GB2332322A

GB2332322A - Detection of vehicle licence plates

Info

Publication number: GB2332322A
Application number: GB9726065A
Authority: GB
Inventors: Brian David Vincent Smith
Original assignee: Pearpoint Ltd
Current assignee: Pearpoint Ltd
Priority date: 1997-12-09
Filing date: 1997-12-09
Publication date: 1999-06-16
Anticipated expiration: 2017-12-09
Also published as: GB2332322A9; GB9726065D0; GB2332322B

Abstract

Apparatus for detecting a vehicle licence plate in a video signal (1) utilises means (2,3,4) for isolating a component (5) of a spatial-frequency spectrum representative of a character set of indicia on the vehicle licence plate. The magnitude of this component is then compared (7) with a predetermined threshold (6) such that the presence of a plate in the signal is indicated if the magnitude of the component (5) exceeds the threshold (6).

Description

_Qetection of vehicle licence-plates 2332322 The present invention relates

to a method and apparatus for the detection of a vehicle licence plate in a video image, particularly although not exclusively obtained from a television signal.

A vehicle licence plate generally consists of a set of characters which may be illuminated so as to Provide a high contrast image against the background. Whilst the details of individual characters differ from letter to letter, there are characteristics which remain invariant across the set of characters. Such characteris%..ics include the stroke width and font type used to form the characters and the character to character spacing.

The present invention stems from the realisation that these characteristics may be analysed over the whole character set so as to generate a spatial-frequency spectrum which is representative of the character set as a whole. This spectrum exists in two dimensions, normally taken horizontally and vertically with respect to the licence plate but not necessarily so.

It is an object of the present invention to provide an improved method and apparatus for the detection of a vehicle licence plate in a video image derived from a television signal or indeed any other suitable source.

According to the present invention. there is provided a method of detecting a vehicle licence plate within a video image comprising the preliminary step of generating a spatial-frequency spectrum representative of d character set for use on the plate, and subsequently filtering a video signal contai-ning said video image to isolate at least one component of said spatial-frequency spectrum and comparing the magnitude of the component with a predetermined value wherein such that the presence of a plate is indicated when the magnitude exceeds a predetermined threshold.

Advantageously, electronic circuits and/or software may be designed to detect th-is characteristic spectrum by using either analogue or digital signal processing techniques or a software algorithm. Such implementations may work on the television signal as it is generated in the natural television scan time-order of top left-hand to bottom right- hand corner, or alternatively may work on a stored video field or frame.

Detecting part of the two dimensional spatialfrequency spectrum, such as the spectrum in the horizontal direction on its own may be sufficient to detect the presence of a licence plate. It could also be combined with a means of detecting the spectrum to be present at the same spatial position on a number of consecutive television lines. Greater detection accuracy will however generally accrue from matching the complete two- dimensional spectrum.

An advantage in implementing electronic hardware to detect the licenceplate spatial -frequency spectrum is that the decision as to whether or not a licence-plate is present in a particular video field or frame may be made by or before the end of the field or frame. This allows subsequent image processing circuitry only to process stored fields or frames which are known to contain a licence-plate.

Further the spatial-frequency detection circuitry or software algorithm my be designed to record the position of an assigned corner of the licence-plate. The subsequent image processing circuitry can therefore be presented with a video f ield or frame known to contain a licence-plate and the horizontal and/or vertical position of the licence plate within the image.

According to a further aspect of the invention, there is provided apparatus suitable for carrying out the above method.

In order to aid in understanding the invention a specific embodiment thereof will now be described by way of example and with reference the accompanying drawings, in which Figure 1 shows a block diagram of apparatus in accordance with the invention.

Referring to the sole Figure, the video signal I is filtered by a bandpass filter 2, the bandwidth and centre frequency of which has been chosen to detect a component of the spatial frequency spectrum of the vehicle licence plate within the television image. The spectrum might be based on the character to character spacing or the letter stroke width within the character or a combination of both. Further multiple band-pass filters may be employed each tuned to a different spatial characteristic. The output of the filter is rectified in rectifier 3 and integrated in integrator 4 to form a measure of the energy present within the frequency response of the band-pass filter 2. if multiple filters are used, then the energy detected by each would be measured, typically by additional rectifier and integrator blocks, with the integrator outputs summed or by use of a summing integrator. The integrator output 5 is then compared against a threshold voltage 6 in comparator 7, which voltage 6 is set such that video fields containing licence-pldtes are above the threshold and vice versa. The comparator 7 output signal 8 thus signifies the presence or absence of a vehicle licence plate on a particular video line. The pragence of the plate on a number of consecutive - 4 video lines can be detected by clocking the output of the comparator 7 into a serial-in, parallel out shift-register 9 with the line sync clock. An AM gate 10 can then detect all the outputs being true together. The length, n, of the shift register 9 is set to however many television lines a licence-plate typically minimally occupies, given the deployment of the camera.

The output of the AND gate 10 can then be used as the Disable signal 11 into a digital counter 12 which is reset on each video field and clocked on each video line, typically by the line synchronisation pulse. At the end of a f ield the counter 12 will hold the count of the last television line at which the plate was detected.

claims:

A method of detecting a vehicle licence plate within a video image comprises filtering a video signal containing said video image to isolate at least one component of a spatial-f requency spectrum representative of a character set of indicia for use on said vehicle licence plate and comparing the magnitude of said component with a predetermined threshold wherein an indication is given that a plate is present if the magnitude of the component exceeds said threshold.

2. A method as claimed in claim 1, wherein the video signal is filtered to obtain at least one bi-dimensional component of said spectrum.

Claims

3. A method as claimed in Claim 1, wherein the video signal is filtered to

obtain at least one singledimensional component of said spectrum.

4. A method as claimed in any preceding Claim, wherein the video signal comprises a series of lines making up a frame holding said video image, each line in turn being filtered and compared such that the presence of a licence plate within the frame is indicated if the threshold is exceeded for a predetermined number of lines.

5. A method as claimed in Claim 4, wherein the presence of a licence plate is indicated if the threshold is exceeded for a predetermined number of consecutive lines.

6. A method as claimed in Claim 4 or Claim 5, including the further step of setting a counter to zero at the start of each frame and incrementing the counter each time a line is filtered and compared and subsequently transferring the value of the counter to a register in the event that the presence of licence plate is indicated within the frame.

7. An apparatus for detecting a vehicle licence plate in a video image held in a f rame made up of a plurality of lines, includes means for filtering a video signal containing said video image to isolate at least one component of a spatial-f requency spectrum representative of a character set of indicia for use on said vehicle licence plate and means for comparing the magnitude of said component with a predetermined threshold wherein an indication is given that a plate is present in said image if the magnitude of the component exceeds said threshold.

8. An apparatus as claimed in Claim 7, wherein said filtering, isolating and comparison means comprises a bandpass filter, a rectifier, an integrator and a comparator in series, wherein the band-pass filter has a bandwidth and a centre frequency selected to detect said component of said spatial frequency spectrum, and the comparator holds a reference voltage such that a filtered, rectified and integrated signal in excess of said voltage is indicative of the presence of a vehicle licence plate in said image.

9. An apparatus as claimed in Claim 7 or Claim 8, f urther including storage means for holding a plurality of consecutive lines of output from said comparison means and logic means arranged to provide a signal indicative of the presence of a vehicle licence plate should the output from the comparison means held in said storage means all exceed said threshold.

10. An apparatus as claimed in Claim 9, wherein the storage means is a serial-in, parallel-out shift-register and the logic means an AND gate which detects whether all the outputs are true together.

11. An apparatus as claimed in Claim 9 or Claim 10, further including a counter which is reset on each video field and incremented on each line wherein the counter is disabled by the signal from said logic means such that at the end of a f ield the counter holds the number of the last line on which a plate was indicated.

12. An apparatus f or detecting a vehicle licence plate in a video image held in a f rame made up of a plurality of lines substantially as described herein with reference to the sole figure of the drawings.