Most of the sherd images on the Atlas pages are produced from images originally generated by a flat-bed
scanner. The following notes describe the procedures and some of the pitfalls. This technique should be
considered experimental.
I first encountered the technique of scanning shallow three-dimensional objects in the context of scanning
botanical samples to produce images for an electronic herbarium. For example:
Not all flat-bed scanners seem to have enough depth of field to capture such an image and some
experimentation is necessary. The software/hardware combination employed for the Atlas images is listed at
the bottom of this page.
My French correspondent tells me that a CCD (charge-coupled
device) scanner gives far better depth of field than a CIS
(contact image sensor) scanner, though the latter are simpler to
manufacture have lower power consumption and are cheaper. The
difference between the technologies is explained on this page (or here). There are several web sites describing experience with
scanning 3D objects with sample scans of (for instance) flint
arrowheads or antique
pistols.
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The initial technique investigated (which had proved successful with the botanical images) was to place the
sherd directly on the glass plate of the scanner and place a shallow box lid with a white interior over this.
The resulting image captures the essentials of the sherd, though it can be rather dark, particularly with
larger sherds, and the background shows as a grey or brown colour. This can be adjusted using image editing
tools, or the sherd can be cut out from the surrounding background.
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Sherd scan using shallow box lid as background
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To counter the problems of the background colour further experiments were conducted by supporting a small
light panel with a daylight balanced tube over the scanner. The resulting images are brighter and have a
paler background.
The light panel can propped over the scanner and sherds on
blocks of wood or polystyrene. The best sherds are clearly
those that are both relatively shallow and flat, though
sherds can also be propped on their edge to enable the
break to be scanned.
The glass surface of the scanner can (and should) be
protected from the sherds by a thin transparent sheet of
plastic film, such as that used on overhead projectors.
When this becomes scratched it can be replaced. Any
light leaking in from the sides into the scanning area
can be blocked with polystyrene blocks (such as offcuts
from insulation boards).
The highest resolution (600dpi) scans can produce a very detailed image similar to that seen when sherds
are examined using a binocular microscope, but these can be too large for current web usage.
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Sherd scan using light panel as back lighting
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Further processing is not usually necessary or desirable,
but the Unsharp Mask (see right) can be used emphasize the
surface or texture of some sherds. This and other
treatments can often be automated using batch tools such as
those in the ImageMagick package without resorting to
time-consuming GUI graphics programs.
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Sherd scan enhanced with Unsharp Mask
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Sherd scan resized with added scale
The final image for the Atlas has a scale added as an
overlay with a perl script using the ImageMagick module.
A clear advantage of these images over those produced with
a digital camera, for instance, is that they are at a
known, fixed scale.
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Hardware
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Epson Perfection 1200U scanner.
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A4 light panel with cold cathode fluorescent tube providing daylight balanced light (Jessops LP812).
Software
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Xsane scanning software. The Sane scanning package allows a scanner
attached to one machine on a network to be shared.
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Gimp the GNU Image Manipulation Program.
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ImageMagick image display,
manipulation and conversion package.
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