Scanning drawings

Mike Dallwitz miked at ENTO.CSIRO.AU
Mon Feb 20 19:09:48 CST 1995


                                                              20 February 1995

> From : Rolf R. van de Pavert rpvt at cbs.nl
>
> The "Rijksherbarium" of Leiden University has an outstanding collection of
> 4500 original diagnostic pencil sketches on orchids made by J.J. Smith in
> the first half of the century. Paper is losing its quality fast and there is
> no money to have this collection properly photographed and digitized. I have
> been asked if I saw any possibility to organize a database to contain this
> material.

The most important thing is to preserve the images by scanning and storing
them in an appropriate way. A database for conveniently accessing the images
is a separate matter. Initially, even an index in a word-processor file would
do.

The Introduction to Computer Images supplied with the DELTA programs (in the
file deltad at .exe) contains recommendations about methods of scanning and
storing images, and about the necessary hardware. An updated version will be
included in the next DELTA release, due in 2 or 3 weeks. Here are of the most
relevant parts of this document.


Creating and editing images

Bitmap images can be produced from existing drawings or photographs by
`scanners', which measure the brightness and/or colour of the original at each
point of a grid. `Flatbed' scanners scan large, usually opaque, material, such
as the page of a book. The maximum resolution is usually 300-600dpi. `Slide'
scanners scan small transparencies, such as 35mm slides or negatives. The
maximum resolution is usually between 1000 and 6000 pixels along the longer
dimension of a 35mm slide. Grey-scale (`black-and-white') scanners should be
capable of distinguishing 256 grey levels if continuous-tone photographs are
to be scanned. Colour scanners should distinguish 256 levels for each primary
colour.

Commercial scanning onto PhotoCD is probably the most cost-effective way of
digitizing slides, when the cost of the necessary equipment, the operator's
time, and the quality of the results are taken into consideration.

Bitmap images are edited with `image editing' or `paint' programs, such as
Adobe Photoshop, Aldus Photostyler, or Micrografx Picture Publisher. Image
editing programs usually have provision for scaling, and conversion between
different formats, but these facilities may be rather limited. Image Alchemy
(Handmade Software, 15951 Los Gatos Blvd, Suite 17, Los Gatos, CA 95032, USA)
is a comprehensive program for format conversion (including JPEG), colour
reduction, and scaling of bitmap images on MS-DOS PC's and Sun computers. A
shareware version of Image Alchemy, and some other shareware or free programs
for viewing and processing bitmap images on MS-DOS PC's, are available on the
Internet by gopher or anonymous ftp from the following Internet hosts.
    muse.bio.cornell.edu  (directory: /pub/delta/graphics)
    spider.ento.csiro.au  (directory: /delta/graphics)

Hardware for displaying and editing images

Software for editing bitmap images makes heavy demands on memory and
processing power, and images can require large amounts of disk space. The
following configuration is suggested as the minimum desirable for this work on
IBM-compatible PC's. Motherboard with 486 66MHz CPU and local bus (VESA or
similar), 16MB RAM, local-bus Super VGA display card with Windows accelerator
and at least 32768 colours at 800x600 (including Windows drivers), high-
resolution monitor (equivalent to NEC 5FG or better), 400 MB hard disk. A high-
speed, high-capacity tape drive is also very desirable. DDS (DAT) tapes have a
capacity of 2GB, and are small and cheap, although the drives are fairly
expensive - currently about AU$1500. Compatible drives are made by several
manufacturers.

Recommendations for storing scanned images

Images should be scanned and stored in such a way that they will be able to
take advantage of improvements in display hardware. The resolution of the
master copies should be as high as possible (preferably at least 1024x768).
Lower-resolution versions can easily be made for display on currently
available hardware.

Line drawings can be stored compactly as GIF or compressed TIFF files with 16
or 256 grey levels. Using 16 or 256 grey levels gives smoother lines than 2
levels (black and white). Using 256 levels does not give noticeably better
results than 16 levels, and produces larger files. However, it is best to use
256 levels until any touching up, such as cleaning up the background and
increasing the contrast, has been done.

Continuous-tone colour images should be stored as TIFF or JPEG, and grey-scale
images as GIF, TIFF, or JPEG. They can be converted to other formats as
necessary. Mapped formats such as GIF should not be used for master copies of
colour images, because of the sometimes severe artifacts and information loss.
If JPEG format is used for master copies, a high quality factor should be
used. Even then the files will contain artifacts, which, though not
perceptible, may have adverse effects on later processing of the image,
particularly sharpening.

Mike Dallwitz                                  Internet md at ento.csiro.au
CSIRO Division of Entomology                   Fax +61 6 246 4000
GPO Box 1700, Canberra ACT 2601, Australia     Phone +61 6 246 4075




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