keep this in mind when choosing
a vector format.
In general, try to store and work with files in the native format
of the program you are using (e.g., AI files for Adobe Illustrator,
VSD for Visio, PSD files for Paint Shop Pro, etc.). The native formats
retain more information about your files and provide more flexibility.
Most screen capture programs have no native format, so
you'd save the files in whatever format you ultimately
need. After you're ready to import the file into a word processing
program, put
the file on the Web, or mail the file, you can save a copy
to your choice of appropriate formats and use the copy as
you choose.
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