Common
file formats
BMP
– This is the Microsoft Windows bitmap format. It's a fairly
compact (compression is optional, but usually turned on) format for
images up to 24 bit. BMP is the native bitmap format for the Windows
environment.
EPS
- Gives excellent quality images, although much larger than tiffs.
can be imported into Quark. Encapsulated Postscript is a flavour of
Postscript (see below) which can be included in other documents –
if your software supports it.
GIF – Graphics Interchange Format is a very
efficient, and still quite popular picture format. There are two flavours
of GIF, the old 87 and the newer 89a. 89a adds several extra features
like transparency (so background graphics can "show through"
the GIF in places) and animation. GIF animations are a very popular
form of Web multimedia, because they're small and display on all current
graphical browsers without needing a special plug-in or taking up
much CPU time.
Unfortunately, GIF pictures can only have 256 colours, or 256 shades
of grey. 256 greys is photo quality so GIF is fine for any monochrome
image, and 256 colour looks OK for many pictures, but it's no use
for professional imaging.
GIF images can also be interlaced, so that you can see a low resolution
version of the picture before downloading very much of it. GIF interlacing
has four passes, which show one out of every eight lines, then another
eighth of the image, then another quarter, then the remaining half.
GIF is a data-stream type format, like JFIF, so you can view partially
downloaded images whether or not they're interlaced – without
interlacing, a 25% downloaded picture gives you the first 25% of the
lines, starting at the top.
JPEG – The JPEG (Joint Photographic Experts
Group) File Interchange Format, commonly called JPEG and with the
filename suffix .JPG, can be the most efficient image storage method
of all – at a price.
The idea of JPEG is that as it compresses the data it throws some
of it away – technically, this is called "lossy compression".
You can configure how lossy you want your JFIFs to be (well, you can
if you're using photoshop); 100% quality gives you almost exactly
the same result as the original picture but also gives you a gigantic,
uncompressible file. 10% quality takes up much less space but looks
dodgy. You have to strike a balance.
JFIF can store up to 24 bit colour, so it's suitable for professional
use, and it can do interlaced display like GIF (called "progressive"
JFIF), which along with its small file sizes makes it the standard
format for Web graphics. Like GIF, JFIF is a data-stream format –
you can view images before you've got all of the data. Also like GIF,
JFIF supports interlacing.
The JFIF format also supports CMYK (process colour – Cyan, Magenta,
Yellow and blacK in a subtractive colour model, as against the
additive Red, Green and Blue more commonly used) images, which makes
it suitable for use in publishing applications. CMYK support was added
in a later version of the standard, though. This means that quite
a few Web browsers, do peculiar things when fed CMYK images. There's
no reason to use CMYK JFIFs unless you're sending the image to a CMYK
output device, which a monitor isn't. Usually, CMYK ones get through
because someone's converted a CMYK image of some other format, like
TIFF, without changing the colour model.
PICT – Pict is the all-in-one Apple Quickdraw
metaformat. It can include bitmapped or vector images, and can use
different compression schemes.
PS – Adobe Systems' Postscript isn't an image
format, per se – it's a page description language, originally
conceived so computers could send very accurate page descriptions
to the then-new high resolution laser printers. You can save black
and white or even colour pictures as Postscript, but you'll end up
with a very large file.
PSD – Adobe Photoshop's native format, which
stores all of its layer and selection and miscellaneous other image
data.
TIF – TIFF (to give the full acronym) stands
for Tag Image File Format. TIFF was a large, unwieldy, 24 bit
format until version 6 came out, which supported compression and made
it less painful. TIFF is, nonetheless, a very popular professional
graphics format that is accepted by Quark.
