GB2105941A - Correlation of representations of a reference and a scene - Google Patents

Abstract

The arrangement comprises stores 2, 4 for storing a current representation of a scene, and a reference representation, a correlator 5 for correlating the representations, a variance evaluator 6 and control devices 3, 7 for controlling the selection of the current representation and the updating of the reference representation. The variance evaluator evaluates the variance of that portion of the current representation which would be used to update the reference representation irrespective of whether the reference representation is to be updated. If the variance is less than a first threshold and the reference representation is to be updated, then a new reference representation of increased size is selected. Updating takes place if (i) a predetermined update criterion independent of variance is met, (ii) the variance is less than a second threshold less than the first, (iii) the variance is less than the first threshold for a preset time. <IMAGE>

Description

SPECIFICATION Correlation arrangements This invention relates to correlation arrangements.

A known correlation arrangement comprises stores for storing, respectively, a representation of a scene and a reference representation and means for correlating the representations. If the arrangement is carried on a vehicle such as a missile and used to track a target towards which the missile moves, the scale of the scene viewed by the camera will increase during the flight time of the missile. The increasing scale causes the degree of correlation of the representations to deteriorate during the aforementioned flight time.

It is an object of the present invention to provide a correlation arrangement in which the adverse effects of increasing scale of the viewed scene are mitigated.

According to the invention, there is provided a correlation arrangement comprising means for storing a representation of a scene and a reference representation, means for correlating the said representations, means for evaluating the variance of at least a portion of the representation of the scene, and means for replacing the said reference representation by a larger reference representation in dependence upon the variance being less than a predetermined value.

"Variance" is the average of the squared deviations of the values of the pixles in the said portion from the mean value of the pixels in the said portion.

For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawing, in which: Figure lisa block diagram of an auto-lock-follow system incorporating the present invention, Figure 2 is a block diagram of a variance evaluation circuit of the system, Figure 3 is a block diagram of an auto-dilation control ofthe system, and Figure 4 is a flow chart of an alternative implementation of the auto-dilation control.

Referring to Figure 1,video information produced at a given rate by a television camera (which may be an infra-red camera) is quantised in an analogue to digital converter 1 and stored in a picture store 2. A control micro processor 3 controls the quantisation of the information and also specifies the portion of the video information to be stored in the store 2. The portion of information stored, hereinafter called the "outer patch" represents an area of the scene viewed by the television camera. The micro processor also specifies and causes a portion, hereinafter called the "inner patch" of the information in the store 2 to be stored in a reference picture store 4. The information stored in the picture store 2 is updated i.e.

replaced by new information every frame, whilst the information stored in the reference picture store is updated, less frequently, in dependence upon a predetermined update criterion e.g. the peak correlation scene falling belowa predetermined level).

In an area correlator 5, the inner patch is correlated with the outer patch in each of a plurality of different positions relative to the outer patch. In each position, the amplitudes of corresponding pixels of the inner and outer patches are compared and the sum of the results of the comparisons produced as the correlation score for that position. The micro processor 3 receives the correlation scores for the positions and determines therefrom the position of best fit between the inner and outer patChes.

The microprocessor uses that determined position of best fit to specify the next outer patch to be stored in the store 2, so that the inner patch will be in the centre of the outer patch. In this way movement of the picture represented by the inner patch in the scene viewed by the TV camera is tracked. When the system is used in a vehicle such a missile tracking a target represented by the inner patch, the position of best fit is used to produce to guidance signal for causing the missile to home in on the target.

When the system is used in a vehicle such as a missile which is moving towards a target, the scale of the scene viewed by the TV camera will increase during the flight time of the missile. In order to take account of this increase in the scale in accordance with this example of the present invention, the size of the inner patch is increased e.g. from say 5 x 5 pixels to 6 x 6 or more pixels in dependence upon the variance of a preselected portion of the outer patch being less than a predetermined threshold value. For this purpose the system of Figure 1 includes a variance evaluator 6, which is shown in more detail in Figure 2, and an auto-dilation control 7, which is shown in more detail in Figure 3.

In the example shown in Figure 2, the statistical variance of the preselected portion of the outer patch is evaluated on each frame. The portion is that portion of the outer patch which would be used to update the inner patch if that inner patch were to be updated on that frame, (irrespective of whether it actually is to be updated or not). The decision to increase the size of the inner patch is taken after correlation is performed (in the correlators) but before the inner patch is updated; furthermore the decision to increase the patch size is taken afterthe decision to update but before an update is effected.

Consider the outer patch to comprise pixels P(i,j) where i = 0,1,2...,j= 0, 1, 2 ..., also, let the inner patch size be X by Y. If the position giving the best correlation score of the current inner patch inside the outer patch is (x, y),then the variance value, V, used for the decision to increase the size of the inner patch is given by::

The variance evaluator 6 shown in Figure 2 includes an address generator 60 which receives from the microprocessor 3 information about the position (x,y) of the current inner patch relative to the current outer patch i.e. the position of best corre lation score and also the size of the current inner patch. r-rom this information, the generator 60 genrato addresses fcr reading out of the picture store 2, the pixels Pij of the outer . aitch which would be used to update the current inner patch The pixel values Pij are squared in a squarer 61 and the squared pixel values summed in an accumulator to form the sum "" (P)j2). That sum is then div,ded in a divider 63 by the patch size XY to form the first term in the expression given above for the variance V.

The second term of that expression is formed by summing the pixel values Pij in an accumulator 64to form the sum ss, Pij, dividing that sum by the patch size Oxy to form the mean amplitude of the pixel values XY SE Pij in a divider 65 and squaring the mean in a squarer 66.

The second term is then subtracted from the first term in a subtractor 67 to form the variance V.

The auto-dilation control 7 shown in Figure 3 compares the value of variance V produced by the evaluator 6 with two threshold values T1 and T2 in respective comparators 70 and 71, T2 being lower than T1.

If the variance V is less than T1, and AND gate 72 is enabled and the size ofthe inner patch is increased by the micro-processor if an update latch 73 is set.

The latch 73 is set via an OR gate 74 under three circumstances: firstly, the latch 73 is set by the occurence of a decision to update the inner patch; secondly, the latch 73 is set by the variance falling below the threshold T2; and thirdly, the latch 73 is set by the variance falling below the threshold T1 but remaining above T2 for a predetermined number TD of frames.

Whenever any of these circumstances arises, the latch 73 is set and the inner patch is updated, whether or not the patch size is also increased, by the micro processor 3.

In order to detect the third circumstance, when the variance Vfalls below T1 a further flag latch 75 is set, thus enabling a counter 76 which counts frames. An arithmetic comparator 77 compares the count ofthe counter 76 with the threshold value TD, and when TD is exceeded sets the update latch 73 via the OR gate 74. Setting the latch 73 clears the counter 76 and resets the flag latch 75, and resets itself after a small delay.

The decisions taken by hardware in the autodilation control could be carried out by software in the micro processor. Figure 4 is a flow chart of such software.

As shown in the flow-chart, t 1e software increases the patch size of, and/or updates, the inner patch in the three circumstances described above.

In Figure 4the following terms are used: (i) VAR The variance V as deiined hereinbefore.

(ii) UPDATE A flag which may be set by preceding processes to request that the inner patch be updated. This flag may also be set by the autodilation process itself.

(iii) PA TCH SIZE The current size ofthe inner patch. The size is changed by the auto-dilation process only ifthe UPDATE flag is set.

(iv) FLAG, T7 FLAG is a flag, internal to the autodilation process, which is set by the variance VAR falling below the predetermined threshold, T1. The value of FLAG is maintained between successive frames.

(vi) COUNTER A Counter, internal to the autodilation process, which is incremented on each frame, providing FLAG is set. The value of COUNTER is maintained between successive frames.

(vii) TD A predetermined threshold on the COUNTER. If the value of COUNTER rises above TD then the UPDATE flag is set.

When the variance VAR falls below the threshold T1 then the flag FLAG is set, indicating that the detail in the scene is becoming dangerously low for the area correlators to work on. If an update is now, or on a subsequent frame, requested, then the size of the new inner patch is increased to include more detail from the scene. If, however, the variance continues to fall, then threshold T2 is crossed, and an update is requested and the patch size increased. T2 should beset approximately to the variance ofthe noise in the system, so that if VAR falls belowT2 then it is known that the correlator 5 is simply correlating noise and that the patch size must be increased immediately. Suppose, however, that after crossing T1,the variance does not cross T2, then an update (and patch size increase) is still requested a number of frames (given by TD) after T1 was crossed. This acts as a sort of 'safety valve' in case other, external update criteria fail. Whenever an update occurs, for whatever reason, COUNTER and FLAG are cleared.

Claims (8)

1. A correlation arrangement comprising means for storing a current representation of a scene and a reference representation, means for correlating the said representations, means for evaluation the variance of at least a portion of the said representation of the scene, and means for replacing the said reference representation by a larger reference representation in dependence upon the variance being less than a predetermined value.

2. A correlation arrangement comprising means for storing a current representation of a scene and a reference representation, means for correlating the representations, means for evaluating the variance of a portion of the current representation, and control means for selecting the current representation in dependence upon the correlation, for replacing the reference representation by a new reference representation in dependence upon a predetermined update criterion, for selecting the said portion, and for increasing the size of the reference representation in dependence upon the variance being less than a predetermined threshold value.

3. An arrangement according to Claim 2, wherein the control means selects, as said portion, that portion ofthe current representation which would be selected as the new reference representation i rrespective of whether or not the reference representation is to be replaced.

4. An arrangement according to Claim 3, wherein the control means increases the size of the reference representation at the time when the reference representation is replaced by the new reference representation.

5. An arrangement according to Claim 3 wherein the control means compares thoyariance with the said threshold value, determines whether the said predetermined update criterion is satisifed, and replaces the reference representation by the new reference representation of increased size if the variance is less than the threshold and the criterion is satisfied.

6. An arrangement according to Claim 3, wherein the control means compares the variance with the said predetermined threshold value and a further less than the said value, determines whether the said predetermined criterion is satisifed, and either replaces the reference representation by the new reference representation of increased size if the variance is less than the said value, and the said criterion is satisifed, or replaces the reference representation by a new such representation of increased size if the variance is less than the further value whether or not the criterion is satisifed.

7. An arrangement according to Claim 6, wherein, if the variance is less than the said threshold but greater than the further threshold for a predetermined time, the control means replaces the reference representation by the new reference representation of increased size irrespective of whether or not the said criterion is satisifed.

8. A correlation arrangement substantiaily as hereinbefore described with reference to Figures 1 and 2 together with Figure 3 or 4 of the accompanying drawings.