Data compression

A video image of a relatively simple nature is produced by scanning using a conventional TV camera (1), and the scan results are converted into a Cartesian co-ordinate form for image analysis. This involves seeking a point of interest during the scan, the number of scan steps along a scan line as indicated by a counter (11) defining the point's location along the line. The point of interest's location in the other co-ordinate is indicated by the setting of a scan line counter (6). After each detection of a point of interest its location is indicated to associated equipment and the scan is resumed. As described this is used in an eye follower of the type described in our U.S. Pat. No. 1,581,018 the image analysis being used to assess the directions in which the eye is looking. The result of this assessment is used to exercise a controlling function or a machine or other device, e.g. to enable a disabled person to control such a machine or device.

This invention relates to data compression, especially where used in 
respect of a video image of a relatively simple nature. 
A single frame of a 625 line TV picture has approximately 
600.times.500=300,000 picture elements (pixels) for a normal resolution 
picture. Each pixel may need up to eight bits to digitally define the 
greay scale, so a single frame needs 2.4 Mbits of storage if compression 
is not used. Where a relatively simple image is to be scanned, e.g. to 
locate a point of interest on that image, the above arrangement is too 
complex and expensive. Hence an object of the invention is to provide a 
data compression technique more suited to relatively simple images. 
According to the invention there is provided a method of processing a video 
image, in which the image is generated by scanning a field of interest 
with a TV camera, the image thus generated being scanned to find the 
position of a point of interest on the image, the point of interest being 
represented by the change from light to dark or vice versa, in which the 
scanning for said point of interest is effected on a line-by-line basis, 
in which the location of a point of interest is indicated in a 
two-co-ordinate manner, in which one co-ordinate is determined by the 
setting of a counter started at the commencement of a line scan and 
stopped when a said point of interest is detected, and in which the other 
co-orginate is determined from the number of the line on which the point 
of interest was detected. 
Thus the indentification of the point of interest is represented in 
Cartesian co-ordinate form. Such an arrangement is suitable for the image 
analysis needed to determine the direction sight of an operator or user. A 
system for making such determination is described in our U.S. Pat. No. 
1,581,018, in which system a closed-circuit TV camera is used to scan an 
operator's or user's eye, and the direction in which the eye is looking is 
determined from an analysis of the image of the eye thus produced.

We now refer to FIG. 1, which is a block diagram of a system for processing 
an image of a human eye to determine from the position of the eye on the 
image the direction in which the user is looking. The output from a video 
camera 1 passes via a frame synchronisation extraction circuit 2 and a 
line synchronisation extraction circuit 3 to an analogue output on which 
there appears the analogue video signal for the image the which the TV 
camera 1 is aimed. This analogue signal is supplied to a threshold 
detector (comparator) 4 the output of which is a two-state non-return to 
zero signal, the two states respectively representing black and white. 
When the image scanning commences, a signal on a frame request input 5 
switches on the frame synchronisation extraction circuit 2, which via the 
connection shown resets to zero a scan line counter 6. This is a ten stage 
binary counter whose setting is advanced by unity for each line scanned. 
When, as is usually the case, the video signal includes noise, it is 
necessary for a change from white to black to be indicated by a number, 
four in this case, of black pixels following four white pixels. To do 
this, the binary video signal is passed into a four-bit shift register 
which is clocked at the pixel rate. The outputs of this register are gated 
by a four-input AND gate 8 to detect four consecutive elements of the same 
kine (1 or 0). This removes short bursts of interference on the video 
image. 
We now revert to the line synchronisation extraction circuit 3: This has 
two further outputs one of which, 9, is used to increment the line counter 
6 so that it steps once per line scanned. The other output 10 is used to 
start a counter 11, which counts the number of steps made along the line 
while the camera is scanning. When the gate 8 gives its output on 
detection of a change to four similar pixels, it sets a latch 12 whose 
output is applied to a five input NOR gate 13 and this, with the 
conditions on its other four inputs sets a second latch 14 when four 
pixels of the other sort have been detected. Thus the setting of the latch 
14 indicates the detection of four black pixels following four white 
pixels. Hence the circuit has detected the position of the point of 
interest. Via its output the latch 14 now stops the counter 11. Hence the 
current state of the two counters indicates the position of the point of 
interest. 
The settings of these two counters are now passed to associated circuitry 
(not shown) where it is stored and processed as desired in response to an 
output 15 from the device. Then a line reset signal on the input 16 resets 
the counter 11 and the latches 12 and 14, and scanning is resumed. Thus 
the position of one or more points of interest are detected and processed. 
The system as used to process an eye image is as shown in FIG. 2. Here we 
see the video camera 1 whose output feeds a processor input board 20 the 
details of which have just been described with reference to FIG. 1. The 
output of this block, which is a word per line scanned, derived as 
described above is applied to an image processor and shape analyser 21, 
whose output is applied to an output driver 22. This controls a display 
23, which may be an array of light-emitting diodes. 
The image processing is software controlled in the manner indicated in FIG. 
3, and the following discussion. On lines which do not intersect the image 
of the eye, the line counter counts to 512 and then stops. These lines are 
ignored when the multi-bit words representing the image are processed in 
the software. The relevant iris limbus A, FIG. 3, is extracted from 
irrelevant eyeball information B by testing the rate of change of the 
tangent to the locus. Thus we have: 
##EQU1## 
Then the output of the processor is determined by comparing the ellipse of 
the iris limbus with look-up tables. 
The image information is stored as an eight-bit count per line scanned and 
processed in software to smooth the data, extract iris information and 
analyse the shape of the eye limbus. The eccentricity and elevation of the 
minor axis of the ellipse E uniquely define the eye position and may be 
compared with the information in the look-up tables to determine the 
appropriate output. 
As has been indicated in our above-mentioned Patent Specification, the 
output which defines the direction in which the eye is looking can be used 
to position a controlled device. This is valuable for the control of 
machinery by disabled persons, and also for remote control of machinery 
where direct contact is inconvenient or not possible.