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The user may open the overview window to show a
reduced version of the entire image. The overview
window helps to keep the synopsis and allows to move to
any image point with a single mouse click. À rectangular
frame is highlighting the region displayed in the toplevel
window.
Data structures
Image data are stored in a fixed four step resolution
pyramid with the following levels of detail:
(1) The icon, that is used by the image browser to
identify the image.
(2) The overview image displayed in the overview
window.
(3) The toplevel image displayed in the toplevel
window.
(4) The image itself that is stored in a tiled image
file format with tiles of 256 x 256 pixels each.
Storing the images in this special tiled format guarantees
access to the image data independent of the original
image size and the position of the desired region within
the image.
The storage overhead for the first three images is less
than 1.5 Mbytes. Any other information like color table or
additional image information are stored in human
readable ASCII files that are totally maintained by the
system.
Data compression could be applied to the image data
with little modification of the code but it would slow down
the image access significantly. Since lossy compression
can not guaranty the requested accuracy only lossiess
methods can be used. The expected storage reduction of
these methods is about 5096. Comparing these figures to
the significant loss of performance we decided not to use
compression so far.
Photogrammetric Applications
Based on its data management and image measurement
functions XLTT allows to manage digital photogrammetric
projects. Embedded into a software environment
including programs for photogrammetric set-up and
bundle adjustment XLTT was used in several
photogrammetric projects. With XLTT and a handful of
programs for data file conversion we are now able to
operate in a completely digital environment. At one end a
scanner (VX3000 film scanning system of VEXCEL
Imaging Corp.) is used to produce digital images. Then
XLTT manages the image data and the coordinate
measurement task. Using the monoscopic and the stereo
measurement tools, which are partly under development,
2D and 3D object coordinates are measured and stored
on data files. These data are imported into CAD systems
or processed by TIN software and lead to the CAD model
of the object.
Outlook
XLTT is still growing and several modules are under
development or in a test phase right now. Some of these
modules are:
(1) Stereo matching tool,
(2) Stereo display tool,
(3) Stereo mensuration tool,
(4) Additional input and output filters (e.g. DXF for
line drawing overlays).
It is possible to translate the IDL code to C or C++ to get
a higher performance so that online image compression
would become possible. Replacing the image database
module by some ‘professional’ data base system would
result in higher flexibility and performance. Multi user
access to a single image data base in a distributed
photogrammetric computing environment would also
need a switch to custom code
References
[1] Ebner, H.; Fritsch, D.; Heipke, Ch.;(Eds.), 1991.
Digital Photogrammetric Systems, Wichmann
Verlag, Karlsruhe.
[2] Leberl, F., Ebner, H., Dowman, l., (Eds.), 1992.
Softcopy Photogrammetric Workstations,
Photogrammetric Engineering and Remote
Sensing, Vol. LVIII, Nr. 1, Bethesda.
[3] deValk, J. P. J. (Ed.), 1992. Integrated Diagnostic
Imaging: Digital PACS in Medicine, Elsevier,
Amsterdam.
[4] Wiesmann, W. et al., 1992. PACS - demand on
spatial and contrast resolution for digital
workstations, in [3], p. 201 ff.
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