image from the neighboring partial picture images as partial picture images so that the end point and the
start points P<s> and end points P(£). start point P'5), which are to be mutually connected
In other words, the outer periphery of the addresses between the contour lines which are separated one from
of the partial picture image memory device is sequen- 5 the other as separate partial picture images are con
tially designated from the origin (0,0) in counter-clock- nected together and is converted into closed loop line
wise direction and the changes of the pixel bits on the segment data consisting as a whole of several closed
outer periphery of the bit pattern are determined. loops.
In the bit pattern here, those bits in the character The signal processing described thus far is identical to
region are expressed by "1" while those in the back- 10 the one shown by the flow chart of FIG. 5 and has been
ground region are expressed by "0". In other words, the already discussed in detail in previously mentioned
pixel bits on the outer periphery of the addresses in the Japanese Patent Application No. 56-180649.
partial picture bit pattern are sequentially read out in The essential part of the method of data compression
counterclockwise direction and the bit corresponding according to this invention is found in the connecting
to "1" when the read-out bit is about to change from 15 means 10 for between different partial picture images
"1"1 to "0" is determined as a start point P® while the shown in FIG. 1 and thereafter.
bit corresponding to "1" when the read out bit is about At the connecting means 10 for between different
to change from "0" to "1" is determined as an end point partial picture images, the start point PW and the end
P'^. point Pt^) corresponding to each of the line segment
The contour line of the bit pattern generally extends 20 data are connected together so that the contour line for
on the periphery of the pixels, but, to simplify the fol- the whole original picture image may be reproduced
lowing description, those pixels or a row of pixels hav- and written into a closed loop line segment group mem
ing a contour line at least on one end are assumed to ory device 11 after being converted into a plurality of
make up a contour line. closed loop line segments.
The coordinate values (x„ y,) of the start point P<-^ 25 The original picture pattern and the contour line of its
and the end point P(^> detected by a start/end point bit pattern are each comprised of a closed loop as shown
detecting means are stored in an appropriate register by an example given in FIG. 2.
and supplied to a contour line tracing means 7 so that a Each closed-loop line segment data is then supplied contour line may be detected with the terminal points to a gradient comparison/elimination means 12 so that defined by the signal from the register. 30 the gradients of the pixel bits lying on the lines defining The pixel bits on the contour line traced by the con- the partial picture images and the contour lines with tour line tracing means 7 are given to a characteristic respect to the neighboring coordinate points are depoint detecting means 8 so that the changes in the char- tected and those pixel points having the same gradients acteristics of the contour line may be detected and ap- ahead and behind are eliminated from the coordinate propriate number of characteristic points on the con- 35 point data.
tour line may be sequentially detected. Specifically, the gradient comparison/elimination
These characteristic points may be classified, for means 12 may be comprised of, for instance, three data
instance, in the following manner according to the type buffer memories 21, 22 and 23, two gradient computing
of the changes in the contour line. circuits 24 and 25, a comparison circuit 26 and a gate
PC): point at which the direction of the contour line 40 circuit 27.
changes abruptly The coordinate point data of a certain closed loop
P<2>: point at which the direction of the contour line line segment group is sequentially stored in the data
changes gradually in one direction buffer memories 21, 22 and 23 as coordinate points P«,
P(3>: point at which the contour line changes from a P„ + l, P„ + 2, • • ■ from the memory device 11 for the
straight line to a curved line 45 closed loop line segment group under the control com
P<4>: point at which the contour line changes from a mand from a closed-loop tracing control circuit 13.
curved line to a straight line The data buffer memories 21 and 22 are connected to
The group consisting of the coordinate points ob- the gradient computing circuit 24 and the data buffer
tained by sequentially tracing the coordinate points of memories 22 and 23 are connected to the gradient com
these characteristic points from the start point to 50 puting circuit 25. And the gradient between the coordi
the end point P^ are written into a line segment data nate points P„ and Pn+i is computed at the gradient
memory device 9 as the compression data of the partial computing circuit 24 while the gradient between the
picture image. coordinate points P„+i and Pn+2 is computed at the
Each of the coordinate points in the line segment data gradient computing circuit 25 so that the gradients thus
memory device is converted into the coordinate point 55 obtained are supplied to the comparison circuit 26.
of the original bit pattern according to the block num- When the gradients of P„P„+i and ... not
ber 00—ij of the memory block MrjO'-M//. in agreement, the output of the comparios circuit 26 is
In other words, for each of the memory blocks supplied to the gate circuit 27 and the output data of the Moo~M,y, the characteristic points p(')-~p(4) and the data buffer memory 22 storing the coordinate point associated start and end points PW and P(£) located at 60 P„+i is sent to the compressed data memory device 14. respective ends of the characteristic points are written Conversely, when the gradients of P„Pn+i and into the line segment data memory device 9, normally, P„+iP„+2 are in agreement, the data corresponding to in the order of the start point PW, the characteristic the coordinate point P„+i is not supplied to the cornpoints P(')~p(4)j ancj the end point P(£) as the com- pressed data memory device 14.
pressed data. 65 For instance, the processing in the gradient cornAfter all the compression data of the line segment parison/elimination means 12 in regards to the data data memory device 9 is collected in regards to all the P„(S), Pn+i(G), P,,+2(^, P«+3(2), P„+4(£). ... of the memory blocks Moo ~ My, the compression data is sup- closed loop line segment group over the memory blocks