FEATURE PARAMETER MEASUREMENT BY LINE SCANNING

3 4

invention, comparing the leading edges of detected sig- Dr)I„„ ncc/,DIDT,m, ~v TIJC A c

. . c .u . i-' j »u c .u BRIEF DESCRIPTION OF THE DRAWINGS nal pulses from the current line scan and those from the

previous line scan (delayed by one line scan period) The invention will now be described by way of exam

and generating a count pulse whenever the two leading pie with reference to the accompany drawings in edges coincide, comparing the trailing edges of de- 5 which:

tected signal pulses from the current line scan and FIG. 1 illustrates a single feature intersected by a those from the previous line scan delayed by one line number of scan lines of a coarse raster on which the scan period, and generating a count pulse whenever the horizontally and vertically projected lengths are detwo trailing edges coincide, comparing the delayed de- noted by H and V respectively,

tected signal pulses with those on the next line scan and 10 FIG. 2 is a block circuit diagram of an associated pagenerating a difference pulse of duration equal to the rameter and ant.-coincidence c.rcuit similar to that of duration of any delayed pulse or part thereof which is FIG- 1 of British Patent Specification No. 1,264 805. not coincident with a detected pulse on the said next . TM -3 ls a block; circuit diagram of a circuit for denv.. ., , . , .j.TM mc difference pulses which when summed produce a line scan, thereby to generate a second series of differ- , „ , 6 , ■ r , . . , , r . ,,

, , i ^ .. „ ^ , , . 15 signal whose magnitude is equivalent to the vertically

ence pulses and integrating the first and second series ". ^ . , iL ^ * ^ J ^ * ^ ^ ■ ^

c , j j i j • c u projected length of detected feature content in the

of difference pulses and said count pulses during a field field

scan to produce a final signal whose magnitude is indie- . ... . . ., c r ■ , .

„*\ . „° , „ . FIG. 4 illustrates the wave forms of signals at the

ative of the perimeter of detected feature content in the , , . ,.. *xti-> . fc,r . ...

F input I and output of the AND-gate of FIG. 3 during the

fleld- 20 line scans 1 to 5 of FIG. 1,

Preferably the pulses forming said first and second FIG 5 .g a block circujt djagram of a circuit for deriv.

series of difference pulses are replaced by a number of jng pulses from whjch a ,ength signa, relatjng to the

count pulses proportional to the duration of the differ- horizontal projected length of detected feature content

ence pulse. This replacement can be effected either by can be cornputed by summation,

electronically chopping the detected signal pulses or 25 FIG. 6 is a modification of the circuit of FIG. 5 which the difference pulses. In either event the integration may be applied to the circuit of FIG. 2, can be performed by simply counting all the count FIG. 7 is a block circuit diagram of a circuit for derivpulses during the field scan. jng shape factor information from signals correspondWhere only one feature lies in the field the integral ing to the horizontal and vertical projected lengths of will relate to the length of its perimeter. Where more 30 features, and

than one feature is detected, the signals arising from FIG. 8 is a block circuit diagram of a circuit for gen

the method according to the preferred feature of the erating a signal whose value is equivalent to the perime

invention are related to the feature from which they ter of detected features.

arise by an associated parameter computer such as de- DETAILED DESCRIPTION OF THE PREFERRED

scribed in British Patent Specifications Nos. 1,264,804 T-ho^t-... ,t-xtto

. , . orw- v EMBODIMENTS and 1,264,805.

The accuracy of the measured value of the perimeter As previously mentioned, FIG. 1 illustrates the hori

will depend upon the spacing between the line scans zontally and vertically projected lengths of a feature in

and the resolution of the measurement effected in the tersected by a number of scan lines of a coarse raster,

line scan direction. The former can usually be ignored Certain of the scan lines are numbered 1, 2, 3 etc. and

but the latter will depend upon the frequency at which the horizontally and vertically projected lengths are de

the pulses are gated to produce count pulse trains in- noted by H and V respectively.

stead of the difference pulses. An error will occur F,G 2 of the drawings corresponds to FIG. 1 of Britwhenever the duration of a detected video signal pulse 45 ish Patent Specifications Nos. 1,264,804 and

is not exactly equal to a whole number multiple of the 1 '26,f-805- T!»« vTM1} comprises a "coincidence cir

. . , ~, . . , , j cult denoted by the dotted outline 11 and an associ

gatins signal period. This is rarely the case and accord- , J ,, , . , ,

■ ° Z „ , c * c *u » • ated parameter computer which comprises the re

mg to a further preferred feature of the present inven- . j .\ j

i. c ^. ■ ^ c j * * j • i i mainder of the circuit. The circuit is supplied with de

tion a further comparison of detected signal pulses , . , . . . , . , „ rr

r ,. r tected video signal pulses obtained from a conventional

from adiacent line scans, separated by one line scan pe- 50 , u /■ , . ^

. , . J , . r J video signal by comparing (in known manner) the

nod, is made and an incrementa count pulse is added ., . . _ ... , . .iL r

,' , , , , , .. ■ video signal amplitude excursions with a reference volt

to the count pulse total whenever the leading edge of an(J ati a ]se onl when tne vjdeo sj ,

a detected video signal pulse on one line occurs within ampIitude exceeds the reference voltage. Since the inone gating signal period of the leading edge of a comci- stantane0us amplitude values of the video signal will dent detected video signal pulse on an adjacent line and 55 correspond to the greyness Gf points in the field, eleclikewise whenever the trailing edges of any two such trjca, puIses whose duration correspond to the length pulses occur within one gating signal period. Where a of the line scan intersections with features whose grey single value increment count pulse is employed a pre- levd djffers from that of the background, can be obferred value is 0.3. tained by setting the reference voltage between the A convenient device for adding 0.3 for each such voltage equivalent to the background grey level and count pulse comprises a separate counter for the incre- that of the feature grey level.

mental count pulses which records three for each count Detected video signal pulses 13 are applied to junc

pulse received and which includes a limit device which tion 10 in FIG. 2 and the leading edge of a detected generates a count pulse for the general perimeter 6J video signal pulse serves as a set signal for a bistable de

countcr whenever the incremental count pulse counter vice 12. The operation of the circuit is as described in

has reached 10 and which simultaneously resets this the two British Patent Specifications Nos. 1,264,804

latter counter to zero. and 1,264,805.

5 6

Assuming that a feature grey level is substantially Specification No. 1, 264,805 is required in which the

constant and that the reference voltage has been cor- module C is adapted to derive the appropriate incre

rectly set, the duration of each detected signal pulse ments of information from each detected video signal

will correspond to the length of the intersection of the pulse.

feature by each line scan. Each detected signal pulse 5 The circuit of FIG. 3 may comprise the computer can therefore be thought of as representing the line module C for the circuit of FIG. 2 and its input and outscan intersect with the feature and these are sometimes put terminals have been marked I and II accordingly, referred to as intersect pulses. In this event the accumulator 45 is reset at the end of As more particularly described in British Patent each modified detected video signal pulse. Where the Specification No. 1,264,805 the associated parameter 10 pulses from AND-gate 42 are not chopped it will be computer comprises a computer module C which pro- seen that by adding the pulses which are obtained on duces an electrical increment of information during or lines 1, 2, 3 and 4 a pulse whose total duration is proat the end of each detected signal pulse, a second com- portional to the vertical projection V of FIG. 1, will be puter module B which receives and holds an accumula- obtained. This addition is performed in the computer tion of increments from previous line scans delayed 15 module A (of FIG. 2).

from the previous line scan by a one line scan period Where the detected signal pulses are chopped at a delay device 28 and a third computer module A which high frequency by gate 43 the addition may be perreceives the signal from computer modules B and C formed by counting the number of chopped pulses and computes a fresh value to include the information which occur on each line scan in the output of the comincrement from the current line scan. The new com- 20 puter module C. In this way digital information is circuputed value is held in the computer module A and is re- lated in the delay device 28 which can therefore be a leased to the delay device 28 when a gate 30 is opened shift register. Preferably the register has sufficient caby a gating signal obtained by differentiating the trail- pacity to enable a parallel word of digital information ing edge of each output pulse from the bistable device to be transmitted so as to allow for close proximity of 12. In this way the gate 30 is opened at the end of each 25 features. The parallel word is then available at the corpulse. The same gating signal can be employed to cause rect instant in time at the output of the line delay decomputer modules B and A respectively to release and vice 28 for transmission as a digital associated parameupdate the information previously supplied to them. ter through the gate 36 when this latter is opened.

A further gate 37 (not described in the two Patent It will be appreciated that size or other form of dis

Specifications Nos. 1,264,804 and 1,264,805 )is closed 3^ crimination can be applied to the information released

by the same pulse which opens gate 36 to prevent the for each feature in a field of view, to allow a size or

final information signal, which appears as the associ- other distribution to be obtained,

ated parameter signal for the feature, from being recir- FIG. 5 illustrates a circuit for computing the horizon

culated once it has been released. In this way informa- tally projected length of detected feature content. A

tion is only circulated around the one line delay device 3^ single count pulse is generated for each intersect pulse

28 and computer modules B and A for as long as is nec- with the feature and the pulses so produced are

essary to complete scanning of a feature. counted in counter 49 the total number representing

FIG. 3 of the drawings illustrates a circuit for obtain- the total horizontally projected length of the detected

ing difference signal pulses which when summed pro- feature content. A single pulse is obtained from each

duce a signal whose magnitude is proportional to the 40 intersect pulse by differentiating circuit 46 and the

vertically projected detected feature content in a field. trailing edge pulses are eliminated by rectifying circuit

Video signal obtained by scanning the field is applied 48.

to terminal 9 for comparision with a reference voltage Signals indicating the total vertically and horizontally

from potentiometer 11 in a comparator 13, this latter projected lengths of detected feature content in a field

serving as a detector and providing detected signal 45 are obtained in the accumulator 45 of FIG. 3 and the

pulses at I. The detected pulses are supplied to one counter 49 of FIG. 5, at the end of scanning the field,

input of an AND gate 42 and to a one line delay device If both circuits of FIGS. 3 and 5 have been operating

40. The delayed pulses from the latter are applied via simultaneously the two projected length signals are

an inverting amplifier 44 to the other input of the AND gQ available at the same point in the time thereby enabling

gate 42. The output of the AND gate is accumulated a value to be determined of the ratio of total vertically

(e.g. integrated) in accumulator 45. projected lengths. This is conveniently done by em

The output pulses from AND gate 42 may be elec- ploying a circuit such as described with reference to

tronically chopped by gating pulses applied to a gate FIG. 7 (to be described later).

43, in which event accumulator 45 can comprise a 5^ When employed as computer module C in FIG. 2 the

pulse counter. counter is reset to zero at the end of each modified de

FIG. 4 of the drawings illustrates in the top half the tected video signal pulse and the counter 49 output

detected signal pulses which appear at junction 10 (ter- provides the signal for module A and is accordingly de

minal I of computer module C) during the five line noted by II.

scans numbered 1 to 5 in FIG. 1 and in the lower half 6Q Alternatively if as shown in FIG. 6 the input signals

the output pulses from AND gate 42. for computer module C are derived from a rectifying

Where the field contains more than one feature the stage 50 connected to terminal HI of FIG. 2 and ar

total vertically projected length signal will relate to all ranged to eliminate the trailing edge pulses, the rectify

the features in the field and will only be available after ing stage 50 and counter 49 constitute the computer

the field has been scanned. 65 module C and the connection between junction 10 and

Where a separate signal is required for each pro- junction I is not required,

jected length measurement for each feature, as associ- Where the vertically and horizontally projected

ated parameter computer as described in British Patent length signals for each feature are required simulta