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IMAGE ANALYSIS : TRANSFER OF TECHNIQUES USED IN ASTRONOMY TO DIFFRACTION
A. Bijaoui
To cite this version:
A. Bijaoui. IMAGE ANALYSIS : TRANSFER OF TECHNIQUES USED IN ASTRON- OMY TO DIFFRACTION. Journal de Physique Colloques, 1986, 47 (C5), pp.C5-63-C5-67.
�10.1051/jphyscol:1986508�. �jpa-00225825�
IMAGE ANALYSIS
:TRANSFER OF TECHNIQUES USED IN ASTRONOMY TO DIFFRACTION
A. BIJAOUI
Observatoire de Nice,
B.P.139, F-06003 Nice Cedex, France
Resume - Le traitement numerique des image's est une technique utilisee depuis de nombreuses annees en Astronomie. Les images 2 traiter sont de nature trbs diverse, obtenues avec une grande panoplie de recepteurs et pour des domaines de longueur d'onde allant des rayons gamma aux ondes d6cam6triques. Cette diversite a conduit B la mise au point de vastes systbmes informatiques de traitement des images.
On indique le materiel et le logiciel n6cessaires, ainsi que les tendances actuelles, principalement liees au traitement des observations obtenues avec le telescope spatial Hubble.
Un survol des differentes operations est effectue, en particulier en ce qui concerne l'analyse de l'image.
On
compare les traitements dans les deux disciplines, en effectuant l'analyse d'un systsme de traitement des images B une fusee B trois dtages. Le premier, technique, consiste dans la construction des bibliothbques necessaires, en grande partie communes entre les deux disciplines. Le second est celui de l'int&gration, conduisant aux commandes utiles. Le dernier consiste dans 1'6criture de procedures d'extraction de l'information, procedures sp6ci- fiques B chacun des domaines.
Abstract - Digital Image Processing is a tool which has been used for many decades in astronomy. Many kinds of image from a wide variety of receivers have to be processed. Therefore huge image processing systems have been achieved.
We outline the hardware and software needed. The Hubble Space Telescope is the incentive behind an international coordination of the software.
A survey of an image processing system is given. Image analysis is the main aim i n astronomical applications.
A comparison of the treatments used in astronomy and diffraction shows that the same problems arise in both disciplines. We conclude by comparing an image processing system to a three stage rocket. The first stage is the technical one containing the construction of all the libraries. In the second one these libraries are integrated for building the commands. The last stage is the scientific one. Specialists of a given discipline crreerned write procedures to process the images that concerns them.
I - INTRODUCTION
Digital Image Processing (DIP) is a tool which has been used for many decades in astronomy. The first applications were essentially image synthesis for radioastro- nomy / I / and Fourier spectrography 121. But with the introduction of computer-driven microdensitometers and digital image receivers DIP began to be used systematically in many other fields of observational astronomy.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986508
JOURNAL
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PHYSIQUEMany different kinds of image have to be processed such as those occurring in direct photography, spectrography, interferograms, etc. The observations of a stellar field, with its set of peaks, give rise to the same sort of problems as those associated with diffraction images.
Different kinds of zeceivers are now used: photographic plates for large fields, electronographic detectors 131, Image Tubes, Charge Coupled Devices (CCD) , cameras /4/ and Image Photon Counting Systems (IPcs) 151.
The range of wavelengths covered is now very large extending from gamma rays to decametric wavelengths.
A large variety of images has to be processed and this has lead to the achievement of huge image processing systems in astronomy 161. We will find many problems i n cormnon between astronomy and neutron diffraction.
I1 - IMAGE PROCESSING SYSTEMS
An image processing system consists of a set of programs for extracting the infor- mation contained in images. The following hardware and software features are needed:
1/Central Unit: even if images can be processed by a micro-computer, only those with 32 bit processors permit the easy development of programs for handl* large images.
2lArray processors are used to reduce data, often in real time.
31 Large storage (tens of megabytes) is always necessary.
Digital video disks are usually only used for archiving images.
4 /
Graphic displays are essential for plotting curves and measuring positions with
cross-wires. They are most frequently used with simple devices.
5 /
An electrostatic (or laser) printer/plotter is used to produce curves and grey-tone images.
61 An image processor allows one to display, on a TV monitor, images with grey- tones, colour, or false colour. Images are stored in memory planes. Some operations can be achieved such as summation, taking ratios, smoothing, histograms, etc. An associated graphic plane allows one to store contours or to plot histograms or profiles. Sometimes an associated disk is required allowing, for example, to make a movie from a set of stored images.
Other peripherals are also often connected with image processing.
DIP requires a very flexible disk operating system. The majbrity of astronomical laboratories now use VMS on
VAXcomputers. The UNIX system, which is available on a large number of computers, is now beginning to be used too.
Fortran 77, or languages derived from this, are widely used. C language is now beginning to be employed.
For graphics, developers have tried to introduce multi-driver packages, allowing one to pass easily from one kind of display to another. The tendency is to use a
GKS-type library 171.
For internal image structure, a format developed in the MIDAS system 181 seems to be accepted by a large number of astronomers. This format allows one to process multi-dimensional images with a set of standard parameters, such as the number of pixels in each dimension, the starting coordinates or sampling steps. Into some programs we can introduce image descriptors, such as the histogram, the calibration curve or point spread function. Access to image data is performed using the
"mapping memory" concept, in which the image is read, or written, only by giving an address on the disk.
In astronomical DIP we often use sets of values which are not images (i.e. sampled functions), but lists of data.
Astructure has been defined in the MIDAS system
(Table structure), using standard parameters, descriptors and mapping memory.
Command languages are generally used in these systems. Often a special language has
been built with a complex grammar. The tendency is to use the tools associated with
the operating system, leading to commands similar to those of the system.
t h e y a r e a d e s c r i p t i o n of a l l t h e s t e p s needed t o e x t r a c t t h e information.
Documentation i s t h e one main f e a t u r e common t o a l l t h e l a r g e systems. The programs a r e o f t e n a v a i l a b l e i n many l a b o r a t o r i e s and used by many astronomers who a r e n o t f a m i l i a r w i t h computer s c i e n c e . So good documentation i s e s s e n t i a l .
About t e n l a r g e systems have now been b u i l t and a r e i n s t a l l e d i n many l a b o r a t o r i e s . These systems a r e used by hundreds of astronomers.
I11
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TENDENCIESThe c o n s t r u c t i o n of an image p r o c e s s i n g system r e q u i r e s so much e f f o r t t h a t an i n t e r n a t i o n a l agreement now e x i t s i n t h e astronomical community t o develop coordi- n a t i o n of t h e software.
The Hubble Space Telescope i s t h e i n c e n t i v e behind t h i s p o l i c y . A European Center F a c i l i t y (ST/ECF) h a s t h e t a s k of c o o r d i n a t i n g t h e e f f o r t s of American and European l a b o r a t o r i e s . I n t h e n e a r f u t u r e ST/ECF w i l l provide packagesfor given a p p l i c a t i o n s .
Some i n t e r f a c e r o u t i n e s have been d e f i n e d i n o r d e r t o f a c i l i t a t e t h e w r i t i n g of software i n d i f f e r e n t r e s e a r c h c e n t e r s .
I V - M A I N CONTENT
A l a r g e image p r o c e s s i n g system c o n t a i n s d i f f e r e n t k i n d s of o p e r a t i o n s : I / Image h a n d l i n g : image e x t r a c t i o n , merging,
...
2 / Image d i s p l a y i n g : w i t h image d i s p l a y o r w i t h g r a p h i c ones (contour, perspec- t i v e ,
....
) . P o s i t i o n s and image v a l u e s can b e measured.3/
Geometric c o r r e c t i o n s : resampling t o o b t a i n an image w i t h c o o r d i n a t e s correspon- ding t o a r e f e r e n c e frame, such a s e q u a t o r i a l c o o r d i n a t e s o r wavelengths.4 / Photometric c o r r e c t i o n s t o reduce t h e image by t a k i n g i n t o account i / non- l i n e a r i t y , ii/ v a r i a t i o n of s e n s i v i t y , iii/ non uniform background o r i v / d e f e c t i v e p i x e l s .
5/ Noise r e d u c t i o n with l i n e a r smoothing o r f i l t e r i n g . Non-linear methods such a s Noise Cheating Enhancement
191,
which t a k e s account of non Gaussian s t a t i s t i c s , a r e a l s o used.6 / Resolution enhancement with l i n e a r deconvolution u s i n g F o u r i e r transform.
I t e r a t i v e p r o c e s s e s , such a s B i r a u d ' s / l o / , Lannes's / I I / , Lucy's /12/, o r t h o s e of Maximum Entropy /13/ a r e used more and more o f t e n t o d e t e c t f a i n t f e a t u r e s .
7 1
C o n t r a s t enhancement, u s i n g g l o b a l o r l o c a l histograms.8/ Image r e s t o r a t i o n , which i s always a very important a p p l i c a t i o n of DIP i n astronomy. F o u r i e r r e s t o r a t i o n i s used f o r r a d i o o b s e r v a t i o n s , o r f o r spectroscopy w i t h a Michelson i n t e r f e r o m e t e r . But some o t h e r k i n d s of r e s t o r a t i o n have been 'achieved. For example u s i n g Walsh-Hadamard o r Radon t r a n s f o r m s
1141.
R e s t o r a t i o n su s i n g t h e Maximum Entropy p r i n c i p l e have been achieved from t h e s p e c t r a l energy d e n s i t y , w i t h o u t phase knowledge ( s p e c k l e i n t e r f e r o m e t r y ) / I S / .
9 / Image Analysis: i t i s t h e main purpose i n a s t r o n o m i c a l DIP. Four k i n d s of a n a l y s i s can b e performed :
-Global a n a l y s i s , w i t h e x t r a c t i o n of some histogram parameters o r v a l u e s of t h e F o u r i e r t r a n s f o m .
-Parametric a n a l y s i s , i n which we compute t h e v a l u e s of some given parameters of i d e n t i f i e d o b j e c t s . As i n t h e d i f f r a c t i o n c a s e , we have t o compute t h e background and then determine t h e p o s i t i o n s and f l u x e s and e x t r a c t t h e p r o f i l e parameters.
Some image modelling i s needed, w i t h s t a t i s t i c a l r u l e s such a s L e a s t Squares, Maximum Likelihood P r i n c i p l e , o r Minimum Variance Bound E s t i m a t o r f o r e s t i m a t i n g t h e parameters. For goodness f i t s we use t e s t s such a s chi-2 o r t h a t of Smirnov-Kolmo- gorov.
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Automated a n a l y s i s , u s i n g d e t e c t i o n with s e a r c h i n g f o r maxima, o r w i t h imageJOURNAL DE PHYSIQUE
segmentation 1161. Many types of contour l i n e s can be used f o r segmentation : i s o ~ h o t e s , L a p l a c i a n o r a n o t h e r edge f u n c t i o n l i n e s 1171, v a l l e y l i n e s 1181 t o s e p a r a t e a l l t h e p i x e l s corresponding t o a g i v e n peak. The e x t r a c t i o n of t h e f i e l d p a r a m e t e r s i s g e n e r a l l y performed i n r e a l time, w i t h computation of moments o r extrema. These p a r a m e t e r s a r e reduced t o o b t a i n p o s i t i o n s , e m i t t e d f l u x e s and p a t t e r n parameters. S e p a r a t i o n between s t a r s and g a l a x i e s i s achieved u s i n g Bayesian c l a s s i f i c a t i o n 1191, C l a s s i f i c a t i o n of n o n - s t e l l a r o b j e c t s i s performed w i t h c l u s t e r a n a l y s i s 1201, w i t h o r without a p r e v i o u s f a c t o r a n a l y s i s 1211. Recognition i s made u s i n g t h i s c l a s s i f i c a t i o n , by s t a t i s t i c a l means. Expert systems a r e beginning t o b e used now 1221.
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Multi-Image a n a l y s i s , f o r o b s e r v a t i o n i n d i f f e r e n t wavelength windows o r a t a l o t of epochs. Geometric correspondence i s e s s e n t i a l . The c l a s s i f i c a t i o n of p i x e l s , which i s t h e main s t e p , i s assumed u s i n g l i n e a r d i s c r i m i n a n t a n a l y s i s 1231 and c l u s t e r a n a l y s i s /24/.Some o t h e r o p e r a t i o n can b e a c h i e v e d i n c e r t a i n c a s e s , such a s image compression o r model s y n t h e s i s .
V
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COMPARISON BETWEEN ASTRONOMY AND DIFFRACTIONWhen we compare t r e a t m e n t s used i n t h e two d i s c i p l i n e s we f i n d t h a t t h e same pro- blems a r i s e , even though t h e u l t i m a t e aims a r e n o t t h e same. I n t h e two c a s e s we f i n d t h e same o p e r a t i o n s o c c u r r i n g such a s background mapping, peak d e t e c t i o n and t h e measurement of p o s i t i o n s and i n t e n s i t i e s . A l o t of image enhancements can b e o b t a i n e d w i t h t h e same t o o l s . Some d i f f e r e n c e s occur, f o r example when t h e pro- c e s s i n g i s very dependent on t h e model f i t t i n g .
S i n c e an image p r o c e s s i n g system i s d i f f i c u l t t o develop, document and maintain, i t i s t h e r e f o r e d e s i r a b l e t o exchange s o f t w a r e between t h e two d i s c i p l i n e s .
An image p r o c e s s i n g system can b e compared t o a t h r e e s t a g e r o c k e t :
11 The f i r s t s t a g e i s t h e t e c h n i c a l one and c o n t a i n s t h e d e f i n i t i o n and c o n s t r u c t i o n of a l l t h e l i b r a r i e s : g r a p h i c (GKS), Input/Output parameter i n t e r f a c e s (command language), f i l e management of images o r t a b l e s , and a l g o r i t h m s t o b e a p p l i e d t o t h e d a t a . Using a v i r t u a l memory system, .it i s e a s y t o s e p a r a t e t h e a l g o r i t h m from t h e f i l e access.An a l g o r i t h m w r i t t e n i n a standard language such a s F o r t r a n 77 i s e a s i l y t r a n s p o r t a b l e . For t h i s s t a g e , we have a s e t of independent packages, which can b e provided by many companies o r l a b o r a t o r i e s .
2/ The second s t a g e i s t h e i n t e g r a t i o n one. I n some l a b o r a t o r i e s , t h e s o f t w a r e from s t a g e one i s used t o b u i l d commands. I f t h e c h o i c e of t h e i n t e r f a c e s i s t h e same i n a l l l a b o r a t o r i e s (GKS, MIDAS f i l e s , s i m i l a r command language), t h e s o f t w a r e t r a n s p o r t from one l a b o r a t o r y t o a n o t h e r i s s i m p l i f i e d . T h i s i s t h e c h o i c e made by many a s t r o n o m i c a l i n s t i t u t e s . It c o u l d a l s o b e made i n d i f f r a c t i o n p r o c e s s i n g i f some a d d i t i o n a l a l g o r i t h m s were introduced.
31 The l a s t s t a g e corresponds t o t h e s c i e n t i f i c one. I n f a c t t h e u s e r of a
DIP
system always needs t o have a s e t of s t a n d a r d procedures. Only s p e c i a l i s t s of t h e d i s c i p l i n e concerned can w r i t e t h e s e .I f we succeed i n u s i n g a common language f o r s o f t w a r e t h a t i s t o b e a p p l i e d i n a common environment, t h e n we w i l l o b t a i n an i n s t r u m e n t f o r d e s c r i b i n g procedures t o e x t r a c t t h e i n f o r m a t i o n c o n t a i n e d i n images which can b e u n i v e r s a l l y understood.
REMERCIEMENTS
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J e r e m e r c i e vivement J. T u l l y pour son a i d e dans l a mise en forme en a n g l a i s de c e t a r t i c l e .REFERENCES
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