Appendix 5:
Tips for scanning images
If you've got some pictures of Jo but are unsure about how to
scan them, here are some hints and tips:
Getting started
- Use a flatbed scanner for best results. Sheet-feed scanners can
be rather tricky to use (you'd have to cut up the magazine for a start)
and hand-held scanners are a big no-no. Reasonable-quality flatbed
scanners can be bought in the UK for less than £100.
- When scanning a newspaper or magazine picture, put a piece of
black paper or card directly behind the image being
scanned. This reduces the amount of "break-through" of the
image on the other side of the page.
- Make sure there's no dirt or dust on the scanner plate (or on the
image itself). The scanner won't know the difference and will assume
that any dust is part of the image.
- Try to get the image reasonably flat. If there are any bumps or
creases, the image won't scan properly.
Selecting the scan resolution
Before we proceed, a quick detour to consider just what it is
you're going to be scanning. If you're in the fortunate position
of having an original photograph or transparency, you can skip this
section and go straight to Performing the scan
below.
However, it's far more likely that you've got a printed
picture in a newspaper or magazine. This sort of picture is composed
of an array of coloured dots printed on the paper (look at a picture
with a magnifying glass and you'll see the dots more clearly).
This dot pattern has a certain resolution, expressed in "dots
per inch" (dpi). A typical magazine picture might use
133dpi or 150dpi resolution, which leads to two problems:
- If you select a scanner resolution that's much higher than the
original printed resolution, the individual dots are going to be
clearly visible in the output image.
- You might get an effect known as moiré patterning (it can
look like regular blotches, or a sort of "venetian blind"
effect) if the pattern of dots is at nearly the same resolution as
the selected scanner resolution. I'll explain how to deal with this
problem later.
You're going to be displaying the final image on a monitor: a
typical 17-inch monitor might have a horizontal viewable area of
roughly 12 inches and you might be running it at a screen
resolution of (say) 1024x768. In other words, 1024 pixels occupy
12 inches, which is roughly 85 pixels per inch. To put it another way,
in order for the picture to appear on the monitor screen the same size
it was in the magazine, you'd need to have the final image at a
resolution of roughly 85dpi (aside: for a 14-inch monitor at
640x480 resolution you'd need 65dpi).
You can sometimes get away with a modicum of enlargement beyond
85dpi, but once you get above the original printed resolution the
output image can only get worse.
Now consider the typical resolution offered by a scanner. Even the
cheaper scanners offer 300dpi, so it should be obvious that you
don't need the resolution of the more expensive
scanners when scanning magazine pictures (you might want some of their
other features, but I digress).
To summarize:
- For magazine pictures you should be aiming for a
final image resolution of 75dpi-100dpi, so that it appears the
same size on the screen as it did in the magazine.
- Don't enlarge the picture too much: it won't look any good.
Performing the scan
- Line up the image properly in the scanner. While most image
manipulation programs will allow you to rotate an image, this usually
results in loss of image quality unless you happen to be rotating it
by exactly 90 degrees or 180 degrees.
- Choose a suitable scan resolution (I would suggest somewhere
between 75dpi and 100dpi to begin with). If you can't select that
resolution directly, scan at a higher resolution with a view to
re-scaling the image after you have scanned it. But
DON'T get carried away when selecting the scan
resolution: if you're aiming for an eventual resolution of somewhere
around 75dpi-90dpi, there's not much point in choosing 600dpi (you'd
be generating a gigantic output file for no good reason).
- If you get moiré patterning in the image, check to see if
your scan software has a Descreen filter facility:
this will be effective in most cases. If that facility is absent, try
scanning at a higher resolution (e.g. 200dpi instead of 150dpi) in the
hope that you can use your image manipulation program to resize the
image and remove the patterning. If you're getting a venetian-blind
effect, you can sometimes get better results by turning the image
through 90 degrees (so that you're scanning it sideways) - it depends
on the scanner.
- Now it's time to save the image. If you're going to have to do
any work on the image (e.g. removing extraneous text, or re-scaling
it) it would be best to save it as a BMP or TIFF image. If you're
happy with the image as it is, you can use JPEG format (but see the
note about JPEG quality settings below).
Whatever you do, never, ever, save a colour photograph as
a GIF image - GIF images always produce poor renditions of
colour photographs compared to other methods.
Processing the image
Once you've got your image as a BMP or TIFF file, you can use your
favourite image manipulation program to make whatever changes you
feel are appropriate. This might include: re-scaling the image,
removing extraneous text, adding a caption, or whatever.
One word of caution: Don't get carried away at this point! There
are numerous examples on the Net of cases where the person doing the
scanning has decided to "improve" the image in all sorts of
weird and wonderful ways (the major culprit is inappropriate use of
the Gaussian Blur function of Photoshop) - the resulting image
could well be regarded as a work of art but it often bears little
resemblance to the original picture in the magazine.
JPEG quality settings
Yes, we've nearly reached the end. All you have to do now is
create the final JPEG image.
At this point I have to talk a bit about what JPEG encoding actually
does to the image. JPEG is known as a lossy image compression
technique: you end up with a much reduced filesize, but some information
has been discarded.
This is controlled by the JPEG quality setting in the program.
Basically, the larger the number, the higher the quality and the larger
the output file. Although it typically holds values between 1 and 100,
it is NOT a percentage! Even with the quality set to
100 you're going to lose some information: it's the way JPEG works.
So, what quality setting to use? This is very much "suck it and
see": it will vary depending on the image being processed (i.e.
whether it's sharp to begin with, or contains text which must be
readable in the final image) and how much of a trade-off you wish to
make between image quality and file size.
Some rules-of-thumb:
- I would suggest you start at a quality setting of somewhere around
75 and try adjusting it up or down until you get the effect you want
(either a small enough file or a sharp enough image).
- Values above 90 increase the file size dramatically with little or
no improvement in perceived image quality.
- Low values (i.e. below about 50) do produce extremely small files:
the downside is that you get artefacts appearing in the image. Any
sharp lines (text etc.) get a sort of "ghosting" effect
rather like watching a picture on a TV with a bad aerial; the image
also tends to take on a "blocky" appearance which is
difficult to describe in words but you'll soon recognise the effect if
you try it out for yourself.
Further reading
For more detailed hints and tips on scanning, I suggest you check out
http://www.scantips.com/