GB2144533A - Image analyser - Google Patents

Abstract

An image analysis system (eg for determining fat in meat or cell analysis) has a T.V. camera 12 whose output is coupled to one input of a fast comparator 18. When the T.V. camera output exceeds the threshold value of the comparator, a clock is started whose pulses are counted at 26 until the output of the camera falls below the threshold value, in order to avoid the previous necessity of providing additional comparators when more than one threshold level is required analysis of the image, the other input of the comparator 18 is connected to the output of a digital to analog converter 20 whereby the threshold value of the single comparator is changed by the D/A converter under the control of a microprocessor at least once during an analysis cycle. With a monochrome image different threshold are used for alternate scans, and with a colour image the three colour signals are switched to the comparator on successive scans using different threshold levels. <IMAGE>

Description

SPECIFICATION Improvements to image analysers The present invention is concerned with image analysers.

Fast analysis of images has for a number of years been carried out by first of all obtaining a television video image, then either operating on this video signal directly, or after digitising the picture and storing the picture in a digital store. Computer analysis of a digitised picture is both slow and expensive, but many sophisticated analytical routines may be attempted.

Analysis of video signals directly ("on the fly") by computers is extremely difficult and consequently very expensive.

A known system for analysis of monochrome video images "on the fly" employs an analogue video comparator which compares the video signal against a pre-set value. When that value is exceeded, a high frequency clock is started and when the signal falls below the pre-set value, the clock is stopped and the number of clock pulses are summed in a digital register. The number and area of any features exceeding the pre-set level may be easily and quickly computed from the data held in such registers. If more than one comparator level is required, then additional comparators and registers may be added.

This type of image analysis has been applied in the past in several fields, but, by way of example, the analysis of fat content in meat in a processing plant will be described. The meat is pre-broken into small pieces and passed on a black conveyor belt below a television camera. Two comparators are employed with pre-set threshold levels for lean (red) meat and fat (white). Two areas are obtained for visible lean and fat and the fat percentage is computed.

Some problems arise in differentiating dark red meat from the black belt and also in differentiating pale meats such as pork from fat. The problem cannot be overcome by employing two monochrome T.V. cameras with different colour filters since differential drift of the cameras results in poor accuracy.

Analysis of colour video signals from a colour T.V. camera is necessary for more complex applications. For instance, in the examination of histological sections and cervical smear stains, the staining process typically shows features of varying shades of pink, blue and purple. The usual method of analysing these features is to use a microscope to measure, by eye, the areas of features and to check their relative disposition. This method is extremely laborious and time consuming and technicians have to be restricted to limited working times.

An object of the present invention is to provide a simple, cheap method of improving the analysis of both monochrome and colour video images "on the fly" and to provide fast analysis of the video signals in a flexible but low cost installation.

According to the present invention there is provided an image analysis system comprising a T.V. camera whose video signal output is coupled to one input of a fast comparator capable of operating at frequencies of one megacycle or greater, the other input of the comparator being coupled to the analog output of a digital to analog converter adapted to provide a plurality of predetermined threshold levels for the comparator under the control of a microprocessor, the ouput of the comparator being coupled via an enabling circuit to a digital clock which is arranged to be started when the comparator determines that the video signal exceeds a selected threshold value set by the digital to analog converter and the enabling circuit is on, the clock pulses being counted into a digital register whose contents are read by the microprocessor at the end of each T.V. frame to provide information to one or more output devices, and the threshold level of the comparator being arranged to be changed at least once during each analysis cycle.

The register, enabling circuit and digital to analog converter are preferably connected separately into the microprocessor controller, which can be of known design. Information stored in the microprocessor is used to decide the value set on the digital to analog converter, and the shape and size of the measuring frame.

Advantageously, the video signal is initially inverted by a video inverter circuit at the output of the camera.

In a preferred embodiment having a monochrome T.V. camera the threshold level of the comparator can be changed at the end of a video frame scan and the register is read by the microprocessor before the next video frame scan.

In certain circumstances, it can be advantageous to have different illuminating lamps to highlight items being measured. Preferably, such different lamps are switched synchronously with the change in threshold level.

Furthermore, different filters can be introduced in front of the camera lens synchronously with the change in threshold level.

In another preferred embodiment having a colour T.V. camera, the three outputs of the camera are switched, in turn, to the comparator whose threshold level can be changed as each video output is switched.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of one embodiment of an image analysis system for monochrome video signals in accordance with the invention; and Fig. 2 is a schematic diagram of one em bodiment of an image analysis system for colour video signals in accordance with the invention.

Referring to Fig. 1 a clock timing circuit 10 generates all the timing signals required to perform the measurement sequence from a master oscillator (not shown) which is crystal controlled. Signals are also provided to synchronise the scan rate of a monochrome television camera 12 so that the system. A derivative of the master oscillator is used as the pixel clock which represents the period of a single unit of area.

The composite video from the T.V. camera 1 2 is processed by 'front end' circuits (not shown) where it is filtered to remove any noise outside the bandwidth of interest and then passed to a D.C. restoration circuit 14 which converts the A.C. coupled video signal to a D.C. coupled signal. A video invert circuit 16 enables the polarity of the video signal to be controlled by a microprocessor (not shown) so that the system can detect either dark objects against a lighter background or vice versa. The output from this circuit 16 is passed to one input of a video rate comparator 18, the other input of which is connected to a 256 level digital to analog convertor 20 which is controlled by the microprocessor.

The microprocessor can thus select one of 256 grey levels from the incoming video signal and pass the result to the measurement circuits in the form of a digital signal (referred to as 'detected video') which represents a '1' or 'O' depending on whether the video level is greater or less than the selected grey level at a particular instant in time.

A Frame Generation circuit 22, which is controlled by the microprocessor, is used to restrict the area of the T.V. picture which is actually measured to eliminate unwanted features before the detected video signal is passed to the measurement circuits.

When commanded to do so by the microprocessor, a measurement period timing circuit 24 generates a measurement period equal to the duration of one complete scan of the T.V. picture and this is used to gate an area count register 26 which then counts the number of area units occupied by the detected video signal. The result of measurement is then read from the register 26 via the microprocessor bus. To enable the operator to see the areas being measured, a video mixer 28 is used to combine the detected video signals with the video from the camera and the results are displayed on a T.V. monitor 30.

Advantageous features of the system lie in the method and speed with which the threshold levels of the video comparator 18 and the frame details may be changed. Also, modulation of the video input to the camera such as by optical filtering, coupled with synchronous de-modulation of the video signal by variation of the threshold level, enables much more information to be obtained without significant increase in instrument cost.

Several applications for image analysis are made easier and more cheaply. The application to determine the amount of fat in meat is achieved more easily and cheaply since only one video comparator need be used and only one accumulating register. The threshold is changed between each video frame and the register is read by the processor at the end of each video frame scan. The area of meat and fat is thus read sequentially which allows different lighting or filtering conditions to be applied in alternate frames. For example, ultra violet light may be applied during the fat scan and red light during the meat scan. The ultra violet light causes fluorescence of the fat, thus enhancing the difference betweem fat and lean and the red light enhances the difference between black belt background and meat.

This provides greater accuracy and allows smaller meat particles, such as minced meat, to be analysed.

In one practical embodiment used for determining the amount of fat in meat, first and second threshold levels are established in a preliminary calibration cycle using reference samples of similar grey scale content to the product which is to be tested for fat content, the two threshold levels being related to areas of fat and meat, respectively. During a subsequent measurement (analysis) cycle, a first video frame (field) is sampled at the first threshold level and the next video frame is sampled at the second threshold level, the threshold level of the comparator being changed between the first and second frame scans. This constitutes one analysis cycle. For the third video frame, the threshold level is returned to the first level and for the fourth frame the threshold level is changed to the second level. This consititutes a second analysis cycle. A plurality of such analysis cycles are repeated (say 250) and the average result is then computed and displayed by the microprocessor.

In other embodiments, more than two threshold levels corresponding to different predetermined grey levels could of course be used in each analysis cycle.

A further example lies in the determination of mean optical density. This is a measurement which is currently performed by digitising an image usually from a microscope view, of cell tissue. From the stored digital data the mean optical density is determined by computer analysis. This analysis can now be done much more quickly and cheaply by taking sequential scans of the image and changing the comparator threshold in equal steps for each scan. An optical density contour can quickly be produced and mean optical density computed.

In the arrangement of Fig. 2 the clock timing circuit 10 is used to synchronise the scan rate of a colour T.V. camera 32 to that of the system. The colour T.V. camera 32 has three outputs, one each for the red, green and blue signals. It is possible to analyse these three signals in parallel in separate systems in the same manner as described hereinbefore for the monochrome signal. This would, however, require three times as much equipment and hence would be uneconomical. A more economic method of achieving the analysis of a colour video signal is to provide a video switching system 34 (Fig. 2) capable of switching each of the three outputs in turn to the detection system. It is possible to select a different threshold value for the comparator 18 when each video output from the camera 32 is switched.

The multiplexed signal from the video switch 34 is, as for the monochrome signal, sent through the D.C. restorer 14 and, if necessary, is inverted by the video inverter 16. The signal is then sent through the comparator 18 whose threshold value can be altered to suit the different components of the multiplexed signal.

Connected to the output of the comparator 18 is a multiplex unit 36 which is able to demultiplex the signal. The multiplex unit 36 is controlled by the microprocessor and assembles the detected video signal into three bit-planes 38 in the memory, each bit-plane 38 representing one complete T.V. frame of each of the red, green and blue signals. Thus the areas of memory allocated to the bitplanes represent areas of the video image which exceeds the selected level of brightness in the red, green and blue spectral regions.

The signals are mapped into the memory as either a logic 0 or logic 1 depending on the X, Y co-ordinates of each pixel. Counters (not shown) are connected to the pixel clock and the T.V. line clock to generate the X, Y coordinates which are used to map the detected video signal into memory.

Provision is made within the circuitry 38 that generates the bit-planes to read data from the memory at the same rate as the T.V. scan, thus emulating a normal 'real time' detected video signal.

The video inverter 16 enables the detection system to select video levels below a certain brightness in each of the red, green and blue spectral regions. By using different combina tions of threshold values for the comparator 18 and normal and inverted video switching, any shade of colour may be detected by subsequent processing of the detected video images held in the three bit-planes 38. Means may be provided for transmitting the information in the bit-planes 38 into a proprietary computer for further processing.

Optionally a fourth bit-plane may be provided which is the result of a logic combination (AND, OR, NOR, NAND) of the three detected bit-planes. The processed bit-plane data can then be read from the bit-plane at the T.V. scan rate and processed by the normal feature counting and area measuring circuitry. This enables automatic framing of features within different colour features. For example, the nucleus of a tissue cell after staining would normally be a dark blue or purple within a pink cytoplasm. Nuclei which are not surrounded by pink cytoplasm can easily be picked out by this means.

Claims (13)

1. An image analysis system comprising a T.V. camera whose video signal output is coupled to one input of a fast comparator capable of operating at frequences of one megacycle or greater, the other input of the comparator being coupled to the analog output of a digital to analog converter adapted to provide a plurality of predetermined threshold levels for the comparator under the control of a microprocessor, the output of the comparator being coupled via an enabling circuit to a digital clock which is arranged to be started when the comparator determines that the video signal exceeds a selected threshold value set by the digital to analog converter and the enabling circuit is on, the clock pulses being counted into a digital register whose contents are read by the microprocessor at the end of each T.V. frame to provide information to one or more output devices and the threshold level of the comparator being arranged to be changed at least once during each analysis cycle.

2. An image analysis system as claimed in claim 1 in which the threshold level of the comparator is changed at the end of a video frame scan and the register is read by the microprocessor before the next video frame scan.

3. An image analysis system as claimed in claims 1 or 2, including a video inverter circuit for selectively inverting the video signal from the camera output during the analysis cycle.

4. An image analysis system as claimed in any of claims 1 to 3, including a frame generation circuit between the comparator and the digital register for restricting the area of T.V. picture which is to be passed to the enabling circuit to thereby select the features of the T.V. picture required for analysis.

5. An image analysis system as claimed in any of claims 1 to 4 in which the T.V. camera is a monochrome T.V. camera.

6. An image analysis system as claimed in any of claims 1 to 4 in which the T.V. camera is a colour T.V. camera.

7. An image analysis system as claimed in claim 6 in which the three video signals from the red, green and blue outputs are connected to a multiplexing switch which selects any one of the signals for coupling to the input of the fast comparator.

8. An image analysis system as claimed in claim 7 in which the video signal multiplexer may be switched during an analysis cycle.

9. An image analysis system as claimed in claim 8 in which a different threshold value can be selected as each component of the multiplexed signal is switched to the comparator.

10. An image analysis system as claimed in any of claims 1 to 9 in which different lamps illuminating the object being analysed and selected from a plurality of lamps having different illumination characteristics, can be switched on synchronously with the change in threshold level.

11. An image analysis system as claimed in any of claims 1 to 10 when used for the determination of the amount of fat in meat products passed under the T.V. camera on a moving conveyor.

12. An image analysis system substantially as hereinbefore described with reference to and as illustrated in Fig. 1 of the accompanying drawings.

13. An image analysis system substantially as hereinbefore described with reference to and as illustrated in Fig. 2 of the accompanying drawings.