All those different types of computer files used for graphics can be
confusing. They all have their strengths and weaknesses and particular
roles to play. We won't bother with what their full names are, just
accept that they are referred to by their file extensions. TIF, BMP, PICT, GIF & JPG files are all raster or bitmap format
files, that is they describe a picture dot-by-dot. TIFs are used by
both PC and Mac and are how most scans and pictures are saved. Large
size is a problem but can be saved in RGB (Red Green Blue, for on screen
or colour laser printing) or CMYK (Cyan, Magenta, Yellow & Keyline,
for process colour printing) form. BMPs are similar but are PC specific
- used for screen backgrounds. PICTs are a Mac format. Both of these
last two have their place but may be better saved as TIFs for cross-platform
compatibility. GIF is a very useful file for Internet use as it only holds information
in up to 256 colours, with less colours and solid colours making smaller
file sizes. Can even hold a sequence of pictures called an 'Animated
GIF'. Another feature of this format is its ability to have transparent
gackgrounds - very useful for Web graphics. JPG files are used both for printing and the Net due to their compression
into a small final size. 10% of a TIF size file or less is often the
case. Not limited to 256 colours but can cover 16.7 million colours
so are used for photos where better colour detail is required but in
a small file size. The more compressed, the more quality will be lost
but most people wouldn't notice the differences at moderate compression. Saving a file in RGB is used for on-screen (Internet, PowerPoint) and
some direct colour printing methods. CMYK is the colour format required
to create separations for offset printing and for most high end colour
printers. Save your files as required for each job or ask your bureau
to convert to CMYK if you don't have the required software. EPS, WMF, CDR & Illustrator files are what's called vector graphic
formats in that they 'describe' a shape such as a circle by diameter,
position, x and y stretch, filled colour and edge thickness and colour.
This usually gives a small file size with the capability to be scaled
up without jagged edges. Note: These file types can also contain bitmap
graphics that will become jagged if over enlarged. WMFs are PC specific and only contain 256 RGB colours so won't separate
to process colour. Great for local printing to lasers due to their size.
Will often cause printer conflicts if printed via postscript, even to
Acrobat format files. CDRs are CorelDRAW files that are better saved as EPS files in most
cases, as are files created in Illustrator. The complexities get worse the more you aim at high end quality but
these guidelines will give you a start. Software determines what you
can and can't do and will often create strange file types. Try to keep
to those described above. Your bureau or printer will be happier and
end results should be closer to what you expect. Remember, talk to your printer and your output service bureau for advice
before you set your job incorrectly. Ask about line screen resolution,
process or spot colour options and whether you need to have everything
in CMYK. Lastly always send ALL your graphic files with any job. That
way, if a problem arises with a graphic, the bureau can usually rectify
it.
GRAPHIC FORMATS
by Colin Johanson (1998)
Raster or 'bit-mapped' graphics need to be scanned or created at sufficient
resolution to avoid the 'jaggies' - stepping visible on angles or curves,
that make a print look rough. A guide is 50 to 100 pixels (dots) per
centimetre (130-260 dpi) of final size graphic for standard to quality
work in colour. 240+ pixels per centimetre (600+ dpi) for line art (diagrams
or cartoons) in limited colours.
EPSs are the standard for high end printing and can be made from a TIF
to create a transparent background - very useful. You will need to print
any file containing EPS graphics via a Postscript printer and driver.