Usng a Scanner
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Preparing the Diskette
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Resolution and Scaling
The aim should be to make the smallest possible file with adequate quality. The final
printed document will come out at 600 dpi, so it is pointless scanning with more
than this. For colour and greyscale, this means a maximum of 200 dpi because the
printer cannot handle any more. For most images a density of 300 dpi will provide
adequate quality.
The final size of the image can also have an effect on the density because scaling
down a scanned image in the document will concentrate the information i.e. reducing
a 600 dpi image to half of its size will increase the density to 1200 dpi.
The way to set up the scanner is:
- Choose lineart as default, only use greyscale or colour if absolutely
necessary.
- Choose the zoom to produce an image of the correct final size (50%
zoom reduces the file size by a factor 4)
- Set the density to 300 for lineart or 100 for greyscale and colour.
Dark Backgrounds
Scanning images with dark backgrounds (like oscilloscope photographs or screen dumps)
produces large files. By using an image inverter (usually an option on the
scanner program) and using the negative, the information remains as clear but
the file is dramatically smaller.
Photographs
Photographs can be very difficult to handle: there is an inevitable loss of resolution
and the drawing the image on the screen may be slow if they have been scanned.
Images from a digital camera usually work well because the distiller is able to
compress them effectively.
Using compressed JPEG images (with e.g. Adobe Photoshop) is
another way to achieve high resolution with minimum file size.
Performance in the PDF file is quite variable and
authors are recommended to use
the test facilities set up by the editorial board if they are in doubt.
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JP 10th
September, 1997