FR2583547A1 - Automatic electronic method of shape analysis, for distinguishing the symmetric sensitive areas of a scene and their centres of symmetry - Google Patents

Abstract

The method of the invention consists in analysing the shapes f of a scene S so as to automatically distinguish among the shapes f symmetric sensitive areas P, linked to auxiliary figures F and to determine their centres of symmetry C. On the basis of a digitised image Id, and of a test point co situated inside the digitised shape fd a series of circles Co<k> are traced out with a diameter increasing around the point co. As soon as a circle co<i> cuts the digitised shape fd a new centre c1 is chosen which is offset with respect to co in a direction opposite to the direction of intersection. Thus a converging series of points cm is constructed by iteration which, if the figure is symmetric, converge towards the centre of symmetry c.

Description

AUTOMATIC ELECTRONIC FORCE ANALYSIS PROCESS
MES TO DISTINGUISH SYMMETRICAL SENSITIVE ZONES
OF A STAGE AND THEIR CENTERS OF SYMMETRY
The present invention relates to optical, digital and electronic methods for analyzing and automatically recognizing shapes on a scene.

It relates more particularly to the case where it is desired to distinguish symmetrical sensitive areas and automatically determine their centers of symmetry; the latter being dispersed randomly on a stage and intersected by auxiliary figures.

The method for automatically determining sensitive areas and for automatically searching for centers of symmetry, proposed by the invention can in particular be implemented in all cases where
- the shapes of the analyzed scene, made up of figures
auxiliaries and sensitive areas can be distinguished from
background of the scene by their gray level after optical capture
in particular using a camera and electronic digitization,
- and where the sensitive areas geometrically differ from the
auxiliary figures which lead to it by the fact, that at least
locally, they have more transverse dimensions
important and that sensitive areas therefore constitute a
bulge vis-à-vis the said auxiliary figures which
intersect.

Various methods are known for the automatic search for symmetrical elements in figures. These are mainly used, in particular, in areas
- reading plans,
- programming assistance, based on reference images,
in the realization of drilling program for machine
numerical control,
- the search for sights for the automatic control of
positioning system and robots.

In the description of the invention, the technical terms are used: “digitize” and “pixels”.

By digitizing is meant the action of expressing in digital form the positions of entities in an image, in particular using an electronic calculation unit.

Moreover, in the context of the present application, as is customary in the field of image processing, by pixel is meant, and by isomorphic association, both
- the elementary structure of an image in digital form,
- the basic electronic structure of a sensor, in particular of
CCD type, capturing images,
- and finally, the elementary structure of a digital image
displayed on a screen or stored after being entered by
a sensor and processes electronically.

The problem of the automatic search for centers of symmetrical shapes is fairly well resolved by the prior art from the moment
- the sensitive areas are simple in shape (circles, squares, ...),
- and where they are distinct from each other and not
combined with auxiliary forms that come into contact
with them.

Thus, French patent application No. 82 00706 (BORNELEC) describes a device and an automatic method for reading pelletizing clichés of printed circuits. This process applies exclusively to the case of center symmetry pellets and more especially circular pellets. The scene must imperatively be absent from any other type of auxiliary figure. The proposed method proceeds by searching for successive medians of segments intersecting the edges of the pellets in orthogonal (X) and (Y) directions. We start from a point located at the edge of the pellet. The development of the horizontal segment of the pellet at this level is measured in the first direction (X). Then, we determine the middle segment of the pellet in the perpendicular (Y) direction, and we consider that the center of the pellet is the middle of the middle segment. To implement this method, it is possible to use a simple optoelectronic cell moved vis-à-vis the pelletizing plate in the (X) and (Y) directions. Its application is therefore simple and efficient. But, it is completely unusable from the moment when, which is the most frequent case, the images include auxiliary figures such as filaments joining the pellets.

Furthermore, US Patent 4,163,212 (WR. BUERGER, K.K.

DIXON and J.F. MONIER), describes a method and a device intended to automatically recognize the centers of the connecting legs of integrated circuits. The sensitive areas representing the legs have an approximately rectangular shape. The recommended method essentially consists in analyzing, in parallel lines, the scene in at least two perpendicular directions (X) and (Y). According to each direction (X) and (Y), the curve of the centers of the segments formed by the intersection of the analysis line with the rectangular sensitive area is determined. It is considered that the center of the sensitive zone is formed by the intersection of said curves of the centers, according to the various directions of analysis. This procedure allows, for simple figures such as - rectangles, to perfectly determine the center of the sensitive zones, on the express condition that they are alone on the stage and independent. If auxiliary figures complete the scene, the process is totally inoperative.

In the case where the sensitive areas, for which the centers of symmetry are sought, are attached to auxiliary figures, the prior art only offers effective solutions in certain very specific cases where the sensitive areas have identifiers.

In particular, French patent application No. 75 09 846 (G. GLIN and J.L. AMIAR) describes a method and a device for aid in programming drilling from printed circuit clichés.

This process applies to clichés having pellets and filaments. It consists of giving the sensitive areas to identify a distinctive appearance. In particular, a transparent appearance is given by leaving a central hole in the middle of the pellets to be drilled. The image of the clichés by windows is digitized, and an analysis and an identification of the areas of the image having the distinctive appearance are carried out. We recognize in particular the central holes of the pellets but also, by mistake, the regions of the bottom located between the filaments. Finally, zones with the distinctive sign are extracted, those which touch the edge of the image window. It is thus possible to identify the only sensitive areas formed by holes in the middle of the pellets. This process therefore works, a priori, perfectly for clichés in which all the pellets are provided with a center hole. On the other hand, it induces errors and is therefore inapplicable in all the images having zones to be etched in the middle of ground planes, because these zones are confused with center points.

In addition, the method is totally inoperative in the most general case where the sensitive areas formed by the pellets do not take a central hole.

Likewise, U.S. Patent 4,295,198 (R.K. COPELAND and R.J.

DIMAGGIO) describes a method and a device for automatically determining the center holes of pads of printed circuit clichés also comprising filaments. The method is based on a series of successive tests to verify correspondence with certain criteria, the selectivity of which is increasing.

The successive sequencing of these tests is intended to increase the speed of operation of the method. When we discover a white figure, we successively carry out each of the tests. The first test, "BURST", consists in verifying if the figure does not have dimensions greater than that of the largest admitted hole. It makes it possible to carry out a very rough first sorting. The second test, nL.PATH ", consists of moving along the shape in a path made up of portions of segments cutting the edges at right angles to also verify a maximum size condition. It tends to eliminate filaments and ensures So a second sort. Finally, the last test, "PATH" recognizes, by searching for the contour, the shapes of sufficiently small dimension, whose contour is closed on itself. The process consists in assimilating all the shapes having passed the three Pellet centering hole tests This method therefore applies exclusively to the detection of pellets through center holes.

The prior art does not propose any universal method for automatically searching, on the image of a scene (such as a printed circuit cliché3, symmetrical sensitive areas (such as pads) connected to auxiliary figures (such as filaments) and of determination of the center of these sensitive areas So that currently no known device is able to perform automatically.

reliable, and adaptable to all types of shape, automatic search for symmetrical sensitive areas, linked to any auxiliary figures.

All existing solutions
- or adapt to specific cases. what in
considerably limits employment.

- either require the addition of identifiers, which complicates
their implementation,
- either are inapplicable as soon as the sensitive forms
symmetrical are confined with auxiliary figures.

The purpose of the invention as claimed hereinafter is to remedy these drawbacks overall.

It solves the problem of
- on any stage, including both
symmetrical sensitive areas, randomly distributed,
and any auxiliary figures, joining and
intersecting sensitive areas
- and as soon as
on the one hand, the shapes formed by said zones
sensitive, possibly combined with figures
auxiliaries can be distinguished from the back of the stage, by their
gray level after digitization; (n) for zones
sensitive and (b) for the background
. and on the other hand, where sensitive areas differ from
auxiliary figures, at least locally, by the fact that they
have larger transverse dimensions
causing a local swelling of the shape,
- to automatically distinguish said sensitive areas without
risk of error,
- and automatically determine with precision the coordinates
of the center of symmetry of said sensitive areas.

The advantages offered by the invention are
- complete automation in the search for shapes
symmetrical, applicable to many areas of industry
such as automatic robot piloting, orientation
automatic object reading, automatic reading of plans,
automatic programming of numerically controlled machines,
- a universality of application of the research process
automatic, for the analysis of scenes of any kind,
comprising sensitive areas and auxiliary figures of
all configurations,
- very high speed and simplicity of operation of the
research process, allowing operation in
real time on microprocessor,
- finally an ease of electronic wiring, of the process
allowing extremely short execution times.

To this end, the general method of the invention consists, after having identified a figure situated inside the scene, by the gray level of one of its points, in determining the existence of a center of symmetry by growing a family of circles inside the shape and determining a maximum inscribed circle. The general method of the invention is based on the following remark: the maximum circle inscribed on a figure of symmetry is always centered on the center of symmetry of this figure.

The main characteristics, variants and advantages of the invention emerge from the description which follows with reference to the appended drawings; which descriptions and drawings are given only by way of nonlimiting examples of use of the method of the invention in the specific field of automatic processing of printed circuit clichés with a view to generating a program for drilling the corresponding printed circuits.

On these drawings
- Figure 1 is a schematic view of a portion of a photograph of
printed circuit, on which is represented the principle implemented
work by the invention to identify a shape of a scene
- Figure 2 is a partial view of the printed circuit cliché
of Figure 1, in which is schematically described the
method of automatic search for the center of symmetry of
the invention
- figure 2 ', shows a detail of a portion of figure 2
digitized
- Figure 3 shows the final stage of identification according to the
method of the invention, a presumed center of symmetry,
- Figures 4, 5 and 6, describe a complementary arrangement
of the method of the invention for confirming the viability of a
presumed center of symmetry.

FIG. 1 shows a portion (Z) of a printed circuit cliché (1). On this photograph (1), made of transparent plastic, the pellets (P1, P2, P3, P4, P5), the filaments (F1, F2, F3, Fq) as well as the mass zones (F7) have been made by a photographic process which made them opaque black.

The scene (S) formed by the cliché (1) is illuminated from below, in particular by a light box system (not shown) so as to make it appear distinctly
- in black, the shapes (f) made up of the pastilles (P1, P2,
P P4, P5), the filaments (F1, F2 F Fq) and the areas of
mass (F7>
- and in white, the backdrop (E).

Clichés (1) have two essential functions in the manufacture of printed circuits
- on the one hand, they serve as a screen printing mask during
the insolation of the sandwiches (copper-epoxy) which after
development and engraving allow the pellets to be released,
circuit connectors and ground planes,
- and secondly, they constitute a metrological reference,
when programming CNC machines
intended for drilling the circuits, to spare the holes
inside which are introduced the legs of the
components.

The method of the invention can advantageously be used to carry out the automatic programming of the drilling of printed circuits from the clichés (1).

Indeed, the scene (S) constituted by the cliché (1) comprises figures (f) composed
- on the one hand, sensitive areas (S) to be identified, these being
made up of the pellets (P1, P2 P3, P4, P5 ...) whose
coordinates of the centers (C'1) must also be determined for
be pierced,
- and on the other hand, auxiliary figures (F) constituted by the
filaments (F1, F2, F3, F4 ...), and ground planes (F7 ...).

The pellets (P1, P2, P3, P4, P5 ...) are placed randomly on the plate (1).

The auxiliary figures (f) are of no use for carrying out the drilling programming and only complicate the process of finding the pads (P1, P2 t P3 s P4, P5 ...) because they intersect at connection points (11 , 12, 13, 14, 15, 16) the pastilles (Pa, P2, P3 P4 P5 ...).

It is nevertheless noted that the pellets (P1, ...) locally have transverse dimensions (d1) greater than the widths (el, e2 ...) of the filaments (F1, F2, ...) which end there. So that they constitute a bulge vis-à-vis the auxiliary figures which intersect them.

In addition, the pellets (P1, P2, P3, P4s P5, etc.) are distinguishable by the fact that they have a central symmetry due to the fact that they are circular.

The principle of automatic shape analysis of the invention aims
- automatically distinguish the pellets (P1, P2 w P3 s Pq,
P5.,. ) with respect to the other auxiliary figures (F1, F2 t F3,
F4> F5, F6, F7 ...) which are not symmetrical,
- and moreover, to automatically determine with precision the
centers (C'1, ...) of the pellets (P1, ...).

To do this, a first step consists in capturing, with a camera (not shown) an image window (1a) covering the portion to be analyzed (Z) and inside which the shapes (f) are located.

Such a window (la) appears in figure 1.

A second step consists in digitizing the window (la) that is to say, in memorizing by an electronic process the image (la) in a digitized image (Id) appearing partially in FIG. 2 '.

Depending on their gray level, the figures (f) and the background (E) are associated with a different binary value. In particular, the blocks (20, 21) covering the shapes (f) and in particular the pellets (P1) and the filaments (F2) are assigned the zero digit corresponding to the color black. In the same way, one assigns to the blocks (23) of the bottom (E) the digit one corresponding to the white color. It is on the digitized image (Fd) that the electronic image processing method of the invention is implemented.

The object of a third step is to fix at least a first test point (co) inside each shape (f) capable of integrating a sensitive zone (P), and in particular a patch (P1).

To do this, a method recommended by the invention is described in FIG. 1. This method consists in traversing the entire scene (S) and discovering all the sensitive areas (P) contained in these shapes (f). To do this, we fix a test grid (G) of constant mesh (M) comprising a plurality of nodes (N1, N2 t N3, ...) and covering the entire scene (S). each node (N1, N2,
N3, ...), of the grid (G), the analysis of the binary level (zero or one) significant of the gray level of the points (N1, N2 ... ). The nodes (N1, N2, etc.) located inside a figure (f) and in particular a patch (P1) and the nodes (N3) located in the background (E) are thus determined. The nodes (N1) located inside a digitized form (fd) constitute subsequent test points for the implementation of the electronic method of analysis of the forms (f).

Thus the point (N1) located inside the pellet (P1) constitutes a test point (co). During a fourth step, detailed in figure 2, we draw around the point (co) a family of concentric circles (Cok) with (k = 1, 2, ...).

The diameter (dk) of the circles is increasing. The circles (cor) are made, inside the digitized image (Id), in a discrete manner, by juxtaposition of pixels. For simplicity, they are drawn in dotted lines in FIG. 2. Thus we have drawn around the test point (co), the circles (col) and (Co3).

Each circle (Cok) is divided into (q) arcs of circles (Qokl) with (1 <q) corresponding to (q) quarters (vol) distributed around the approximate center (co). In the present case, we have chosen: q = 4.

The circles (Co1, Co2, ...) are divided into four quarters (Qo1,
Qo12, ..., Qo2,1, Qo2,2, ...) corresponding to four quarters (001) distributed around the test point (co).

For each circle (Cok) drawn, a test is carried out to determine whether or not the circle (Cok) is entirely contained within the corresponding digitized shape (fd). As long as the circles (Cok) are entirely contained within the digitized form (fd), as is the case for the circle (Col), we successively increase, during a fifth step, the diameter (dk) circles (Cok, Cok 1, ...).

On the contrary, as soon as a circle (Coi) of the family, such as for (Co2), intersects the digitized form (fdj, i.e. at least one point (e) of the circle ( Co, ...) is in the outer zone of the digitized form (fd), corresponding to the digitized background (Ed), we determine the number (qo) of arcs of circles (Qoij) of the circle (Coi) intersecting the digitized form (fd).

When, which is the most frequent case, a single arc (Qo2,1) of the circle (Co2) intersects the digitized shape (fd), we choose, during a seventh step, a new test point ( cl), shifted in the opposite direction to the direction of the arc (Qo2,1) intersecting the digitized shape (fd). Steps four to seven of the process developed above are then iterated from this new test point (c1). We thus create a series of test points (cO, c1, c2, c3, cn, ...). It sometimes happens, as is the case for the test point (cq) that the first circle (C413) intersects the shape (fd) by two arcs (Q413'1) and (Q41312). It is then recommended by the invention to choose a new test point (C5) offset in a direction opposite to the two arcs (Q413,1) and (Q413,2) secant to the digitized form (fd).

Then the steps of the process are iterated again.

It can be seen that from point (C5) the series of test points (cn) becomes stationary around the midpoint (c).

The application of the method of the invention then consists
- to consider that the analyzed form (f) includes
probably a suspected pellet (P),
- and that the presumed center of this pellet is (c).

When it is desired to process patches with two axes of symmetry, such as oblong patches, it happens that when the sequence (cn) converges towards the center of symmetry, symmetrical and opposite arcs intersect the digitized form. To take this situation into account (not shown), an even number (q) of arcs is chosen, in particular four. And for all the families of circles (Cnk) we group the arcs (Qnkl) in opposite symmetrical pairs. Each time, that during the growth of the diameters (dk), a first circle (cry) of a family of circles (Cnk) centered on a test point (cn), cuts a digitized shape (fd), we is then interested in the symmetrical pairs of arcs of which a single arc (Qnil) intersects the digitized form (fd). And, if there exists at least one pair of arcs of this type, with disymmetric intersection, we choose a new test point (cn + 1), offset with respect to the test point (cn) in a direction opposite to that of the only arcs (Qnil) interceding the digitized form (fd).

In all cases, as shown in figure 3, the evolution of the series of test points (cn) included in the digitized form (fd) is stopped and made stationary, as soon as for one of the points (cn)
- on the one hand there is a circle (Cnk), centered on this point
test (cn) completely included inside the form
digitized (fd),
- and on the other hand that, the following circle (Cnk 1) of the family,
centered on the test point (cn), has a number (N) of arcs
(Qnk 1 l) intersecting the digitized form (fd) greater than one
minimum acceptance threshold (A) with (A <q).

We then consider that the test point (cn) is a presumed center of symmetry (c) of a sensitive zone (P) included in the shape (f) analyzed.

In the case described in figure 3, we have chosen a number of arcs (q = 4). In addition, a minimum acceptance threshold was chosen (A = 4).

The center (cn) satisfies the following double condition
- there is a circle (Cnk), center on the test point (cn),
located entirely inside the digitized form (fd),
- and all the arcs (Qnk + ll, Qnk + î, 2, Qnk + 1.3 Qnk + 1.4) of
circle, intersect the digitized shape (fd).

The point (cn) is considered as the center of symmetry (c) of a presumed sensitive zone (P) located at the intersection of the form (f).

The performance of automatic searches for centers of symmetry (c), using the method of the invention can reach any required level. They essentially depend
- the size of the pixel tiles (20, 21 ...);
- the relative displacement step of the successive test points
(we,
- the evolution of the diameters (dk) of the test circles.

It is recommended by the invention to set the following constraints
- pixel size (20, 21 ...) less than or equal to the precision
(s),
- increment of diameters (dk), (dk + 1 - dk), less than or equal to
the precision (s),
- elementary displacement between two successive test points
(cn) and (cn + 1) less than the presicion (s).

In particular, with reference to FIG. 2, it can be seen that the most frequent displacement between two test points (Co, C1) is a vector (V1) which can take four directions opposite to the arc (Qokl) intersecting the shape. In particular, we notice that the displacement between the centers (Co) and (C1) is a vector (V1) bisector at the quarter (003) opposite to that (flight) of the arc (Qo21) intersecting the digitized form (fd) .

When two non-opposed arcs of a first circle (Cn) centered on a test point (cn) intersect the digitized form (fd), we can combine two elementary displacement vectors as appears for displacement (C4, C5).

In most cases, we choose a series of diameters (dk) of the families of circles (Cnk) constituted by a geometric progression (dk) = Do x (k + ko) identical for each family of circles (Cnk). That is to say that the increase between two successive diameters (dk, dk ...) is constant. We also choose the constant (ko) and the argument (Do) such that the first diameter (d1) Do x (1 + ko) is appreciably less than the transverse dimension (do) of the smallest pellet (P2 ). The argument (Do) is less than or equal to the absolute precision (s) sought in the determination of the centers of symmetry (c).

In addition, the test grid (G) is preferably square mesh (M) with side (e) less than the dimension (do) of the smallest pellet (P2). In this case, it is recommended that the argument (Do) and the side (e) verify the relation (e + Do <do), in order to be sure that at least a first circle (Col) of a family of circles centered on a test point (co) is entirely located inside each shape (f) analyzed.

When a presumed exact center of symmetry of a presumed sensitive area (P) is discovered within a shape (f), an additional confirmatory test is carried out as described in Figures 4, 5 and 6.

To do this, we carry out the line-by-line analysis of the digitized form (fd) in at least one direction (X X ') and preferably two directions, one (X X') and the other (Y Y ').

In the case of the analysis according to the direction (X X '), the width (L) of the digitized form (fd) is measured at each ordinate (Y) of the second direction (Y Y'). We study (figure 6), the variation of the width (L) of the digitized form as a function of the level (g).

Likewise, we study the variation of the height (M) of the shape, according to the direction (Y Y ') as a function of the abscissa x (figure 5).

Due to the assumption made that a pellet (P) is larger than a filament (F) which ends up there, it can be seen that the evolution curves of FIGS. 5 and 6 both present
- a zone (Z1, Z'1) of growth of the amplitude of the shape,
- and a zone (Z2, Z'2) of decrease.

It is recommended by the invention to confirm a presumed center of symmetry (C) only if it responds in both directions (X X ') and (Y Y') to this condition of phases of growth and decrease.

It is also recommended to subject the presumed center (c) to a second test relating to the maximum amplitude. Indeed, the shapes (f) containing a pellet (P) have a maximum amplitude (A, A ') greater than the diameter (do) of the smallest pellet (P2). We therefore only confirm a presumed center of symmetry if, according to the two directions (X X ') and (Y Y'), the evolution of the amplitude (L, H) of the form (f) presents a zone d 'outgrowth (H H') of amplitude (A, A ') greater than the transverse dimension (do) of the smallest pellet (P2>.

A means of complementary confirmation of an assumed center of symmetry (c) consists in comparing the diameter (dn) of the last circle (Cn) included inside a digitized form (fd) with the transverse dimension (do) of the smallest pellet (P2> of the scene (S) and similarly, to compare the diameter (dn + 1) of the circle (Cn + 1) with the transverse dimensions (D) = (dl) of the largest pellet ( P1) ~
One then confirms the presumed center of symmetry only if the following relations are verified
- (dn)> (do),
- etiou (dn + 1) <(D) or any equivalent relation. We can then consider that the form (f) does indeed contain a pellet (P) with a center (c) and an inscribed circle of diameter (dn).

When a center (c) of a circular pellet (P) has been determined and confirmed according to the previous methods, a verification of the diameter is carried out, To do this, the central amplitude of the shape is determined, it is i.e. the distance (A1 CA'1) in several directions (a1, a'1), (a2 a'2), (a3, a'3), (a4, a'4) and in particular four. And if, the values (A1 CA'1> of the central amplitude are confirmed at a value (Ao) at (y%) at least (N times), (Y) and (N) being parameters probably chosen, we consider that the diameter of the pellet (P) is (Ao).

The method described above, implemented electronically, makes it possible to automatically detect any symmetrical sensitive areas (P) on a scene (S) as well as their center of symmetry (c), with absolute precision. The determination can be made regardless of the presence of auxiliary figures (F).

This method is particularly advantageous in use in all fields of shape processing and automatic image analysis. It finds a particularly effective application for automatically programming the drilling of printed circuits from clichés.

The invention having now been described, and its interest justified on a detailed example, the applicants reserve the exclusivity thereof throughout the term of the patent, without limitation other than that of the claims below.

Claims (12)

1) Electronic process for analyzing shapes (f) of a scene (S) and in order to automatically distinguish among the shapes (f), symmetrical sensitive areas (P), intersected by auxiliary figures (F), arranged randomly on the scene (S) and determine the center of symmetry (e) of these sensitive areas (P), said sensitive areas (P) and possibly the auxiliary figures (F) being distinguishable from the background (E) of the scene (S) by their gray level ending after digitization at a different binary level for the different shapes (f) and for the background (E); this method consisting in identifying whether a shape (f) locally contains a sensitive area (P). - a1> to be entered with a camera, a window covering it the inspection zone of the shape (f), - a2> to digitize said window (la) in the form of an image digital work (Id) containing the digitized form (fd), - a3> and to set a test point (co) located inside of the digitized form (fd), Said method being characterized in that - a4) we trace, around the test point (co), a family of concentric circles (Cok), of diameters (dk) increasing, centered on the test point (co), each circle (cok) being divided into (q) arcs (Qokl) with (1 d q) corresponding to (q) districts (Qo1) distributed around the test point (co) - a5> as long as the circles (Cok) of the family are fully contained within the form digitized (fd), we successively grow the diameter of the circles (Cok, Cok + 1, ...); - a6) as soon as a circle (Col), of the family intersects the shape digitized (fd), i.e. at least one point (e) of the circle (Coi) is in the outer area of the form (fd), corresponding to the digitized background (-Ed), we determine the number (qo) of arcs (Qoij) of the circle (Coi) cutting the digitized form (fd) ;; - a7) if, which is the most frequent case, only one arc (Qoij) of the cutting circle (Coi), intersects the shape (fd) digitized, - then we choose a new test point (cl), offset in the opposite direction to the arc direction (Qoii) intersecting the form (fd), - and we iterate from this new test point (C1) steps (a4) to (a7) of the process, - by creating a convergent sequence (cn) of points of tests, as long as the uniqueness of intersection of the arcs (Qnkl) from circles (Cnk) to the digitized form (fd) is maintained, - a8) if the sequence (cn) converges to a limit point (c) or becomes stationary in the neighborhood of a point (c), we consider that - the form (f) probably includes a zone suspected sensitive (P), - centered on the presumed center (c). 2) Electronic method, according to the preceding claim 1, for automatic detection of symmetrical sensitive areas (P) and automatic determination of the centers of symmetry (c) of these sensitive areas (P), characterized in that a number is chosen even (q) of arcs (Qnkl), and in particular four arcs, for all families of circles (Cnk), the arcs (Qnkl) being defined by quarters (Qn1) of fixed directions, and the arcs (Qnk) being for each circle (Cnk) grouped together by symmetrical pair of opposite arcs. 3) Electronic method, according to the preceding claim 2, for automatic detection of symmetrical sensitive areas (P) and automatic determination of the centers of symmetry (c) of these sensitive areas (P); this method being characterized in that each time that during the growth of the diameters (dk), a first circle (Cni) of a family of circles (Cnk) centered on a test point (cn), intersects the shape digitized (fd) analyzed, - one determines the symmetrical pairs of arcs of which only one arc (Qnil) intersects the digitized form (fd), - and, if there is at least one pair of arcs of this type, to asymmetric interfectxon, we choose a new point test (cn + l) offset from the test point previous (cn) in a direction opposite to that of the only arc (Qnil) interceding the digitized form (fd). 4) Electronic method according to one of the preceding claims 1 to 3, for automatic detection of symmetrical sensitive areas and for automatic determination of centers of symmetry (c) of these sensitive areas (P), characterized in that - we stop and make stationary the evolution of the series test points (cn) included in a digitized form (fd), as soon as, for a test point (cn) of the series . on the one hand, there is a circle (Cnk), centered on said test point (cn) completely included within the digitized form (fd) analyzed, . and on the other hand, the following circle (Cnk + l) of the family, centered on the test point (cn), has a number (N) of arcs (qnk + l'l) intersecting the digitized form (fd), greater than a minimum acceptance threshold A (A 4 q). - and we then consider that the last test point (cn) is a presumed center of symmetry (c) of a sensitive area (P) included in form (f). 5) Electronic method, according to the preceding claim (4), for automatic detection of sensitive areas (P) and automatic determination of the centers of symmetry (c) of these sensitive areas (P), characterized in that - we stop and make stationary the evolution of the series test points (cn) included in a shape (f), from the instant when, for a test point (cn) in the series, on the one hand, there is a circle (Cnk), centered on said test point (cn), fully included within the digitized form (fd), and on the other hand, all the arcs (Qnk + l'l) of the circle following (Cnk + i) of the family, centered on the point test (cn), intersect the digitized form (fd). - and we then consider that the last test point (cn) is a presumed center of symmetry of a sensitive area (P) included in form (f). 6) Electronic process. according to one of the preceding claims 1 to 5, characterized in that a series of diameters (dk) of the families of circles (Cnk) is used, consisting of a geometric progression (dk) = (Do) x (k + ko), identical for each family of circles, where the constant (ko) is chosen such that the first diameter (dl) = (Do) x (1 f ko) is less than the transverse dimension (do) of the most small sensitive area sought, and where the argument (D) of the geometric progression is substantially equal to the absolute precision required (s) in the automatic determination of the centers of symmetry (c). 7) Electronic method according to one of the preceding claims 1 to 5, characterized in that to obtain a precision (s) one chooses an elementary displacement between two test points (cn), (cn + 1), .. .., made up of a vector (cn, en + 1) - oriented substantially according to the direction of the arc (# Qnk + l #l ') opposite to that (Qnk + l # l) intersecting the digitized form (fd), - and of constant measurement (1) less than or equal to the precision (s) absolute required. 8) Electronic method according to one of the preceding claims 1 to 7, for automatic determination of symmetrical sensitive areas (P) characterized in that, in order to travel through the entire scene (S) and to discover all the sensitive areas (P) symmetrical contained in the shapes (f) of the scene, as well as the center of symmetry of each sensitive area (P), an analysis of the level is systematically carried out on the digitized global image (Id) of the entire scene (S) binary significant of the gray level of a plurality of test points distributed according to the nodes (N1, N2. ..) of a test grid (G) with constant mesh, covering the digitized scene (Sd) and one proceeds to the implementation of the method of the invention by taking successively as first test point (Co) the test points (N1, N2, ...) of the binary level grid (G) corresponding to the figures ( f). 9r # Method according to the preceding claim 8 for determining the centers of symmetry of sensitive areas of minimum transverse dimension (do) characterized in that one uses a test grid (G) with a square mesh of side (e) <( do). 10) Method according to one of the preceding claims 4 and 5, of confirming a center of symmetry (c) presumed exact of a sensitive area (P) presumed located inside a shape (f) said process intended to avoid errors due to the presence of auxiliary figures (F) intersecting the sensitive area (P) being characterized in that - in at least a first direction (X X '), we measure, on the digitized image, the evolution according to a second direction (Y Y ') (perpendicular to the first), of the width of the digitized shape (fd), - and we do not confirm said presumed center of symmetry (c) as well as the existence of a sensitive area (P), that if the evolution of the width of the shape according to (Y Y ') present a growth phase, followed by a phase of decrease. 11) Method according to the preceding claim 10, for confirming a center of symmetry (c) assumed to be exact of a sensitive area (P), said method being characterized in that one does not confirm a center of symmetry (c) assumed that if, between said growth phase and said decay phase, the digitized form (fd) exhibits an area of protuberance of greater width than the transverse dimension (do) of the smallest sensitive area of the scene. 12) Method according to one of the preceding claims 4 and 5 of automatic determination of the sensitive areas (P) and of automatic determination of their center of symmetry (c), said method being characterized in that, for each presumed center of symmetry ( c) found, - we compare the diameter (dn) of the last circle (Cn) included with inside, with the minimum transverse dimension (do) of the smallest sensitive area of the scene (S), - and where we compare the diameter (dn + 1) of the circle (Cn + 1) with the dimensions transverse # ales (Do) of the largest zone sensitive of the scene (9), - and we only confirm that the presumed center of symmetry is a exact center that Si ~ (donation)>, (do), . eLlou (dn + I) <(Do) or any equivalent comparison. - we consider that the forr. le (f) contains a sensitive area symmetrical (P) with center (c) and inscribed circle of diameter (dn). 13) Method according to one of the preceding claims 1 to 12, for automatically determining the diameter of circular sensitive areas (P), distinguished inside the scene (S), said method being characterized in that after determination and confirmation from the center (c) of the sensitive zone (P), the central amplitudes of the digitized form (fd) in (R) directions are determined. The statistical distribution of said central amplitudes is analyzed, - and if the values of the central amplitudes are confirmed1 to a value (A) up to (y 90), at least N times, we consider that the diameter (d) of the sensitive area (P) is equal to (A).