Electronic image processing

An electronic image processing system, using image dissection by solid state devices or by scanning and sampling, applies a signal representing a succession of signal values on an array of picture elements to complementary high and low pass electronic filters to produce an output signal in which a desired image feature is enhanced and noise is suppressed. The high pass filter, e.g. FIG. 1, comprises at least one matched pair of signal weighting gradient detectors, e.g. FIGS. 3 and 5, arranged back-to-back to detect a brightness transition 10, as shown in FIGS. 4 and 6, and having their outputs cored independently. Each pair will be sensitive to gradients in a particular direction in the array, e.g. horizontal, vertical or diagonal. The system is applicable to two-dimensional spatial arrays, three dimensional spatial arrays, and also to the temporal dimension, e.g. succession of moving pictures.

This invention relates to electronic image processing systems using image 
dissection, for example, by solid state devices or by scanning and 
sampling. 
Electronic image processing systems are known in which signals from a 
predetermined array of picture elements, or pels, are summed and averaged 
using electronic circuits so as to generate a signal or signals displaying 
selected components of the spatial fluctuations in image brightness. 
Appropriate weighting or multiplying factors are applied to the signals 
from the individual pels constituting the array in order to produce the 
desired response. One such known circuit generates a signal displaying 
mainly the large scale spatial fluctuations of the image brightness, the 
circuit acting as a low pass spatial filter. Another known circuit acts as 
a high pass spatial filter and generates a signal displaying mainly the 
small scale fluctuations that characterize the fine detail of the image. 
In practice, a set of filters having a high pass characteristic are 
utilised to generate signals corresponding to respective selected 
components of the spatial fluctuations in image brightness. 
These circuits can be used at the same time on the signals from one array 
of pels, and if the combined weighting on each of the pels due to the low 
pass circuit and the set of circuits having the high pass characteristic 
is zero on all but one pel of the array, then these filters are 
complementary. Under this condition, addition of the output signals from 
the filters regenerates the input signal unchanged, except for any change 
in gain. 
The two-dimensional spatial frequency response of such complementary 
filters may be illustrated diagrammatically as performing a partitioning 
of the spatial frequency plane into two areas, a central area surrounding 
and including the zero frequency origin and representing the pass band of 
the low pass filter, and the area outside this central region and up to 
the band limit of the system representing the pass band of the high pass 
filter. The use of such complementary low pass and high pass spatial 
filters at the same time enables the high frequency components of the 
final scanned image to be selectively modified by amplification or 
attentuation for the purposes of image enhancement or noise suppression. 
Electronic circuits performing the function of spatial filters as described 
have also been employed to detect the occurrence of predetermined image 
features. Each particular set of pel weightings, sometimes called a mask, 
is chosen to match a desired image feature. Thus, a one pel, or point, 
feature detector on a 3.times.3 pel array can be formed by the set of pel 
weightings: 
______________________________________ 
-1 -1 -1 
-1 8 -1 
-1 -1 -1 
______________________________________ 
This set of weightings, known in the prior art, can also be considered to 
form a high pass spatial filter. The complementary low pass filter would 
be formed by the set of weightings. 
______________________________________ 
1 1 1 
1 1 1 
1 1 1 
______________________________________ 
so that the two sets sum to form 
______________________________________ 
0 0 0 
0 9 0 
0 0 0 
______________________________________ 
In this case all weightings should be divided by 9 to give unity overall 
gain. 
Similarly, particular vertical and horizontal line detectors on a 3.times.3 
pel array, known in the prior art, are formed respectively by the sets of 
weightings 
______________________________________ 
-1 2 -1 -1 -1 -1 
-1 2 -1 and 2 2 2 
-1 2 -1 -1 -1 -1 
______________________________________ 
which can also be considered to form high pass spatial filters. The use of 
spatial filtering techniques for feature detection thus involves summing 
the signals from an array of pels after weighting the signals in 
accordance with the characteristics of the feature required. If the 
absolute value of such a weighted sum exceeds a predetermined threshold, 
then the particular image feature associated with that set of weightings 
is assumed to have been detected. 
Among the most important constituents of images are edges or brightness 
transitions. In order to detect such transitions, particular vertical and 
horizontal edge detectors on a 3.times.3 pel array, known in the prior 
art, are formed respectively by the sets of weightings: 
______________________________________ 
-1 0 1 -1 -1 -1 
-1 0 1 and 0 0 0 
-1 0 1 1 1 1 
______________________________________ 
which also can be considered to form horizontal and vertical gradient 
detectors, respectively. It will be noticed that both of these sets 
include pairs of next-adjacent (as distinct from adjacent) linear strings 
of pel weightings, that all the weightings in a string are of the same 
sign, that the weightings on the next-adjacent strings are of opposite 
sign, and that the weightings sum to zero. Summing the signal values from 
the corresponding pels after weighting in this way will, therefore, 
produce a signal characteristic of the first-difference on next-adjacent 
pels taken in a horizontal or a vertical direction, respectively. 
It is also possible to have a gradient detector with a pair of linear 
strings of pel weightings satisfying the foregoing requirements but with 
the two strings being adjacent to one another. In this case, summing the 
signal values after weighting in this way will produce a signal 
characteristic of the first difference on adjacent pels. 
Following such detection at a particular image location, the image 
processing procedure applied at that location may be modified. For 
example, where the point feature detector referred to above is employed 
together with the complementary low pass filter, the high pass output 
signal from the detector might be added to the low pass signal to form a 
regenerated signal when the detector output is greater in absolute 
magnitude than the predetermined threshold. This known technique has been 
described as signal coring with reference to its application in noise 
suppression systems. 
Signal coring is most directly applied in image processing applications 
where the cored signal is provided by a set of high pass filters that is 
complementary to the low pass filter with which the set is associated. The 
regenerated signal thus formed from the combination of the cored high pass 
signal and the low pass signal substantially relates to but one pel of the 
array. If these filters are not complementary, some bands of frequencies 
may be omitted from the regenerated signal, or some bands of frequencies 
may be duplicated in the regenerated signal causing an undesirable 
increase in signal level within these duplicated bands. As a result, 
undesirable artifacts are developed in image locations corresponding to 
adjacent pels, e.g., forming unwanted lines adjacent the detected feature. 
Appreciating that edges are of most interest, both line detectors and edge 
detectors are responsive to the brightness transitions represented by 
edges. While either detector may therefore appear useful, each has its 
inherent limitations. A line detector provides inherently poorer 
signal-to-noise performance than an edge detector. However, the edge 
detector requires an arrangement of pel weightings that prevents 
substantial complementarity to the low pass filter with which it is 
associated. 
While available edge and line detectors both possess undesirable 
attributes, they are nonetheless the conventional means for processing 
edge constituents of an image. The invention approaches this problem from 
the novel viewpoint that a high pass filter can be reconstituted by 
combining one or more pairs of matched gradient (or edge) feature 
detectors. This premise provides a solution to edge processing 
difficulties by utilizing the desirable attributes of each detector, 
namely, that a gradient (or edge) detector can provide a considerably 
better signal-to-noise performance for the detection of image edges or 
brightness transitions than can the high pass filter (e.g., a line 
detector) described above while the high pass filter can provide 
considerably better complementarity to to a low pass filter than can the 
gradient (or edge) detector described above. 
Thus in accordance with the present invention an electronic image 
processing system includes a high-pass electronic spatial filter 
comprising a pair of matched gradient detectors arranged back-to-back to 
detect a brightness transition such as an edge with their outputs being 
cored independently before being combined to comprise the output of the 
reconstituted high-pass filter. 
This enables brightness transitions, such as image edges perpendicular to 
the opposed directions of sensitivity of the pair of gradient detectors, 
to be detected with improved signal-to-noise performance without adding 
undesirable artifacts to the reconstituted high pass signal. 
The matched pair of gradient detectors are arranged back-to-back so that 
they are sensitive to oppositely directed gradients, and have one of the 
component strings of pel weightings in common. They therefore respond to a 
scanned edge which is parallel to the linear strings of pel weightings 
making up the pair of gradient detectors. The pair of gradient detectors 
may be disposed to sense an edge which appears to extend horizontally, or 
vertically or diagonally in the image. 
The high-pass filter of the invention may comprise more than one pair of 
matched gradient detectors. One pair may detect an edge extending in one 
direction, e.g. horizontally, while another pair, or pairs, may detect an 
edge, or edges, extending in a second, or further direction. Each gradient 
detector output will be cored independently. 
Although there are clear advantages to be obtained by the use of a 
plurality of pairs of gradient detectors sensitive to different spatial 
directions, it should be appreciated that use may be made of more than one 
pair of gradient detectors which are sensitive in the same direction. This 
may be advantageous where the two or more pairs have different pass bands 
together making up the desired high-pass characteristic. If the pel 
strings for one such pair are adjacent then they will form a high-pass 
filter, while if they are next-adjacent or more widely spaced they will 
form a bandpass filter. 
The principle of the invention still applies even if each of the two 
strings of a gradient detector includes only a single pel weighting. 
Image enhancement in accordance with the invention may also be made 
responsive to temporal variations in pel signals in cases where temporal 
sampling is effected as in frame sequential moving picture systems such as 
for example in television. Application of the principle can then ensure 
that inter-frame averaging can be used in uniform areas while avoiding, or 
reducing, image smear in areas of image motion or change. 
An electronic image processing system in accordance with the invention may 
include a network which cores first difference weighted signals from 
adjacent or from next adjacent pels of an array of at least three pels and 
then generates second difference signals from the cored first difference 
signals to provide an appropriate image feature enhancing signal. 
The principles of the invention set out in the preceding paragraphs may be 
used in conjunction with signals from 3.times.3, 5.times.5 or larger 
rectangular arrays and may also be modified for use with known arrays of 
hexagonal or other shape. 
Although it is preferred that the high pass filter, formed by the sum of 
the gradient-detecting filters, and the low pass filter, are complementary 
in that the combined weightings of these filters on corresponding pels sum 
to zero on all pels except the central pel, it may be found that useful 
improvements in signal to noise ratio are still obtainable where 
relatively small residual weightings remain on some pels, particularly on 
the outer pels, of the array after the weightings have been combined. The 
filters may then be said to be substantially complementary.

The invention will now be described in more detail with reference to the 
accompanying drawings in which: 
FIG. 1 shows a known high pass spatial filter or line detector; 
FIG. 2 shows the output waveform caused by scanning the filter of FIG. 1 
over an edge image 10; 
FIGS. 3 and 5 show individually known gradient, or edge-feature, detectors; 
FIGS. 4 and 6 respectively show the output waveform caused by scanning the 
detectors of FIGS. 3 and 5 over the edge image 10; 
FIG. 7 is a network constituting one of a complementary pair of horizontal 
gradient detectors which are spaced one pel apart in a first embodiment of 
the invention; 
FIG. 8 shows an alternative network to FIG. 7, a single element delay unit 
being used in the gradient detector to derive difference signals between 
adjacent pels, and also shows means for deriving the output of the 
complementary horizontal gradient detector; 
FIG. 9 shows a high pass spatial filter, and 
FIG. 10 shows a matched complementary pair, E and F, of horizontal gradient 
detectors derived therefrom; 
FIG. 11 shows a band pass spatial filter, and FIG. 12 shows a matched pair, 
G and H, of horizontal gradient detectors derived therefrom; 
FIG. 13 shows a third network in accordance with the invention, using the 
principle of the network of FIG. 7, but applied to next-adjacent pels, for 
providing spatial filtration using the pair of gradient detectors G and H 
of FIG. 12; 
FIG. 14 shows a fourth network in accordance with the invention, using the 
principle of the network of FIG. 8 but applied to next-adjacent pels, for 
providing spatial filtration using the pair of gradient detectors G and H 
of FIG. 12; 
FIG. 15 shows a band pass diagonal line detector for a 5.times.5 pel array, 
and FIG. 16 shows a matched pair, L and M, of diagonal gradient detectors 
derived therefrom; 
FIG. 17 shows a complementary pair, N and P, of diagonal gradient detectors 
for a 3.times.3 pel array; 
FIG. 18 shows a fifth network in accordance with the invention and applying 
its principles to an image in three spatial dimensions; 
FIG. 19 shows a sixth network in which temporal sampling of a sequence of 
pictures is effected; 
FIG. 20 shows a seventh network arranged to derive a tempero-spatial 
average signal for noise suppression; and 
FIG. 21 shows an eighth network arranged to derive a tempero-spatial 
gradient signal for modifying the output of FIG. 20 when horizontal image 
motion takes place between successive pictures. 
In considering the invention in more detail attention is invited to FIG. 1 
of the accompanying drawings where a high pass filter or line detector for 
a 3.times.3 pel array is shown. An this filter is scanned over an edge 
image 10 there is generated an output, shown in FIG. 2, consisting of a 
negative pulse 11 immediately followed by a positive pulse 12. 
However the high pass filter or line detector of FIG. 1 may be separated 
into two back-to-back gradient, or edge-feature, detectors spaced one pel 
apart as shown respectively in FIGS. 3 and 5. Their respective individual 
outputs on being scanned over the same edge image 10 are the spaced 
negative pulse 13 and positive pulse 14, seen respectively in FIGS. 4 and 
6. 
It will be noted that the negative pulse 13 is in the same spatial location 
in relation to the edge 10 as the negative pulse 11, and similarly with 
the positive pulse 14 and 12. The outputs of the two gradient detectors of 
FIGS. 3 and 5 therefore do not overlap for the same edge, and they can be 
thresholded or cored independently. As the noise level of their outputs is 
lower by a factor of 
##EQU1## 
for a white gaussian noise input, than that of the line-feature detector 
of FIG. 1, the threshold or coring level used in the subsequent signal 
processing can either be reduced, leading to better signal transmission, 
or, with the same threshold level, better noise rejection can be achieved. 
Thus there are advantages in substituting the pair of matched gradient 
detecting spatial filters of FIGS. 3 and 5, spaced one pel apart, for the 
high pass filter of FIG. 1. 
A single horizontal gradient detector constituting one of this pair of 
filters is shown in the network of FIG. 7. Here the incoming video signal 
S from an image scanning system is applied to a 1 element delay 20, to a 
differencer 21, and to a 1 line delay 22. The 1 element delayed signal is 
also applied to the differencer 21 and the result is applied, together 
with the corresponding results for the two pels of the next two lines (by 
way of 1 element delay 23, differencer 24, and by way of 1 line delay 25, 
1 element delay 26, and differencer 27) to the summer 28. The sum, 
representing the output of the gradient detector of FIG. 5 for example, is 
applied to the amplifier 29 which performs the thresholding operation to 
produce the cored signal C1. A second similar network, not shown, may be 
used to produce a cored output C2, not shown, for the other horizontal 
gradient detector of the pair, or, more economically this output may be 
produced by delaying signal C1 with a one element delay, not shown, and 
inverting it with an inverter, not shown in this particular Figure. The 
two cored signals C1 and C2 are then summed to provide an output which 
will be suitable for enhancing the image of the vertical edge 10. 
As an alternative, the first difference signals may be obtained prior to 
the use of the line delay units. FIG. 8 shows another horizontal gradient 
detector in which the first difference signals between adjacent pels are 
obtained prior to the line delay units. Here the input signal S is applied 
only to the 1 element delay 30 and the differencer 31. The 1 element 
delayed output is also applied to the differencer 31, and the resultant 
difference between one pel and the adjacent pel is applied to the summer 
32 and also to the 1 line delay 33 and thence to the 1 line delay 34, the 
outputs of these line delays also being applied to the summer 32. 
Amplifier 35 received the output of summer 32 to produce thresholded 
output signal D1. As before, another network, not shown, may be used to 
supply the cored output D2 of the other horizontal gradient detector, or 
this may be derived more economically as shown by delaying signal D1 with 
a one element delay 36 and inverting it in inverter 37. The two cored 
signals D1 and D2 are then summed to provide an output which will be 
suitable for enhancing the image of the vertical edge 10. 
For a vertical gradient detector, the networks shown could be used after 
interchanging the line and element delay units. Again, the cored outputs 
of the pair of gradient detectors are summed to provide an output which 
will be suitable for enhancing the image of a horizontal edge. 
The above examples use pel weightings of +1 and -1 but the weightings used 
need not be either equal or unity. Thus from the high pass filter of FIG. 
9 there may be formed a matched pair of gradient detectors E and F of FIG. 
10. The necessary signals for horizontal gradient detection may be 
obtained using the networks of FIG. 7 or FIG. 8, with the addition of the 
appropriate weighting factors shown in FIG. 10. The cored outputs of the 
two gradient detectors E and F are summed to provide an output which is 
enhanced where a vertical line or edge is scanned. 
It should be noted that the pattern of weightings of the high pass filter 
of FIG. 9, in addition to forming the matched pair of gradient detectors E 
and F, can be rotated through 90.degree. to form an additional filter. In 
this form it can be used to generate a second matched pair of gradient 
detectors sensitive to gradients at right angles to those sensed by the 
pair E and F shown in FIG. 10. Use of this second pair additionally will 
provide enhancement of horizontal image edges in addition to the 
enhancement of vertical image edges provided by the first pair. 
The combined output from these two pairs of gradient detectors make up the 
output of the high pass filter, and is added, possibly after amplification 
or attenuation, to the output of the complementary low pass filter 
______________________________________ 
1 1 1 
1 1 1 
1 1 1 
______________________________________ 
to form the enhanced video signal. 
The examples so far described have used adjacent strings of pel weightings 
but this is not a necessary restriction. In FIG. 11 is shown a band pass 
filter, this time for a 5.times.5 pel array, which uses next-adjacent 
strings. This can be replaced by the pair of gradient detectors G and H of 
FIG. 12, which also have next-adjacent strings of pel weightings with one 
string in common and whose outputs, after coring, together give an 
advantage in signal to noise ratio. The required signals can be generated 
by networks of the types shown in FIGS. 7 and 8 if each of the single 
element delay units there shown is replaced by a pair of such units. 
Alternatively the required signals J1 and J2 from one such pair of 
gradient detectors may be generated by a network similar to that shown in 
FIG. 13. Here single delay units are used but the outputs of adjacent 
pairs of pels are summed. This arrangement uses the principle of FIG. 7 
but derives both output signals as shown. FIG. 13 shows element delay 
units 40, 41, 42, 43, 44, 45, 58 and 59; line delay units 46, 47, 48 and 
49; differencers 39, 50, 51, 52 and 53; summers 55 and 56; thresholding 
amplifier 57; inverter 54; the incoming signal is S and the output of one 
detector is J1 while that of the other detector of the complementary pair 
is J2. 
Because they comprise a band pass filter the pair of filters G and H do not 
form a complete set with respect to a complementary low pass filter, in 
the sense that the pair of filters E and F does with the rotated version 
referred to above with reference to FIG. 9. Other filters must be used in 
addition to make up such a complete set with a high pass characteristic, 
although in practice it may be desirable to omit some of the component 
filters. 
Yet another alternative network is shown in FIG. 14, corresponding to FIG. 
8. Here a differencer 70 is connected to the first element delay unit 60 
and summers 71, 72, 73, 74 and 75 replace the differencers 39, 50, 51, 52 
and 53 of FIG. 13. In other respects these arrangements are the same, the 
other element delay units being identified at 61, 62, 63, 64, 65, 78 and 
178, the line delay units at 66, 67, 68 and 69, the thresholding amplifier 
77 receiving the output of summer 76, and an inverter 79. The pair of 
cored outputs of the high pass filter are K1 and K2. 
The described gradient detectors have been horizontal, or, in some cases, 
vertical detectors. The same principle can be applied to diagonal gradient 
detectors. In FIG. 15 is shown a band pass diagonal line detector for a 
5.times.5 pel array. This can be replaced by the pair of filters L and M 
of FIG. 16. 
FIG. 17 shows a pair of diagonal gradient detecting filters N and P for a 
3.times.3 array. The high pass filter from which they were derived is not 
shown. It will be appreciated that the pairs of diagonal filters of FIGS. 
16 and 17 require networks similar in principle to FIGS. 7, 8, 13 or 14 
but with the delay units re-arranged to select the signals from the 
appropriate picture elements. 
The above embodiments of at least one pair of gradient detectors replacing 
a single high pass filter will normally be used in conjunction with the 
complementary or substantially complementary, low pass filter. 
In some instances the inclusion of such particular signals in the final 
sum, or the processing of signals, can be made dependent on the presence 
or absence of other predetermined signals. Whether or not such an adaptive 
system is advantageous will depend upon the characteristics desired. 
The invention is applicable to analogue and to digital systems. In a 
digital system it is desirable but not essential that the weightings are 
limited to powers of 2. 
The foregoing examples are concerned with two-dimensional arrays of pels 
but it should be understood that the present invention is not limited to 
such arrays. 
One extension of the invention is to the multi-dimensional scanning of a 
stationary object. In this case at least three high pass spatial frequency 
filters may be formed, each comprising at least one pair of back-to-back 
gradient detectors as referred to above, where the signal gradients are 
now directed in multi-space and are derived from pairs of weighted sums 
over two-dimensional sheets or planes of pels, rather than one-dimensional 
strings of pels, and the complementary low-pass filter comprises a 
weighted sum over all pels within a corresponding 3-dimensional region. In 
any particular application the choice of weighting patterns within the 
principles described above will depend on the characteristics of the 
images to be enhanced. 
An example of a network suitable for 3-dimensional application is shown in 
FIG. 18. To assist in appreciating the relationship of this network to 
those described above it should be noted that in the network of FIG. 8, 
the first signal differences in one dimension, generated by element delay 
30 and difference 31 are summed over a second dimension by line delay 
units 33 and 34 and summer 32 before being cored. In the network of FIG. 
18 the first signal differences in one dimension are generated from the 
incoming signal S by an element delay unit 80 and differencer 81 and 
summed over a second dimension by line delay units 83 and 84 and summer 
85. In this application a third spatial dimension is being scanned in 
sequential fields, and the output of summer 85 is summed over this third 
spatial dimension by field delay units 86 and 87 and summer 88. The summer 
output is applied to amplifier 89 which effects a coring or thresholding 
operation to provide an output R1 of one gradient detector network. An 
element delay 90 and inverter 91 provide the output R2 for the other 
gradient detector network of the pair. R1 and R2 are summed to provide an 
image enhancing signal. The two directions of summing, by summers 85 and 
88, define a summing plane that is normal to the direction in which the 
first differences are generated. Other similar circuits could be used to 
generate difference signals directed in other directions, with summing 
over other planes normal to these other directions. 
The stage at which the first differencing operation is performed may be 
changed so that FIG. 18 could be based on the principle of FIG. 7 rather 
than FIG. 8. 
In the three-dimensional application described above with reference to FIG. 
18, the summing of weighted signal values is effected over two dimensions 
which together define a summing plane that is normal to the direction in 
which the first signal differences are generated. This latter direction 
has been referred to as a third spatial direction in the three-dimensional 
application. In fact it is not necessary for this direction to be spatial, 
and it may be temporal. Any signal differences in this temporal direction 
represent differences between successive views of a changing image or 
moving object which is being scanned sequentially. These views could be 
successive frames from a succession of moving pictures, and the temporal 
differences may be obtained by use of a temporal processing network 
including gradient detectors and temporal averager including at least two 
picture storage elements so that signal values on any pel can be compared 
over at least three successive pictures. 
Referring now to FIG. 19, the example of a temporal processing network 
shown applies a video signal S in turn to two picture delay units 92 and 
93. A temporal average summer 94 derives the average low pass signal value 
for a pel over the succession of three pictures, and a pair of 
differencers 95 and 96 function as gradient detectors to generate the 
respective first difference weighted signals across the first and the 
second picture delay units 92 and 93. These first difference signals are 
independently thresholded or cored by amplifiers 97 and 98, and the 
resultant signals applied to a summer 99 to derive the grain suppressed 
second difference temporal gradient signal. This temporal gradient signal 
is small or zero for image areas with little or no temporal change, and 
increases with the rate of image change. Combining this signal in summer 
100 with the signal from the temporal averager 94 causes the noise 
suppression produced by the averaging process to be reduced in areas of 
image change with the greatest reduction being obtained in areas of 
maximum change. It is to be noted that, looking in a temporal direction, 
the pair of differencers 95 and 96 form back-to-back gradient detectors 
providing signals that constitute a high pass signal when combined in the 
adder 99. 
The network of FIG. 19 can be used alone as indicated in a single temporal 
dimension. Additionally it can be used in conjunction with networks 
performing summing over one or more spatial dimensions, which would be 
appropriate when the image change arises from image motion. In this case 
the network of FIG. 19 would be preceded by a network performing summing 
over a linear array of elements arranged normally to the specified 
direction of motion. The output from summer 94 would then be a 
tempero-spatial average while the output from 99 would be a 
tempero-spatial image-enhancing signal suitable for the reduction of image 
smear due to image motion in the specified direction. For maximum freedom 
from image smearing the spatial gradients sensed should extend in 
horizontal, vertical and diagonal directions, and appropriate pairs of 
gradient detectors in the form of spatial filters should be used. 
Thus a network which generates a tempero-spatial difference signal output 
characteristic of the gradient in only one spatial direction using line or 
element delay units, will probably not provide freedom from image smear 
due to movement in other directions. Nevertheless, such a network may find 
useful applications, and since horizontal movement in motion pictures is 
more common than vertical movement, a tempero-spatial network now to be 
described, sensitive to horizontal gradients may be particularly useful. 
Alternatively the same result could be achieved with other networks, for 
example those of FIGS. 20 and 21 where spatial sampling is effected 
horizontally, in accordance with a filter weighting pattern not shown, 
together with temporal sampling. FIG. 20 shows a network for deriving the 
tempero-spatial average signal from video signal S. This video signal is 
applied in succession to field storage elements 110 and 111, and in 
succession to pairs of line delay units 112 and 113, with their associated 
summer 114; units 115 and 116 with their associated summer 117; and units 
118 and 119 with their associated summer 120. The outputs from the three 
summers are combined by summer 121. 
FIG. 21 shows a network for deriving the tempero-spatial gradient detector 
high pass output for combining with the low pass output of FIG. 20. In 
FIG. 21 the video signal S is applied in succession to field storage 
elements 130 and 131, and to a pair of line delay units 132 and 133 with 
their associated summer 134. A second pair of line delay units 135 and 136 
and their associated summer 137, and a third pair of line delay units 138 
and 139 and their associated summer 140 receive the video signal delayed 
by one and two picture intervals respectively. These pairs of line delay 
units and their associated summer are coupled in pairs to differencers 141 
and 142 which generate the first difference tempero-spatial signals which 
are then thresholded or cored by amplifiers 143 and 144. These signals are 
added by summer 145 whose outputs constitutes the high pass output for 
addition to the output of summer 121 of FIG. 20. 
The networks of FIGS. 20 and 21 could be simplified by carrying out the 
multi-line summing in a single unit, having a pair of line-delay units and 
a summer, before applying the signal to the field delay units. The first 
differencers are then connected directly to the field delay units. 
Multi-dimensional scanning has been described above with reference either 
to spatial coordinates in three dimensions or a temporal coordinate plus 
spatial coordinates in two dimensions. However these embodiments are 
merely exemplary of applications for the invention and are not intended as 
a restriction. For example, sets of matched gradient detectors may be 
scanned over an object in four dimensions, including three spatial 
dimensions and a fourth temporal dimension as well, to provide an image 
enhancement signal suitable for enhancing the brightness transitions in 
each of the four dimensions. 
With all of the arrangements described above it will normally be the 
practice to apply the video signal to a low pass filter and to the pair of 
pairs of gradient-detectors constituting the complementary high pass 
filter. The combined output signal from the pair or pairs of 
gradient-detectors provides an image-enhancing signal to be added back to 
the output of the low pass filter in such manner as to provide an 
apparently improved image with greater discrimination between wanted image 
features and unwanted noise.