Herbert Jahn 
  
STEREO MATCHING FOR PUSHBROOM STEREO CAMERAS 
Herbert JAHN 
DLR 
Institute of Space Sensor Technology and Planetary Exploration 
Herbert.Jahn G dlr.de 
Working Group III/2 
KEY WORDS: Photogrammetry, Stereopsis, CCD Cameras, Image Matching, Parallel Processing. 
ABSTRACT 
A parallel stereo matching algorithm is presented which is mainly thought for the processing of images from pushbroom 
stereo cameras. The algorithm is designed for non-epipolar geometry, because of disturbances of flight attitude and 
velocity. Existing epipolar algorithms can give a first estimation of disparities in epipolar (x-) direction, but the 
recursive algorithm can also start with zero-disparity initial condition if the disparities are not too big. The algorithm 
minimizes locally a certain least squares distance of a stereo image pair using the method of steepest descent leading to 
a recursive disparity updating. To diminish ambiguities a pyramid with Gaussian image smoothing together with other 
measures (e.g. exploiting the ordering constraint and applying edge preserving disparity smoothing) is used. The 
presented matching algorithm is parallel in space and sequential in time. Therefore, when suitable parallel processing 
hardware (with one processing element assigned to each pixel) will be available then real-time stereo processing 
becomes possible. Some examples demonstrate the capabilities of the algorithm but also the remaining difficulties. 
KURZFASSUNG 
Ein paralleler Algorithmus zur Stereo-Bildzuordnung, der hauptsüchlich für die Verarbeitung von Bilddaten von 
Pushbroom-Zeilenkameras gedacht ist, wird prásentiert. Der Algorithmus wurde wegen vorkommender Stórungen der 
Fluglage und —-geschwindigkeit für nicht-epipolare Geometrie konzipiert. Vorhandene epipolare Verfahren kónnen für 
eine erste Schátzung der Parallaxen in epipolarer (x- ) Richtung verwendet werden, aber der rekursive Algorithmus 
erbeitet auch ohne derartige Schátzwerte, wenn die Parallaxen nicht zu grof sind. Der Algorithmus minimiert lokal 
einen gewissen Abstand eines Stereo-Bildpaares durch Anwendung der Methode des steilsten Abstiegs. Dies führt zu 
einem rekursiven updating der Parallaxen. Zur Verminderung von Mehrdeutigkeiten wird eine GauBsche Pyramide 
zusammen mit anderen MaBnahmen (Reihenfolgebeschrinkung — ordering constraint, kantenerhaltende Glittung von 
Parallaxen) verwendet. Der Algorithmus ist räumlich parallel und zeitlich sequentiell und kann daher, wenn geeignete 
Parallelverarbeitungs-Hardware (mit einem Prozessorelement pro Pixel) verfügbar ist, Echtzeit-Stereoverarbeitung 
gewährleisten. Einige Beispiele zeigen die Fähigkeiten des Verfahrens und seine Mängel. 
1 INTRODUCTION 
The successful experiments with the digital stereo cameras HRSC (High Resolution Stereo Camera), WAOSS (Wide 
Angle Optoelectronic Stereo Scanner), WAAC (Wide Angle Airborne Camera) and with a prototype of the first 
commercial digital aerial camera ADC (Airborne Digital Camera) performed at the Institute of Space Sensor Systems 
and Planetary Exploration of the German Aerospace Center (DLR) have shown that high quality stereo reconstruction 
with pushbroom cameras is possible. 
To generate cost efficient and high quality 3D data products the key problem continues to be the matching of two or 
more image stripes. One needs a very fast matching algorithm in order to process efficiently the huge data amounts 
generated. 
Because of aircraft attitude and velocity variations the image geometry is not strictly epipolar. Therefore, available 
efficient epipolar algorithms, e. g. the algorithm of Gimel’farb (1999) using dynamic programming techniques, can be 
used only as a first approximation. For refinement a very fast and precise non-epipolar algorithm is needed. 
  
436 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.