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.