Jökull - 01.12.1994, Síða 49

Fig. 7 Block diagram of the IBAS image analysis system.
7. mynd. Yfirlitsmynd af myndgreiningarkerfinu IBAS við
Jarðfrœðistofnun Danmerkur.
MEASUREMENT OF GRAIN SHAPE BY
ELECTRONIC IMAGE ANALYSIS
The image analysis system used is an IBAS 2
(manufacturer Kontron Bildanalyse GmbH). The
IBAS image analysis system is a two processor sys-
tem with a serial microcomputer as the control unit
and an array processor as the image handling unit.
The image handling unit consists of a 4 Mb image
memory (VMB), a memory address controller (MAC)
and a 10 MIPS microprogrammable array processor
(MIAP). The image analysis configuration is shown
in Fig. 7. The analogous signal generated in the TV-
scanner (Bosch TYK 9B) mounted on a Zeiss Tesso-
var (photomacrographic zoom system) is digitized in
the analog/digital (A/D) converter and stored in the
VMB with a spatial resolution of 512x512 pixels and
a grey value resolution of 8 bit (256 grey values). The
stored images can be displayed on a conventional
monitor via the D/A converter. The image manipula-
tion is carried out by microprogram orders and micro-
program sequences selected from the control comput-
er. The programming is menu-oriented and is con-
trolled via the keyboard and/or the digitizer tablet.
The image analysis sequence can be subdivided
into six main routines. These are calibration, image
grabbing, shade correction, segmentation, measure-
Fig. 8 The image analysis sequence.
8. mynd. Flœðirit yfir myndgreiningarferlið.
ment and data storage (Fig. 8).
Before an analysis is initiated, the system must be
calibrated. The various buffers are cleared and the
mm to pixel conversion factor is calculated for the se-
lected magnification. Given the size range of the
tephra grains (0.5 mm to 1.0 mm) a magnification re-
sulting in a conversion factor of about 0.02 mm/pixel
is appropriate. The sample is labelled using a well de-
fined identification code.
Three images of the actual field of view are digi-
tized and stored and the average image is computed.
This is done to reduce the effect of TV-scanner arti-
facts (noise) on the stored image.
An electronic illumination correction (shading) is
necessary because the set-up of lamps does not easily
produce evenly illuminated fields of view. A prerequi-
site for the correction is knowledge about the grey
value distribution of the entire background. A simple
way of obtaining that information is to record an
empty field of view. Another method to generate the
grey value distribution of the background is by apply-
ing an electronic filter in order to eliminate the tephra
grains and overwrite them with interpolated back-
ground values. By setting grey value thresholds a
rough discrimination of the background is achieved.
JÖKULL, No. 44
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