Method and device for artificial afterglow in a digital image converter

A method and a device for artificial afterglow in a digital image converter for a radar system wherein there is applied to the image converter a general law of decrease of the after glow constituted by a series of laws, each able to be different, each applied during one antenna revolution. These laws are selected according to the data to be processed, in particular according to their brightness level.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns processing artificial persistence or 
afterglow in a digital image converter. It also concerns devices operating 
this process. 
The essential role of an image converter is to convert an image at 
relatively slow renewal into a television type image, which is very 
luminous, allowing its exploitation in bright surroundings. 
This slow renewal image is generally a radar image but it can also be 
images issuing fromaa sonar, an infra red sensor, an echography device, 
that are desired to be visualized on screens functioning as television 
receiver screens. 
According to the prior art, the image converters initially used memory 
tubes generally comprising two guns, a writing gun controlled in radar 
sweeping, for example, and a reading gun controlled in television 
sweeping. Thereafter, digital image converters D.I.C) were introduced, 
that used digital circuits. 
FIG. 1 represents schematically a digital image converter to which the 
present invention applies. 
Such a converter essentially comprises a circuit 1 forming the radar 
interface in which the incident video is processed, this circuit receiving 
the radar video signals VI jointly with radar synchronization signals SY, 
and a circuit 2 for conversion of the polar coordinates, .theta. of the 
radar video in Cartesian representation XY. Circuits 1 and 2 are connected 
to a digital random access memory 3 (RAM) through an addressing circuit 7. 
Afterglow circuit 4, the object of the invention, is interposed between 
interface 1 and memory 3. A TV sweeping generator 5 is connected, through 
the addressing circuit 7, to memory 3. It is also connected to a 
television monitor 6, which itself is connected to memory 3. 
The functions of the DIC above are the following: 
Interface circuit 1 samples and puts into digital form the radar video 
signals VI that are applied to it. It can include a video compression 
circuit allowing the acquisition of video signals received by the radar 
after its emission of a pulse for a defined angle of the antenna or aerial 
in rotation, corresponding to a synchronization pulse SY, and the reading 
of these video signals, in delayed time and at a different speed, so as to 
be adapted to the access time of the image memory 3; 
circuit 2 for the conversion of the polar coordinates into Cartesian 
coordinates allows the calculation of the address of each image element in 
Cartesian coordinates from radar data received in polar coordinates; 
the image memory 3 has a capacity adapted to the television standard used. 
It can have, for example, 1 024 lines of 1.024 memory points. The 
luminence of each point or dot can be coded, for example with the use of 3 
bits, authorizing eight levels of video intensity for each dot. For this 
memory, the television reading phase and the radar writing phase are 
asynchronous. Reading has priority and during a reading phase, conversion 
is interrupted; 
circuit 5 carries out the following operations: 
generation of television synczhronization signals, 
simultaneous reading of several dots in the image memory 3, in such a way 
as to respect the access time of the circuits used and to allow writing in 
this single memory; 
analog-digital conversion of this video intensity data read in the image 
memory in order to generate a television video signal for the television 
monitor 6 on which the visualized data appears. 
Afterglow circuit 4 acts to restore, with respect to digital data, for 
which the afterglow does not exist, an afterglow effect comparable to that 
produced on an analog memory tube. The recorded data is not attenuted by 
itself as a function of time in a digital memory that, without this 
particular process called artificial afterglow, has a tendency to perform 
indefinitely, the television image tending towards saturation. On the 
contrary, on a tube, the brightness of a spot starts to drop once it has 
been recorded. For a digital memory therefore, the afterglow circuit 4 
creates a similar effect, sometimes with a delay of one antenna revolution 
and a decrease of the level quantified at each revolution. 
According to the prior art, the artificial afterglow applied to a D.I.C. 
follows a law of fixed decrease, i.e. the law of composition between the 
incident video and the video already recorded in the memory, follows a law 
which is not variable in time. If the incident video is called Vi, issuing 
from interface 1, the video existing in the memory 3 is Vm, and the 
resulting video Vr, i.e. that which will be rewritten in the memory 3, the 
law that follows the artificial afterglow is the following: 
If Vi.gtoreq.Vm, Vr=Vi, i.e. if the incident video has an amplitude 
superior or equal to the video in the memory, for the memory cell 
involved, the value of the incident amplitude (Vi) is rewritten into the 
memory as the value of the resulting video. 
If Vi&lt;Vm, Vr=Vm-k, i.e. if the amplitude of the incident video is inferior 
to that of the video in memory, Vm-K is chosen as the amplitude of the 
video signal to be rewritten in memory, the value of the signal already in 
the memory Vm decreased by a determined constant value k that is called 
the "decrement factor". 
This operation must occur only once for each antenna revolution. In order 
to check it, each elementary cell of the memory comprises a bit called an 
afterglow bit which, at each registration operation, is compared to a 
"revolution bit" that changes value at each antenna revolution. In the 
case where the incident video signal Vi has an amplitude inferior to the 
amplitude of signal Vm already put into memory, the retentivity bit is 
compared with the revolution bit. 
If there is inequality between these two bits, signifying that the 
decrementation of the video was not yet carried out at the antenna 
revolution involved, for the memory cell analyzed, the new value of the 
video signal put in memory is Vr=Vm-k. The afterglow bit is thus changed, 
assuming its other logic value, allowing to avoid subsequently at the same 
antenna revolution a further decrementation for that cell. 
If there is egality of the afterglow bit and of the revolution bit, this 
means that operation Vm-k has already been carried out during the antenna 
revolution involved for the cell concerned and it will not be repeated. 
The decrementation process being carried out simultaneously with the write 
operations, i.e. in synchronization with the rotation of the antenna for a 
radar (in a more general framework it would be in synchronization with the 
arrival of the data from position of a sensor), the effect obtained on the 
image is very close to that obtained with an analog image converter. On 
these latter, the signal decreases once it is written, whereas in a 
digital image converter the decrease begins at the following antanna 
rotation with a decrementation quantum at each rotation. 
FIG. 2 shows the evolution of the brightness level of a given dot, supposed 
to be initially at the maximum level equal to 7, in relation with the 
number of antenna revolutions, thus taking into account the evolution of 
the afterglow in the cases of an analog image (curve 1) and digital (curve 
II) converter. In the second point, it will be quantified, with k=1. 
It will also be noted that the term "k" allows fixing of the decrease law 
of the video in memory. For a video expressed on n bits, the video signal 
disappearance occurs after 2.sup.n -1 antenna rotations if k=1. 
FIG. 3 is analogous to FIG. 2 and shows that for k=2 and a video signal 
expressed on eight levels, the decrease is obtained on four rotations. 
According to the prior art, it is also possible to vary the term k from 1 
to 2.sup.n for a video expressed on n bits. However, for a given 
equipment, the term k once selected remains constant. 
It can also be noted that for certain values of k, the time necessary for 
the disappearance of the data does not follow a regular law as a function 
of the initial level of the incident video. 
FIG. 4 is analogous to FIG. 2 and represents a decrease example of a video 
signal expressed on eight levels with k=4. 
It will be observed that a video data entered with levels 7, 6 and 5 
remains in memory during two antenna rotations, while the data entered 
with levels 4, 3, 2 or 1 only remains there during one rotation. 
Thus, according to what is set out herein-above, it is possible to obtain 
decrease laws of video data that are close to the natural decrease 
observed in the analog image converters. However, the decrease law that 
was selected for a converter remains fixed with time. 
It can, however, be selected to be more or less rapid and in this case, it 
has been observed that for certain relatively high values of k, the 
decrease of brightness of the data was not regular, the high level of data 
remaining in the memory longer than the low level data. Furthermore, all 
the data, whatever their input level, are processed in the same way. 
There are, however, certain cases where it would be worthwhile modifying, 
in a more selective manner, the decrease law of the afterglow in a digital 
image converter with the purpose of obtaining an improved exploitation of 
the data displayed on the screen of the television monitor. Thus, in the 
case where the radar data are relative to afterglow slow moving targets, 
like ships, it is difficult, when a DIC is used for which the decrease of 
the afterglow is adjusted according to the prior art, to separate the last 
echo received from the preceding ones (it is the phenomenon called 
"fusion"). As a matter of fact, such a target is almost at the same place 
from one antenna revolution to the next; thus, the echos of that target 
are displayed substantially on the same area on the TV monitor, producing 
a large spot. In that case, it is very difficult for the observer to 
separate the last echo from the spot, and the data concerning the 
trajectory and the direction of the target are lost. Further, prior art 
devices are not suitable for marine surroundings due to sea clutter, on 
which, weak echoes are difficult to select, and also due to some 
particular fixed targets like a buoy which give rise to a fixed echo but 
which can be sometimes invisible due to waves. 
It is an object of the invention to provide a processing of the afterglow 
in a digital image converter so that the decrease law that is applied is 
variable with time and adaptable to the surroundings. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a method for artificial 
afterglow on an image having a renewal period, said image being in the 
form of an incident video signal (V.sub.i), said incident video signal 
being processed and then stored (V.sub.m) in a digital memory, said method 
comprising a step of comparison of said incident video signal with said 
stored signal, and a step of rewritting in said memory a signal (V.sub.r) 
resulting from said comparison and from the application of one elementary 
afterglow law out of a plurality of elementary afterglow laws forming a 
general afterglow law, said elementary afterglow law controlling the 
decreasing of the stored signal in relation with said incident signal and 
with time, each of said elementary afterglow law being applied during one 
renewal period, said general afterglow law thus being applied during a 
plurality of renewal periods called a cycle.

DETAILED DESCRIPTION OF THE INVENTION 
As mentioned above, it is an object of the present invention to create, in 
a digital image converter, a decrease law of the artificial afterglow that 
is a function of the surroundings, i.e. both the amplitude of the 
processed video signals and time, for favoring certain echoes with respect 
to others, and this differently, for example, at each antenna revolution 
and on a certain number of revolutions. The interconnection of the laws 
can be any type, according to what is required. However, it must be 
cyclic. 
In FIG. 5 parallel lines represent successive echoes of a given slow moving 
target. 
According to the prior art, as discussed above, these successive echoes 
have a tendency to form a large luminous and indistinct spot. 
In FIG. 5, the scale has not been respected, since in practice the lines 
represented are not seen. 
In a defined operational situation, it is worthwhile separating the last 
echo registered from the preceding ones. In order to do this, it is 
necessary to decrement rapidly the high brightness levels of the video 
data. The level of the last echo having not yet been decremented, it will 
be detached from the mass. 
In order to thereafter obtain on the image the different phases of targets 
displacement, i.e. to restore the trajectory notion, it is necessary to 
maintain longer in the memory the video data of the intermediary levels. 
Furthermore, in order to avoid as many of the drawbacks as possible caused 
by the noise, i.e. to improve the detection, it is necessary to reduce as 
much as possible the low levels by applying a rapid decrease law. 
The following description gives an example of a combination of decrease 
laws being a function of the amplitude of the video signals concerned and 
the time, which provides a solution to the problem that was raised, for 
example, for slow-moving targets. It is presumed that the video is coded 
on three bits supplying eight brightness levels. In the following 
description, four laws are described. 
A first law called L1 is such that it brings back high levels, here levels 
6 and 7 for example, to level 5 and maintains constant the brightness data 
that are at levels inferior or equal to 5. 
Contrary to what has been explained above for the prior art, where the 
video signal (Vi) is directly memorized if it presents a brightness 
superior to that of video Vm already stored in the memory, according to 
the teaching of the invention, this conditions is arranged in the 
following manner by what is called a "weighted recording". This weighting 
is the following: 
EQU if V.sub.i &gt;V.sub.m and V.sub.i .gtoreq.level 3, V.sub.r =V.sub.i 
EQU if V.sub.i &gt;V.sub.m and V.sub.i &lt;level 3,V.sub.r =(V.sub.i +V.sub.m)/2 
This registration allows integration, from one antenna revolution to the 
next, of the brightness of an average luminous intensity echo, for 
example, at level 3, the position of which does not vary from one antenna 
revolution to the next. After a certain number of antenna revolutions, the 
brightness of this echo has sufficiently increased for it to become 
visible. In analogous conditions, a noise echo of a comparable brightness 
level, but which is random, i.e. that is not present in the same position 
from one antenna revolution to the next, cannot be integrated and 
furthermore finishes by disappearing. In these conditions, therefore, an 
average echo becomes visible and a noise echo is suppressed. 
The second law called Law L2 is such that it brings back high levels 6 and 
7 to level 5, that it decrements level 5 and that it maintains constant 
the levels lower than level 5. Furthermore, the recording of the incident 
video Vi is weighted as before. 
A third law, Law L3, is such that it brings back high levels to level 5, 
that it decrements by a single level (K=1) all the levels inferior or 
equal to level 5. The recording of the incident video is weighted as 
before. 
A fourth law, Law L4, is such that it brings back high levels 6 and 7 and 
decrements with a factor k=2, the levels inferior or equal to level 3. 
Other aims can be defined to be applied according to the operational 
situation observed and what it is desired to obtain. 
According to the present invention, these laws L1, L2, L3, L4 can be 
successively applied during each renewal period of the incident image, 
i.e. each revolution of the antenna in the case where a radar is involved, 
with a determined sequence, allowing accentuation of one effect more than 
another. It will be noted that the sequence does not exclude the 
repetition of one or several of these laws. Such a sequence forms a 
general afterglow law, which is thus variable with time. 
FIG. 6 represents the brightness decrease of a dot, which is initially at a 
maximum brightness (level 7), during the successive antenna revolutions, 
due to application of a particular combination of the preceding laws, 
namely L.sub.3 -L.sub.1 -L.sub.2 -L.sub.1 forming a general afterglow law. 
During the first antenna revolution, the considered dot is at level n=7. 
During the second revolution, provided that law L.sub.3 is applied, the 
brightness level decreases to level 5. During the following revolution, 
the law L.sub.1 maintains the level 5. During next revolution, the level 
decreases to level 4 due to law L.sub.2, and so on. During the 18th 
revolution, the dot desapears. 
This diagram is shown in the absence of any input video signal (V.sub.i) 
for the considered dot. 
FIG. 7 represents a variation during the successive antenna revolutions 
which is similar to the one of FIG. 6, under the same conditions, where 
the amplitude of the echoes (dB) replaces brightness levels. 
It will be noted that all the laws involved bring the upper levels to level 
5. The effect of this is to separate from the luminous spot the last echo 
received, that becomes perfectly visible with respect to the preceding 
echoes the brightness level of which is weaker. Law L2 doses the decrease 
of intermediary level 5 and Law L1 ensures the maintenance of all the 
video data the intensity of which is inferior or equal to level 5. 
FIG. 8 represents, in a way similar to FIG. 6, the general law obtained 
with a sequence of L3-L1-L1-L2-L1-L1. 
A distribution of the average datas levels is observed, for example, 3, 2 
and 1, that gives an observer a better notion of the direction and 
trajectory of a target. 
FIG. 9 represents, in a way similar to the one of FIG. 7, the evolution of 
the amplitude of the echoes when the law of FIG. 8 is applied. 
FIG. 10 represents this displacement notion by a fading of the brightness 
that decreases from left to right. During the first three antenna 
revolutions, the image is very luminous. It decreases during the following 
three revolutions, further decreases during the following six revolutions 
and so on. The brightness decrease is suggested by points that are 
progressively spaced further apart. 
FIG. 11 represents another example of the general law obtained with a 
sequence L3-L4-L4-L4-L4-L4-L4. In this case, a spreading out of the spot's 
tail for intermediary levels 5, 4 and 3 and a relatively rapid extinction 
of the datas of levels 2 and 1 is observed. 
FIG. 12 represents the 3 evolution of the amplitude of the echoes when the 
decrease law of FIG. 11 is applied. 
FIG. 13 is a symbolic representation similar to the one of FIG. 10 and 
where the points are replaced by bands. 
From what follows, it is possible to resume the afterglow law resulting 
from the combination of Laws L1, L2, L3, while making abstraction of time: 
For Vm.gtoreq.5 
if Vi.gtoreq.5, Vr=Vi 
if Vi&lt;5, Vr=5 
For 3.ltoreq.Vm&lt;5 
if Vi.gtoreq.Vm, Vr=Vi 
if Vi&lt;Vm, Vr=Vm 
For 0.ltoreq.Vm&lt;3 
if Vi.gtoreq.3, Vr=Vi 
if Vi&lt;3, V.sub.r =(V.sub.i +V.sub.m)/2 
FIG. 14 represents a partial diagram of one embodiment of a DIC according 
to the invention. 
The processing circuit 1 of the incoming data VI delivers the incident 
video Vi to a circuit 8 called afterglow logic that determines the 
resulting video Vr to be stored in the image memory 3. According to the 
chosen afterglow law, such as explained herein-above, circuit 8 carries 
out the comparison between the incident video Vi, delivered by circuit 1, 
and video Vm actually stored in memory 3 that the latter transmits through 
connection 9. Through connection 10, memory 3 transmits to circuit 8 the 
afterglow bit b.sub.r that must prevent a rewritting of a video signal 
already stored and decremented from being made several times per antenna 
rotation. A logic circuit called control logic 11, controlled by an 
angular reference signal e.g. the north signal (N) in the case of a radar, 
is connected to circuit 8 and controls the changes of the decrementation 
laws at each antenna rotation, i.e. at each image renewal period, i.e. as 
a function of time. Circuits 8 and 11 form circuit 4 in FIG. 1. 
FIG. 15 represents a schematic diagram of one embodiment of circuit 11 of 
FIG. 14. 
The control logic 11 comprises a down counter 12, controlled by north 
signal N, an addressing up counter 13, also controlled by signal N, a 
memory 14 comprising the decrementation laws that can succeed one another 
according to the invention at each operations cycle. The afterglow logic 8 
can be a programmable read-only memory (PROM) or a random access memory 
(RAM), which contains the general afterglow law and is connected to memory 
14. It receives the incident video Vi, the afterglow bit b.sub.r from a 
flip-flop 15, controlled by the north signal (N), data Vm and bit b.sub.r 
contained in image memory 3. 
The operation of control logic 11 is as follows: 
At the beginning of a cycle, i.e. of a plurality of afterglow laws (e.g. 
four laws in FIG. 6 or six laws in FIG. 8) which will succeed one another 
and which will be repeated during the following cycles, the down counter 
12 contains the number of laws in one cycle. At the end of the down count, 
a pulse is sent to up counter 13 and to down counter 12, in order to 
recharge the latter thus allowing it to drive a new cycle. 
Counter 13 is started by signal N and the data it delivers is sent to 
memory 14. That data constitutes an address of an area in memory 14 where 
is stored the reference (e.g. the number) of the actual law to be applied. 
The end pulse from counter 12 resets counter 13. 
Thus memory 14 furnishes data concerning the law to be applied to circuit 
8. The latter furnishes signal Vr upon further reception of signals Vi, 
Vm, br. 
In the herein-above description, some examples of laws that can be aplied 
in a given operational situation are cited. Other examples of laws can be 
stored in the equipment, at the disposal of an operator or control 
function operator, which allows modification of the sequence of laws to be 
applied according to the operational situation. 
It will also be noted that the decrement factor k is variable and, 
according to the invention, for each law applied, its value can be 
selected according to the effect desired. There is a possibility of 
selecting K=0 in a particular operational situation. This can be, for 
example, the case of an echo whose trace is not to be lost and for which 
it is required to be stored continuously in the image memory. In this 
case, selection of K=0 means suppressing the decrease of the afterglow for 
a certain category of echoes of determined level and, due to this fact, 
from one antenna revolution to the next, the registered echo does not have 
its brightness diminished. 
The present description concerns a processing of the afterglow in a digital 
image converter as well as an embodiment given by way of non-limitative 
example. In practice, the embodiment can be carried out by using a 
microprocessor.