Method for generating quadratic curve signal

Abstract

Assuming that a given equation representing a quadratic curve is:F(x, y)=ax.sup.2 +bxy+cy.sup.2 +dx+ey+f=0,the method for generating quadratic curve signals repeatedly selects a point close to F (x, y)=0 in only one of either the region of F (x,y).gtoreq.0 or the region of F (x,y)<0. This method allows to generate quadratic curve signals by using only a few parameters and without using complicated calculations. A hardware implementation is also disclosed.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for generating signals representing a quadratic curve such as a circle, an ellipse or a parabola, and more particularly to a method for generating quadratic curve signals best suited for use in a CRT display unit or a plotter.

2. Description of Prior Art

Known as a conventional method for generating signals representing a quadratic curve by repeating steps that select a new point from among eight points (x+1, y+1), (x+1, y), (x+1, y-1), (x, y-1), (x-1, y-1), (x-1, y), (x-1, y+1) and (x, y+1) adjacent to a current point (x, y) in a Cartesian coordinates system, is a method disclosed by a paper entitled "Algorithm for drawing ellipses or hyperbolae with a digital plotter" by M. L. V. Pitteway, Computer Journal, Vol. 10, November 1967, pp. 282-289.

This method first selects one octant from among the first octant in which point (x+1, y+1) or (x+1, y) can be selected, the second octant in which point (x+1, y) or (x+1, y-1) can be selected, the third octant in which point (x+1, y-1) or (x, y-1) can be selected, the fourth octant in which point (x, y-1) or (x-1, y-1) can be selected, the fifth octant in which point (x-1, y-1) or (x-1, y) can be selected, the sixth octant in which point (x-1, y) or (x-1, y+1) can be selected, the seventh octant in which point (x-1, y+1) or (x, y+1) can be selected, and the eighth octant in which point (x, y+1) or (x+1, y+1) can be selected. Then, by assuming that selectable points in the selected octant are (X.sub.1, Y.sub.1) and (X.sub.2, Y.sub.2) (e.g., X.sub.1 =x+1, Y.sub.1 =y+1, X.sub.2 =x+1 and Y.sub.2 =y in the first octant), that the equation of the quadratic curve is

F(x, y)=ax.sup.2 +bxy+cy.sup.2 +dx+ey+f=0, and that X.sub.3 =(X.sub.1 +X.sub.2)/2 and Y.sub.3 =(Y.sub.1 +Y.sub.2)/2, either (X.sub.1, Y.sub.1) or (X.sub.2, Y.sub.2) is selected according to the sign of D(x,y)=F(X.sub.3, Y.sub.3). Consequently, the next point is selected whether it be in the region of F (x,y).gtoreq.0 or in the region of F (x,y)&lt;0.

The method described in the above paper requires many parameters, complicated operations, and many operations for changing of parameters when changing the octant. And, it has a problem that it is difficult to be realized on hardware.

SUMMARY OF THE INVENTION

An object of this invention is to provide a method for generating quadratic curve signals which requires relatively few parameters, can generate signals representing a quadratic curve with only simple operations, and can be easily realized in hardware.

To attain the above objects, according to this invention, signals representing a line approximating a quadratic curve F (x, y)=0 are generated by repeatingly selecting a new point close to F (x,y)=0 from points in only one of either the region of F (x,y).gtoreq.0 or the region of F (x,y)&lt;0.

If the point to be selected is limited to only in the positive or only in the negative region of F (x,y), as described above, the next point is a point which does not change the sign of F (x,y) but if possible it reduces the absolute value of F (x,y). So the selection of a point is performed only by determining the sign.

For example, it is assumed that two candidate points (X.sub.1, Y.sub.1) and (X.sub.2, Y.sub.2) are selected in the octant selection step, from eight points around the current point. ((X.sub.0, Y.sub.0) is the current point.) Then let

F(X.sub.1, Y.sub.1)-F(X.sub.0, Y.sub.0)=.alpha. (the accrual of F when point (X.sub.1, Y.sub.1) is selected), and F(X.sub.2, Y.sub.2)-F(X.sub.0, Y.sub.0)=.beta. (the accrual of F when point (X.sub.2, Y.sub.2) is selected).

Then, if points only in the region of F (x, y).gtoreq.0 are to be selected, the following steps are sufficient to decide the choice of the next point:

(1) Check the sign of .alpha. or .beta.,

(2) Check the sign of F (X.sub.2, Y.sub.2) if .alpha..gtoreq.0(.beta.&lt;0),

(3) Check the sign of F (X.sub.1, Y.sub.1) if .alpha.&lt;0(.beta..gtoreq.0),

(4) Select (X.sub.2, Y.sub.2) if F (X.sub.2, Y.sub.2).gtoreq.0 or F (X.sub.1, Y.sub.1)&lt;0,

(5) Select (X.sub.1, Y.sub.1) if F (X.sub.2, Y.sub.2)&lt;0 or F (X.sub.1, Y.sub.1).gtoreq.0.

If points only in the region of F (x, y)&lt;0 are to be selected, the following steps are sufficient to decide the selection of the next point:

(1) Check the sign of .alpha. or .beta.,

(2) Check the sign of F (X.sub.1, Y.sub.1) if .alpha..gtoreq.0 (.beta.&lt;0),

(3) Check the sign of F (X.sub.2, Y.sub.2) if .alpha.&lt;0 (.beta..gtoreq.0),

(4) Select (X.sub.1, Y.sub.1) if F (X.sub.2, Y.sub.2).gtoreq.0 or F (X.sub.1, Y.sub.1)&lt;0,

(5) Select (X.sub.2, Y.sub.2) if F (X.sub.2, Y.sub.2)&lt;0 or F (X.sub.1, Y.sub.1).gtoreq.0.

It should be noted that in the above steps only signs are checked. Thus, it is possible to provide symmetry to the flow of operations, which allows an easy realization with hardware.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing one embodiment of a method for generating quadratic signals according to the invention.

FIGS. 2(a)-(d) and 3(a)-(d) are diagrams illustrating the basic principle of the invention.

FIGS. 4(a)-(h), are diagrams illustrating eight octants.

FIG. 5 is a diagram illustrating .alpha. and .beta. changes accompanying the octant changes.

FIG. 6 is a diagram showing a sequence of dots in drawing a circle of F=x.sup.2 +y.sup.2 -36=0 in the region of F.gtoreq.0 according to the method of FIG. 1.

FIG. 7 is a diagram showing a sequence of dots in drawing a circle of F=x.sup.2 +y.sup.2 -36=0 in the region of F&lt;0 according to the method of FIG. 1.

FIGS. 8A, 8B, 8C, 8D, 8E, 8F, 8G and 8H show steps to draw a circle of F=x.sup.2 +y.sup.2 -72=0 in the region of F&lt;0 according to the method of FIG. 1.

FIGS. 9A, 9B, 9C, 9D, 9E and 9F show steps to draw an ellipse of F=x.sup.2 +4y.sup.2 -156=0 in the region of F&lt;0 according to the method of FIG. 1.

FIGS. 10A, 10B, 10C, 10D, 10E and 10F show steps to draw an ellipse of F=10x.sup.2 -16xy+10y.sup.2 -288=0 in the region of F&lt;0 according to the method of FIG. 1.

FIGS. 11A, 11B, 11C, 11D, 11E, 11F and 11G show steps to draw a parabola of F=4y-x.sup.2 +2=0 in the region of F.gtoreq.0 according to the method of FIG. 1.

FIG. 12 is a block diagram showing one exemplary configuration of an apparatus used for performing the method of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a flowchart showing an embodiment of the method for generating quadratic curve signals according to the invention. Prior to the description the embodiment of the invention shown in FIG. 1, basic principles of the invention will be described by referring to FIGS. 2 and 3.

FIG. 2 shows the method for selecting the next point in the region of F (x,y).gtoreq.0. In the figure, (X.sub.0, Y.sub.0) indicates the current point, (X.sub.1, Y.sub.1) and (X.sub.2, Y.sub.2) the two candidates for the next point. In the case of FIG. 2(a), because both (X.sub.1, Y.sub.1) and (X.sub.2, Y.sub.2) are in the region of F (x, y)&lt;0, (X.sub.2, Y.sub.2) which is closer to F (x, y)=0 is selected. In the case of FIG. 2(b), although (X.sub.2, Y.sub.2) is closer to F (x,y)=0 than (X.sub.1, Y.sub.1), (X.sub.1, Y.sub.1) is selected because (X.sub.2, Y.sub.2) is in the region of F (x, y)&lt;0. In the case of FIG. 2(c), because both (X.sub.1, Y.sub.1) and (X.sub.2, Y.sub.2) are in the region of F (x, y)&lt;0, (X.sub.1, Y.sub. 1) being closer to F (x, y)=0 is selected. In the case of FIG. 2(d), although (X.sub.1, Y.sub.1) is closer to F (x, y)=0 than (X.sub.2, Y.sub.2), (X.sub.2, Y.sub.2) is selected because (X.sub.1, Y.sub.1) is in the region of F (x, y)&lt;0.

FIG. 3 shows the method for selecting the next point in the region of F (x, y)&lt;0. In the case of FIG. 3(a), because both (X.sub.1, Y.sub.1) and (X.sub.2, Y.sub.2) are in the region of F (x, y)&lt;0, (X.sub.1, Y.sub.1) being closer to F (x, y)=0 is selected. In the case of FIG. 3(b), although (X.sub.1, Y.sub.1) is closer to F (x, y)=0 than (X.sub.2, Y.sub.2), (X.sub.2, Y.sub.2) is selected because (X.sub.1, Y.sub.1) is in the region of F (x, y)&lt;0. In the case of FIG. 3(c), because both (X.sub.1, Y.sub.1) and (X.sub.2, Y.sub.2) are in the region of F (x, y)&lt;0, (X.sub.2, Y.sub.2) which is closer to F (x, y)=0 is selected. In the case of FIG. 3(d), although (X.sub.2, Y.sub.2) is closer to F (x, y)=0 than (X.sub.1, Y.sub.1), (X.sub.1, Y.sub.1) is selected because (X.sub.2, Y.sub.2) is in the region of F (x, y)&lt;0.

In the embodiment shown in FIG. 1, the following parameters are used:

Decision parameter: F (=ax.sup.2 +bxy+cy.sup.2 +dx+ey+f) Direction parameters: .alpha., .beta. (dependent of x, y, a, b, c, d, e, octant) Shape parameters: a, b, c (coefficients of x.sup.2, xy and y.sup.2 in the quadratic equation) Deviation parameters: T1, T2, T3 (dependent of a, b, c, octant)

.alpha. and .beta. depend on the octant. There are eight octants. FIG. 4(a) shows the first octant in which a point (x+1, y+1) or (x+1, y) can be selected as the next point to the current point (x, y), FIG. 4(b) shows the second octant in which a point (x+1, y) or (x+1, y-1) can be selected as the next point, FIG. 4(c) shows the third octant in which a point (x+1, y-1) or (x, y-1) can be selected as the next point, FIG. 4(d) shows the fourth octant in which a point (x, y-1) or (x-1, y-1) can be selected as the next point, FIG. 4(e) shows the fifth octant in which a point (x-1, y-1) or (x-1, y) can be selected as the next point, FIG. 4(f) shows the sixth octant in which a point (x-1, y) or (x-1, y+1) can be selected as the next point, FIG. 4(g) shows the seventh octant in which a point (x-1, y+1) or (x, y+1) can be selected as the next point, FIG. 4(h) shows the eighth octant in which a point (x, y+1) or (x+1, y+1) can be selected as the next point.

In the first octant, .alpha. and .beta. are:

.alpha.=F(x+1, y+1)-F(x, y) .beta.=F(x+1, y)-F(x, y)

In the second octant:

.alpha.=F(x+1, y-1)-F(x, y) .beta.=F(x+1, y)-F(x, y)

In the third octant:

.alpha.=F(x+1, y-1)-F(x, y) .beta.=F(x, y-1)-F(x, y)

In the fourth octant:

.alpha.=F(x-1, y-1)-F(x, y) .beta.=F(x, y-1)-F(x, y)

In the fifth octant:

.alpha.=F(x-1, y-1)-F(x, y) .beta.=F(x-1, y)-F(x, y)

In the sixth octant:

.alpha.=F(x-1, y+1)-F(x, y) .beta.=F(x-1, y)-F(x, y)

In the seventh octant:

.alpha.=F(x-1, y+1)-F(x, y) .beta.=F(x, y+1)-F(x, y)

In the eighth octant:

.alpha.=F(x+1, y+1)-F(x, y) .beta.=F(x, y+1)-F(x, y)

It should be noted that, by these definitions, .alpha. changes while .beta. does not, in a transition between the first and second octants, or between the third and fourth octants, or the fifth and sixth, or the seventh and eighth octants. Similarly, .beta. changes but .alpha. does not, in any transition between the second and third, or the fourth and fifth, the sixth and seventh, or the eighth and first octants. Thus, in any transition between adjacent octants, only one of the parameters .alpha. and .beta. will change in value and must be updated.

As illustrated later, T1 is a parameter which must be added to .beta. after selecting a point that displaces by (+1) or (-1) along either X or Y direction from the current point (x, y). T1 has the following values:

In the first octant, 2a(=.beta.(x+1, y)-.beta.(x, y)), In the second octant, 2a(=.beta.(x+1, y)-.beta.(x, y)), In the third octant, 2c(=.beta.(x, y-1)-.beta.(x, y)), In the fourth octant, 2c(=.beta.(x, y-1)-.beta.(x, y)), In the fifth octant, 2a(=.beta.(x, y-1y)-.beta.(x, y)), In the sixth octant, 2a(=.beta.(x-1, y)-.beta.(x, y)),

In the seventh octant, 2c(=.beta.x, y+1)-.beta.(x, y)),

In the eighth octant, 2c(=.beta.(x, y+1)-.beta.(x, y)).

Thus, T1 is 2a in the first, second, fifth and sixth octant, and is 2c in the third, fourth, seventh and eighth octants. In other words, T1 has only two values for all octants. Therefore, in the following, T1 is referred as T1 (=2a) for the first, second, fifth and sixth octant, and T1' (=2c) in the third, fourth, seventh and eighth octants.

As illustrated later, T2 is a parameter which must be added to .alpha. after selecting a point that displaces by (+1) or (-1) along either X or Y direction from the current point (x, y), and must be added to .beta. after selecting a point that displaces by (+1) or (-1) in X direction and by (+1) or (-1) in Y direction, from the current point (x, y). T2 has the following values:

In the first octant,

2a+b(=.alpha.(x+1, y)-.alpha.(x, y)=.beta.(x+1, y+1)-.beta.(x, y)),

In the second octant,

2a-b(=.alpha.(x+1, y)-.alpha.(x, y)=.beta.(x+1, y-1)-.beta.(x, y)),

In the third octant,

2c=b(=.alpha.(x, y-1)-.alpha.(x, y)=.beta.(x+1, y-1)-.beta.(x, y)),

In the fourth octant,

2c+b(=.alpha.(x, y-1)-.alpha.(x, y)=.beta.(x-1, y-1)-.beta.(x, y)),

In the fifth octant,

2a+b(=.alpha.(x-1, y)-.alpha.(x, y)=.beta.(x-1, y-1)-.beta.(x, y)),

In the sixth octant,

2a-b(=.alpha.(x-1, y)-.alpha.(x, y)=.beta.(x-1, y+1)-.beta.(x, y)),

In the seventh octant,

2c-b(=.alpha.(x, y+1)-.alpha.(x, y)=.beta.(x-1, y+1)-.beta.(x, y)),

In the eighth octant,

2c+b(=.alpha.(x, y+1)-.alpha.(x, y)=.beta.(x+1, y+1)-.beta.(x, y)).

As illustrated later, T3 is a parameter which must be added to .alpha. after selecting a point that displaces by (+1) or (-1) in X direction and by (30 1) or (-1) in Y direction, from the current point (x, y). T3 has the following values:

In the first octant,

2a+2c+2b(=.alpha.(x+1, y+1)-.alpha.(x, y))

In the second octant,

2a+2c-2b(=.alpha.(x+1, y-1)-.alpha.(x, y))

In the third octant,

2a+2c-2b(=.alpha.(x+1, y-1)-.alpha.(x, y))

In the fourth octant,

2a+2c-2b(=.alpha.(x+1, y-1)-.alpha.(x, y))

In the fifth octant,

2a+2c+2b(=.alpha.(x-1, y-1)-.alpha.(x, y))

In the sixth octant,

2a+2c-2b (=.alpha.(x-1, y+1)-(x, y))

In the seventh octant,

2a+2c-2b (=.alpha.(x-1, y+1)-.alpha.(x, y))

In the eighth octant,

2a+2c+2b(=.alpha.(x+1, y+1)-.alpha.(x, y))

Thus, T3 is 2a+2c+2b in the first, fourth, fifth and eighth octants, and is 2a+2c-2b in the second, third, sixth and seventh octants. In other words, T3 has only two values for all octants. Therefore, in the following, T3 is referred to as T3 (=2a+2c+2b) for the first, fourth, fifth and eighth octants, and T3' (=2a+2c-2b) in the second, third, sixth and seventh octants.

Table 1 below shows the values of .alpha., .beta., T1 (T1'), T2 and T3 (T3') in the eight octants.

In Table 1, the equations in the change column (either the .alpha. or .beta. column) are:

.alpha.=2.beta.-.alpha.+2c .alpha.=2.beta.-.alpha.+2a .beta.=.alpha.-.beta.+b .beta.=.alpha.-.beta.-b

These are equations for finding .alpha. and .beta. for the next octant by using .alpha. and .beta. for the current octant, when changing the octant. Three digits in parentheses in the octant column are codes indicating each octant.

It should be noted that the above equations, for finding .alpha. and .beta. for the next octant, apply for transitions between two adjacent octants in either direction. This is because these equations express a symmetrical function, the sum, of the old and new values of the changing parameter (.alpha. or .beta.) in terms of other parameters that do not change in the subject transition, as is easily seen.

TABLE 1__________________________________________________________________________Octant .alpha. .beta. T1 T2 T3__________________________________________________________________________First 2ax + bx + by + 2cy + 2ax + by + a + d 2a 2a + b 2a + 2c + 2b(111) a + b + c + d + eChange .alpha. 32 2 .beta. - .alpha. + 2cSecond 2ax - bx + by - 2cy + 2ax + by + a + d 2a 2a - b 2a + 2c - 2b(110) a - b + c + d + 3 (T3')Change .beta. = .alpha. + bThird 2ax - bx + by - 2cy + -bx - 2cy + c - e 2c 2c - b 2a + 2c - 2b(010) a - b + c + d + e (T1') (T3')Change .alpha. = 2 .beta. - .alpha. + 2aFourth -2ax - bx - by - 2cy + -bx - 2cy + c - e 2c 2c + b 2a + 2c + 2b(000) a + b + c - d - e (T1')Change .beta. = .alpha. - bFifth -2ax - bx - by - 2cy + -2ax - by + a - d 2a 2a + b 2a + 2c + 2b(100) a + b + c - d - eChange .alpha. = 2 .beta. - .alpha. + 2cSixth -2ax + by - by + 2cy + -2ax - by + a - d 2a 2a - b 2a + 2c - 2b(101) a - b + c - d + e (t3')Change .beta. = .alpha. - .beta. + bSeventh -2ax + bx - by + 2cy + bx + 2cy + c + e 2c 2c - b 2a + 2c - 2b(001) a - b + c - d + e (T1') (T3')Change .alpha. = 2 .beta. = .alpha. + 2aEighth 2ax + bx + by + 2cy + bx + 2cy + c + e 2c 2c + b 2a + 2c + 2b(011) a + b + c + d + e (T1')Change .beta. = .alpha. - .beta. - bFirst 2ax + bx 30 by + 2cy + 2ax + by + a + d 2a 2a + b 2a + 2c + 2b(111) a + b + c + d + e__________________________________________________________________________

Now referring to FIG. 1, the preferred embodiment of the invention is described. First, the start point (X.sub.s, Y.sub.s) is to be given. Then, as shown in the block 2, values for F, .alpha., .beta., T1, T1' and b are obtained at the start point and an octant is selected. For example, when drawing a circle

F=x.sup.2 +y.sup.2 -36=0, if it is assumed that the start point is (-5, 5) and the initial octant is the first octant, then (by Table 1) F=(-5).sup.2 +5.sup.2 -36=14 .alpha.=2x(-5)+2x5+2=2 .beta.=2x(-5)+1=-9 T1=T1'=2 b=0 are set. And, as shown in the block 4, values for T3, T3' and T2 are found from the following equations (by Table 1): T3=T1+T1'+2b T3'=T1+T1'-2b T2=T1(T1').+-.b (-sign for octants 2, 3, 6 and 7) For the above example, T3=T3'=4 T2=2.

Table 2 below shows .alpha., .beta., T1 (T1'), T2 and T3 (T3') in each octant for F=x.sup.2 +y.sup.2 -36.

TABLE 2______________________________________ T1 T3Octant .alpha. .beta. (T1') T2 (T3')______________________________________First 2x + 2y + 2 2x + 1 2 2 4(111)Second 2x - 2y + 2 2x + 1 2 2 4(110)Third 2x - 2y + 2 -2y + 1 2 2 4(010)Fourth -2x - 2y + 2 -2y + 1 2 2 4(000)Fifth -2x - 2y + 2 -2x + 1 2 2 4(100)Sixth -2x + 2y + 2 -2x + 1 2 2 4(101)Seventh -2x + 2y + 2 2y + 1 2 2 4(001)Eighth 2x + 2y + 2 2y + 1 2 2 4(011)______________________________________

Then, as shown in the block 6, the signs for .alpha. and .beta. are checked. If .alpha. and .beta. have different signs, the octant first selected is a correct octant. In the above example, since .alpha.=2, .beta.=-9 and the signs for .alpha. and .beta. are different, the octant is the correct one.

If .alpha. and .beta. have equal signs, the octant change process shown in the block 8 is performed. As clearly seen from Table 1, changing the value of .alpha. according to the equations in Table 1 while maintaining .beta. is sufficient to change from the first octant to the second octant, from the third to the fourth, from the fifth to the sixth, or the seventh to the eighth. Also, changing the value of .beta. according to the equations in Table 1 while maintaining .alpha. is sufficient to change from the second octant to the third octant, from the fourth to the fifth, from the sixth to the seventh, or the eighth to the first. In particular, when the octant is continuously changed, changes of .alpha. and .beta. are caused alternately (see FIG. 5). Then, by checking whether .alpha. was changed in the last octant change or not, in the block 10, it is found which one of .alpha. and .beta. should now be changed in this octant change. For example, if the current first octant is now to be changed for the second octant, it is found that change of .alpha. is now required because .beta. was (or would have been) changed in the last octant change.

If the necessity of change of .alpha. is detected, it is decided whether the current octant is the first or fifth octant, or not, in block 12. If so, as shown in the block 14, an operation

.alpha.=2.beta.-.alpha.+2c is performed to change the value of .alpha.. This means that the current octant is changed to the second or the sixth octants, respectively. In the above example, this changes the first octant to the second octant. If in the block 12 it is decided that the current octant is not the first or the fifth octant, it is the third or the seventh octant, so that an operation .alpha.=2.beta.-.alpha.+2a is performed in the block 16 to change the value of .alpha.. This means that the current octant is changed to the fourth or the eighth octant.

However, when the block 10 provides an affirmative result in judgment, the necessity of change of .beta. is detected, and then, as shown in the block 18, it is judged whether the current octant is the second or sixth octant, or not. If so, as shown in the block 20, an operation

.beta.=.alpha.-.beta.+b is performed to change .beta.. This means that the current octant is changed to the third or the seventh octant. If the block 18 provides a negative decision, the current octant is the fourth or the eighth octant, so that an operation .beta.=.alpha.-.beta.-b is performed to change .beta., as shown in block 22. This means that the current octant is changed to the fifth or the first octant.

Along with the change of octant as described above, the value of T1 (T1'), T2 and T3 (T3') are also changed according to Table 1, as briefly indicated in block 24 of FIG. 1. It is clear from Table 1 that new values for all of them corresponding to the new octant can be determined using the values set in the block 2 or 4.

Then, the signs of the new .alpha. and .beta. are checked, again in the decision block 6. If .alpha. and .beta. have different signs, the point selection process in block 39 is performed. If they still have the same sign, the octant change process in block 8 is again performed. This process continues until .alpha. and .beta. have different signs.

When .alpha. and .beta. have different signs, it is first judged in the block 32 whether F and .alpha. have the same or different signs. It is equivalent to the checking of signs of F and .beta. because, when it is intended to draw a curve in the region of F.gtoreq.0, F is positive (including zero), so the fact that F and .alpha. have the same sign means that .alpha. is positive (or zero) and .beta. is negative. When it is intended to draw a curve in the region of F&lt;0, F is negative, so the fact that F and .alpha. have the same sign means that .alpha. is negative and .beta. is positive (or zero).

If it is judged in block 32 that they have the same sign, the signs of F and F+.beta. are compared, as shown in block 34. If the same sign, the point that displaces by (+1) or (-1) along either X or Y direction is selected, as shown in the block 36. Thus, if it is assumed to be the first octant, (X+1, Y) is selected. If F and F+.beta. are judged in block 34 to have different signs, the point that displaces by (+1) or (-1) in the X direction and (+1) or (-1) in the Y direction is selected, as shown in the block 42. Now, if it is assumed to be the first octant, (X+1, Y+1) is selected.

If F and .alpha. are judged in block 32 to have different signs, the signs of F and F+.alpha. are compared in the block 40. If the same sign, the point that displaces by (+1) or (-1) in the X direction and (+1) or (-1) in the Y direction is selected as shown in the block 42. If F and F+.alpha. are judged to have different signs, the point that displaces by (+1) or (-1) along either X or Y direction is selected, as shown in the block 36.

After the process of block 36 is executed, the values of parameters are updated, as shown in the block 38, according to the equations:

F=F+.beta. .alpha.=.alpha.+T2 .beta.=.beta.+T1 (T1'). After the process of the block 42 is executed, the values of parameters are updated, as shown in the block 44, according to the equations: F=F+.alpha. .alpha.=.alpha.+T3 (T3') .beta.=.beta.+T2. Then, returning to the block 6, the signs of .alpha. and .beta. are checked. If they are different, the point selection process of block 30 is again performed. If, however, the signs are the same, the octant change process of block 8 is performed next, as described above.

FIG. 6 shows a circle of F=x.sup.2 +y.sup.2 -36=0 that is drawn in the region of F.gtoreq.0 according to the method of FIG. 1 by assuming the start point of (-5, 5). Tables 3 and 4 below, taken together as one table, show F, .alpha., .beta. and the octant change when drawing the curve of FIG. 6, also recalling Table 2 above.

TABLE 3__________________________________________________________________________ Point NextF .alpha. .beta. selection (x, y)__________________________________________________________________________P1 14 2 -9 (x + 1, y) (-4, 5)P2 5 4 -7 (x + 1, y + 1) (-3, 6) (F + .beta.) (.alpha. + T2) (.beta. + T1)P3 9 8 -5 (x + 1, y) (-2, 6) (F + .alpha.) (.alpha. + T3) (.beta. + T2)P4 4 10 -3 (x + 1, y) (-1, 6) (F + .beta.) (.alpha. + T2) (.beta. + T1)P5 1 12 -1 (x + 1, y) (0, 6) (F + .beta.) (.alpha. + T2) (.beta. + T1) 0 14 1 (F + .beta.) (.alpha. + T2) (.beta. + T1)P6 0 -10 1 (x + 1, y) (l, 6)(Change of (.alpha. = 2.beta. - .alpha. + 2c)octant)P7 1 -8 3 (x + 1, y) (2, 6)P8 4 -6 5 (x + 1, y) (3, 6)P9 9 -4 7 (x + 1, y - 1) (4, 5) 5 0 9P10 5 0 -9 (x + 1, y - 1) (5, 4)(Change ofoctantP11 5 4 -7 (x + 1, y - 1) (6, 2)P12 9 8 -5 (x, y - 1) (6, 2)P13 4 10 -3 (x, y - 1) (6, 1)P14 1 12 -1 (x, y - 1) (6, 0) 0 -10 1 (x, y - 1) (6, -1)P15 0 -10 1 (x, y - 1) (6, -1)octant__________________________________________________________________________ TABLE 4______________________________________ Point NextF .alpha. .beta. selection (x, y)______________________________________P16 1 -8 3 (x, y - 1) (6, -2)P17 4 -6 5 (x, y - 1) (6, -3)P18 9 -4 7 (x - 1, y - 1) (5, -4) 5 0 9P19 5 0 -9 (x - 1, y - 1) (4, -5)Change ofoctantP20 5 4 -7 (x - 1, y - 1) (3, -6)P21 9 8 -5 (x - 1, y) (2, -6)P22 4 10 -3 (x - 1, y) (1, -6)P23 1 12 -1 (x - 1, y) (0, -6) 0 14 1P24 0 -10 1 (x - 1, y) (-1, -6)(Change ofoctantP25 1 -8 3 (x - 1, y) (-2, -6)P26 4 - 6 5 (x - 1, y) (-3, -6)______________________________________

FIG. 7 shows a circle of F=x.sup.2 +y.sup.2 -36=0, which is drawn in the region of F&lt;0 according to the method of FIG. 1 by assuming the start point of (-4, 4). Table 5 below shows F, .alpha., .beta. and the octant change when drawing the curve of FIG. 7, while also recalling Table 2 above.

TABLE 5__________________________________________________________________________ Point NextF .alpha. .beta. selection (x, y)__________________________________________________________________________Q1 -4 2 -7 (x + 1, y + 1) (-3, 5)Q2 -2 6 -5 (x + 1, y) (-2, 5) (F + .alpha.) (.alpha. + T3) (.beta. + T2)Q3 -7 8 -3 (x + 1, y) (-1, 5) (F + .beta.) (.alpha. + T2) (.beta. + T1)Q4 - 31 100 - 1 (x + 1, y) (0, 5) (F + .beta.) (.alpha. + T2) (.beta. + T1) -11 12 1 (F + .beta.) (.alpha. + T2) (.beta. + T1)Q5 -11 -8 1 (x + 1, y) (1, 5)(Change of (2 .beta. - .alpha. + 2c)octant)Q6 -10 -6 3 (x + 1, y) (2, 5) (F + .beta.) (.alpha. + T2) (.beta. + T1)Q7 -7 -4 5 (x + 1, y) (3, 5) (F + .beta.) (.alpha. + T2) (.beta. + T1)Q8 -2 -2 7 (x + 1, y - 1) (4, 4) (F + .beta.) (.alpha. + T2) (.beta. + T1) -4 2 9 (F + .alpha.) (.alpha. + T3) (.beta. + T2)Q9 -4 2 -7 (x + 1, y - 1) (5, 3)(Change of (.alpha. - .beta. + b)octant)Q10 -2 6 -5 (x, y 31 1) (5, 2) (F + .alpha.) (.alpha. + T3) (.beta. + T2)Q11 -7 8 -3 (x, y - 1) (5, 1) (F + .beta.) (.alpha. + T2) (.beta. + T1)Q12 - 10 10 -1 (x, y - 1) (5, 0) (F + .beta.) (.alpha. + T2) (.beta. + T1)__________________________________________________________________________

FIGS. 8A, 8B, 8C, 8D, 8E, 8F, 8G and 8H show steps to draw a circle of F=x.sup.2 +y.sup.2 -72=0 in the region of F&lt;0 according to the method of FIG. 1 by assuming the start point of (0, 8). Table 6A, 6B, 6C, 6D, 6E, 6F, 6G and 6H show F, .alpha., .beta., the octant, T1, T1', T2, T3 and T3' corresponding to FIGS. 8A to 8H, respectively.

TABLE 6A__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________0 FFFF8 FFFF2 00001 2 002 002 002 004 0041 FFFF9 FFFF4 00003 2 002 002 002 004 0042 FFFFC FFFF6 00005 2 002 002 002 004 0043 FFFF2 FFFFA 00007 2 002 002 002 004 0044 FFFF9 FFFFC 00009 2 002 002 002 004 0045 FFFF5 00000 FFFF5 3 002 002 002 004 004__________________________________________________________________________ TABLE 6B__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________6 FFFF5 00004 FFFF7 3 002 002 002 004 0047 FFFF9 00008 FFFF9 3 002 002 002 004 0048 FFFF2 0000A FFFFB 3 002 002 002 004 0049 FFFFC 0000E FFFFD 3 002 002 002 004 00410 FFFF9 00010 FFFFF 3 002 002 002 004 00411 FFFF8 FFFF2 00001 4 002 002 002 004 004__________________________________________________________________________ TABLE 6C__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________12 FFFF9 FFFF4 00003 4 002 002 002 004 00413 FFFFC FFFF6 00005 4 002 002 002 004 00414 FFFF2 FFFFA 00007 4 002 002 002 004 00415 FFFF9 FFFFC 00009 4 002 002 002 004 00416 FFFF5 00000 FFFF5 5 002 002 002 004 00417 FFFF5 00004 FFFF7 5 002 002 002 004 004__________________________________________________________________________ TABLE 6D__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________18 FFFF9 00008 FFFF9 5 002 002 002 004 00419 FFFF2 0000A FFFFB 5 002 002 002 004 00420 FFFFC 0000E FFFFD 5 002 002 002 004 00421 FFFF9 00010 FFFFF 5 002 002 002 004 00422 FFFF8 FFFF2 00001 6 002 002 002 004 00423 FFFF9 FFFF4 00003 6 002 002 002 004 004__________________________________________________________________________ TABLE 6E__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________24 FFFFC FFFF6 00005 6 002 002 002 004 00425 FFFF2 FFFFA 00007 6 002 002 002 004 00426 FFFF9 FFFFC 00009 6 002 002 002 004 00427 FFFF5 00000 FFFF5 7 002 002 002 004 00428 FFFF5 00004 FFFF7 7 002 002 002 004 00429 FFFF9 00008 FFFF9 7 002 002 002 004 004__________________________________________________________________________ TABLE 6F__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________30 FFFF2 0000A FFFFB 7 002 002 002 004 00431 FFFFC 0000E FFFFD 7 002 002 002 004 00432 FFFF9 00010 FFFFF 7 002 002 002 004 00433 FFFF8 FFFF2 00001 8 002 002 002 004 00434 FFFF9 FFFF4 00003 8 002 002 002 004 00435 FFFFC FFFF6 00005 8 002 002 002 004 004__________________________________________________________________________ TABLE 6G__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________36 FFFF2 FFFFA 00007 8 002 002 002 004 00437 FFFF9 FFFFC 00009 8 002 002 002 004 00438 FFFF5 00000 FFFF5 1 002 002 002 004 00439 FFFF5 00004 FFFF7 1 002 002 002 004 00440 FFFF9 00008 FFFF9 1 002 002 002 004 00441 FFFF2 0000A FFFFB 1 002 002 002 004 004__________________________________________________________________________ TABLE 6H__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________42 FFFFC 0000E FFFFD 1 002 002 002 004 00443 FFFF9 00010 FFFFF 1 002 002 002 004 00444 FFFF8 FFFF2 00001 2 002 002 002 004 004__________________________________________________________________________

FIGS. 9A, 9B, 9C, 9D, 9E and 9F show steps to draw an ellipse of F=x.sup.2 +4y.sup.2 -156=0 in the region of F&lt;0 according to the method of FIG. 1, by assuming the start point of (0, 6). Table 7A, 7B, 7C, 7D, 7E and 7F show F, .alpha., .beta., the octant, T1, T1', T2. T3 and T3' corresponding to FIGS. 9A to 9F, respectively.

TABLE 7A__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________0 FFFF4 FFFD3 00001 2 002 008 002 00A 00A1 FFFF5 FFFD5 00003 2 002 008 002 00A 00A2 FFFF8 FFFD7 00005 2 002 008 002 00A 00A3 FFFFD FFFD9 00007 2 002 008 002 00A 00A4 FFFD6 FFFE3 00009 2 002 008 002 00A 00A5 FFFDF FFFE5 0000B 2 002 008 002 00A 00A__________________________________________________________________________ TABLE 7B__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________6 FFFEA FFFE7 0000D 2 002 008 002 00A 00A7 FFFF7 FFFE9 0000F 2 002 008 002 00A 00A8 FFFF0 FFFF3 00011 2 002 008 002 00A 00A9 FFFF1 FFFF5 00013 2 002 008 002 00A 00A10 FFFF6 FFFFF 00015 2 002 008 002 00A 00A11 FFFFB 00001 FFFEA 3 002 008 008 00A 00A__________________________________________________________________________ TABLE 7C__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________12 FFFFC 0000B FFFF2 3 002 008 008 00A 00A13 FFFEE 00013 FFFFA 3 002 008 008 00A 00A14 FFFE8 FFFFB 00002 4 002 008 008 00A 00A15 FFFEA FFFF3 0000A 4 002 008 008 00A 00A16 FFFF4 FFFFB 00012 4 002 008 008 00A 00A17 FFFEF 00005 FFFFB 5 002 008 002 00A 00A__________________________________________________________________________ TABLE 7D__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________18 FFFF4 0000F FFFED 5 002 008 002 00A 00A19 FFFE1 00011 FFFEF 5 002 008 002 00A 00A20 FFFF2 0001B FFFF1 5 002 008 002 00A 00A21 FFFF3 0001D FFFF3 5 002 008 002 00A 00A22 FFFF6 0001F FFFF5 5 002 008 002 00A 00A23 FFFF5 00029 FFFF7 5 002 008 002 00A 00A__________________________________________________________________________ TABLE 7E__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________24 FFFEC 0002B FFFF9 5 002 008 002 00A 00A25 FFFE5 0002D FFFFB 5 002 008 002 00A 00A26 FFFE0 0002F FFFFD 5 002 008 002 00A 00A27 FFFDD 00031 FFFFF 5 002 008 002 00A 00A28 FFFDC FFFD7 00001 6 002 008 002 00A 00A29 FFFDD FFFD9 00003 6 002 008 002 00A 00A__________________________________________________________________________ TABLE 7F__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________30 FFFE0 FFFDB 00005 6 002 008 002 00A 00A31 FFFE5 FFFDD 00007 6 002 008 002 00A 00A32 FFFEC FFFDF 00009 6 002 008 002 00A 00A33 FFFF5 FFFE1 0000B 6 002 008 002 00A 00A34 FFFD6 FFFEB 0000D 6 002 008 002 00A 00A35 FFFE3 FFFED 0000F 6 002 008 002 00A 00A__________________________________________________________________________

FIGS. 10A, 10B, 10C, 10D, 10E and 10F show steps to draw an ellipse of F=10x.sup.2 -16xy+10y.sup.2 -288=0 in the region of F&lt;0 according to the method of FIG. 1, by assuming the start print of (6, 8). Table 8A, 8B, 8C, 8D, 8E and 8F show F, .alpha., .beta., the octant, T1, T1', T2, T3 and T3' corresponding to FIGS. 10A to 10F, respectively.

TABLE 8A__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________0 FFFC8 FFFDC 00002 2 014 014 024 008 0481 FFFCA 00000 FFFDA 3 014 014 024 008 0482 FFFCA 00048 FFFFE 3 014 014 024 008 0483 FFFC8 FFFCC 00012 4 014 014 004 008 0484 FFFDA FFFD0 00026 4 014 014 004 008 0485 FFFAA FFFD8 0002A 4 014 014 004 008 048__________________________________________________________________________ TABLE 8B__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________6 FFFD4 FFFDC 0003E 4 014 014 004 008 0487 FFFB0 FFFE4 00042 4 014 014 004 008 0488 FFFF2 FFFE8 00056 4 014 014 004 008 0489 FFFDA FFFF0 0005A 4 014 014 004 008 04810 FFFCA FFFF8 0005E 4 014 014 004 008 04811 FFFC2 00000 FFFAE 5 014 014 004 008 048__________________________________________________________________________ TABLE 8C__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________12 FFFC2 00008 FFFB2 5 014 014 004 008 04813 FFFCA 00010 FFFB6 5 014 014 004 008 04814 FFFDA 00018 FFFBA 5 014 014 004 008 04815 FFFF2 00020 FFFBE 5 014 014 004 008 04816 FFFB0 00024 FFFD2 5 014 014 004 008 04817 FFFD4 0002C FFFD6 5 014 014 004 008 048__________________________________________________________________________ TABLE 8D__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________18 FFFAA 00030 FFFEA 5 014 014 004 008 04819 FFFDA 00038 FFFEE 5 014 014 004 008 04820 FFFC8 FFFDC 00002 6 014 014 024 008 04821 FFFCA 00000 FFFDA 7 014 014 024 008 04822 FFFCA 00048 FFFFE 7 014 014 024 008 04823 FFFCB FFFCC 00012 8 014 014 004 008 048__________________________________________________________________________ TABLE 8E__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________24 FFFDA FFFD0 00026 8 014 014 004 008 04825 FFFAA FFFD8 0003A 8 014 014 004 008 04826 FFFD4 FFFDC 0003E 8 014 014 004 008 04827 FFFB0 FFFE4 00042 8 014 014 004 008 04828 FFFF2 FFFE8 00056 8 014 014 004 008 04829 FFFDA FFFF0 0005A 8 014 014 004 008 048__________________________________________________________________________ TABLE 8F__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________30 FFFCA FFFF8 0005E 8 014 014 004 008 04831 FFFC2 00000 FFFAE 1 014 014 004 008 04832 FFFC2 00008 FFFB2 1 014 014 004 008 04833 FFFCA 00010 FFFB6 1 014 014 004 008 04834 FFFDA 00018 FFFBA 1 014 014 004 008 04835 FFFF2 00020 FFFBE 1 014 014 004 008 048__________________________________________________________________________

FIGS. 11A, 11B, 11C, 11D, 11E, 11F and 11G show steps to draw a parabola of F=4y-x.sup.2 +2=0 in the region of F.gtoreq.0 according to the method of FIG. 1, by assuming the start point of (-8, 18). Table 9A, 9B, 9C, 9D, 9E, 9F and 9G show F, .alpha., .beta., the octant, T1, T1', T2, T3 and T3' corresponding to FIGS. 11A to 11G, respectively.

TABLE 9A__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________0 0000A 0000B FFFFC 3 FFE 000 000 FFE FFE1 00006 0000B FFFFC 3 FFE 000 000 FFE FFE2 00002 0000B FFFFC 3 FFE 000 000 FFE FFE3 0000D 00009 FFFFC 3 FFE 000 000 FFE FFE4 00009 00009 FFFFC 3 FFE 000 000 FFE FFE5 00005 00009 FFFFC 3 FFE 000 000 FFE FFE__________________________________________________________________________ TABLE 9B__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________6 00001 00009 FFFFC 3 FFE 000 000 FFE FFE7 0000A 00007 FFFFC 3 FFE 000 000 FFE FFE8 00006 00007 FFFFC 3 FFE 000 000 FFE FFE9 00002 00007 FFFFC 3 FFE 000 000 FFE FFE10 00009 00005 FFFFC 3 FFE 000 000 FFE FFE11 00005 00005 FFFFC 3 FFE 000 000 FFE FFE__________________________________________________________________________ TABLE 9C__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________12 00001 00005 FFFFC 3 FFE 000 000 FFE FFE13 00006 00003 FFFFC 3 FFE 000 000 FFE FFE14 00002 00003 FFFFC 3 FFE 000 000 FFE FFE15 00005 00001 FFFFC 3 FFE 000 000 FFE FFE16 00001 00001 FFFFC 3 FFE 000 000 FFE FFE17 00002 FFFFF 00003 2 FFE 000 FFE FFE FFE__________________________________________________________________________ TABLE 9D__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________18 00001 FFFFD 00001 2 FFE 000 FFE FFE FFE19 00002 00003 FFFFF 1 FFE 000 FFE FFE FFE20 00001 00001 FFFFD 1 FFE 000 FFE FFE FFE21 00002 FFFFF 00004 8 FFE 000 000 FFE FFE22 00001 FFFFD 00004 8 FFE 000 000 FFE FFE23 00005 FFFFD 00004 8 FFE 000 000 FFE FFE__________________________________________________________________________ TABLE 9E__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________24 00002 FFFFB 00004 8 FFE 000 000 FFE FFE25 00006 FFFFB 00004 8 FFE 000 000 FFE FFE26 00001 FFFF9 00004 8 FFE 000 000 FFE FFE27 00005 FFFF9 00004 8 FFE 000 000 FFE FFE28 00009 FFFF9 00004 8 FFE 000 000 FFE FFE29 00002 FFFF7 00004 8 FFE 000 000 FFE FFE__________________________________________________________________________ TABLE 9F__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________30 00006 FFFF7 00004 8 FFE 000 000 FFE FFE31 0000A FFFF7 00004 8 FFE 000 000 FFE FFE32 00001 FFFF5 00004 8 FFE 000 000 FFE FFE33 00005 FFFF5 00004 8 FFE 000 000 FFE FFE34 00009 FFFF5 00004 8 FFE 000 000 FFE FFE35 0000D FFFF5 00004 8 FFE 000 000 FFE FFE__________________________________________________________________________ TABLE 9G__________________________________________________________________________NO F .alpha. .beta. Octant T1 T1' T2 T3 T3'__________________________________________________________________________36 00002 FFFF3 00004 8 FFE 000 000 FFE FFE37 00006 FFFF3 00004 8 FFE 000 000 FFE FFE38 0000A FFFF3 00004 8 FFE 000 000 FFE FFE39 0000E FFFF3 00004 8 FFE 000 000 FFE FFE40 00001 FFFF3 00004 8 FFE 000 000 FFE FFE41 00005 FFFF3 00004 8 FFE 000 000 FFE FFE__________________________________________________________________________

FIG. 12 shows a configuration of an apparatus used for implementing the method of FIG. 1. First, the parameters F, .alpha., .beta., T1, T1' and b representing a curve to be drawn as well as the octant are given through a data bus 50 and a multiplexer 52. The parameters F, .alpha., .beta., T1, T1' and b are stored in an F register 60, .alpha. register 54, .beta. register 56, T1 register 62 , T1' register 64 and b register 58, respectively. The octant is provided to an octant section 74. A pair of start coordinates (X.sub.s, Y.sub.s) is set in an X counter 84 and a Y counter 86, respectively.

Then, an adder control circuit 78 receives an instruction to perform operation according to the following equations through the data bus 50 and the multiplexer 52:

T3=T1+T1'+2b T3'=T1+T1'-2b T2=T1(T1').+-.b

According to the instruction, an adder 80 performs the above operations using output from the T1, T1' and b registers 62, 64 and 58, respectively, and supplies the results to T3, T3' and T2 registers 68, 70 and 66, respectively.

Then, a first sign judging section 72 receives outputs from the .alpha. and .beta. registers 54 and 56 and compares the signs of .alpha. and .beta.. The first sign judging section 72 supplies an octant change request signal to the octant section 74 through a line 73 if the signs of .alpha. and .beta. are the same The octant section 74 also receives through a line 75 a signal indicating whether change of .alpha. was performed in the last octant change or not. However, it is unknown whether .alpha. was changed in the last octant change when the octant is first provided. So a signal indicating whether change of .alpha. should be assumed in the last octant change or not is supplied at the same time when an octant is provided from outside.

When the octant section 74 receives a signal indicating that a change of .alpha. was (or would have been) performed in an octant preceding to the given octant, it causes the adder 80 to perform an operation

.beta.=.alpha.-.beta.+b through the adder control circuit 78 if the given octant is the second, third, sixth or seventh octant, and supplies the result to the .beta. register 56. The octant section 74 causes the adder 80 to perform an operation .beta.=.alpha.-.beta.-b through the adder control circuit 78 if the given octant is the first fourth, fifth or eighth octant, and supplies the result of the .beta. register 56.

If the section 74 receives a signal indicating that the change of .alpha. was not performed in an octant preceding to the given octant, it causes the adder 80 to perform an operation

.alpha.=2.beta.-.alpha.+2c through the adder control circuit 78 if the given octant is the first, second, fifth or sixth octant, and supplies the result to the .alpha. register 54. if the given octant is the third, fourth, seventh or eighth octant, it causes the adder 80 to perform an operation .alpha.=2.beta.-.alpha.+2a, and supplies the result to the .alpha. register 54. Also, it causes the adder 80 to perform an operation of T2=T1(T1').+-.b. The octant section 74 generates a code representing the new octant which becomes the current octant after the change.

If the signs of .alpha. and .beta. become different after the octant change, the first sign judging section 72 does not issue the octant change request signal any more. Then, the second sign judging section 76 receives the outputs of the .alpha. register 54 and the F register 60 and checks the signs of F and .alpha.. If they are the same, the section 76 instructs the adder control circuit 78 to perform an operation to generate F+.beta.. According to this, the adder 80 receives the outputs of the F and .beta. registers 60 and 56, performs the operation (F+.beta.), and supplies the result to a step control circuit 82, through the multiplexer 52.

The step control circuit 82 is also supplied with the output of the F register 60, and a signal representing the current octant from the octant section 74. The step control circuit 82 generates output as listed in Table 10 below.

TABLE 10______________________________________ Signs forOctant F and F + .beta. X up X down Y up Y down______________________________________First Same on off off off Different on off on offSecond Same on off off off Different on off off onThird Same off off off on Different on off off onFourth Same off off off on Different off on off onFifth Same off on off off Different off on off onSixth Same off on off off Different off on on offSeventh Same off off on off Different off on on offEighth Same off off on off Different on off on off______________________________________

If the second sign judging circuit 76 detects that the signs of F and .alpha. are different, it instructs the adder circuit 78 to perform an operation to generate F+.alpha.. The adder 80 receives the outputs of the F and .alpha. registers 60 and 54, performs the operation (F+.alpha.), and supplies the result to the step control circuit 82. In this case, the step control circuit 82 generates as listed in Table 11.

TABLE 11______________________________________ Signs forOctant F and F + .alpha. X up X down Y up Y down______________________________________First Same on off on off Different on off off offSecond Same on off off on Different on off off offThird Same on off off on Different off off off onFourth Same off on off on Different off off off onFifth Same off on off off Different off on off offSixth Same off on on off Different off on off offSeventh Same off on on off Different off off on offEighth Same on off on off Different off off on off______________________________________

The X and Y counters 84 and 86, respectively, increase or decrease the values of X and Y by one according to output supplied from the step control circuit 82. The output of the step control circuit 82 is also supplied to the adder control circuit 78. When the step control circuit 82 outputs a signal to increment only one of either X or Y by .+-.1, the adder control circuit 78 causes the adder 80 to perform the following operations to update the values of F, .alpha. and .beta..

F=F+.beta. .alpha.=.alpha.+T2 .beta.=.beta.+T1 (T1')

When the step control circuit 82 outputs signals to increment both X and Y by .+-.1, the adder control circuit 78 causes the adder 80 to perform the following operations to update the values of F, .alpha. and .beta..

F=F+.alpha. .alpha.=.alpha.+T3(T3') .beta.=.beta.+T2

Thereafter, the next point will be obtained using the new parameters. When the values of the X and Y counters 84 and 86 reach the end point coordinates set in X and Y end point registers 88 and 90, respectively, drawing of the curve is terminated by signals from a stop check circuit 92.

Since the above embodiment changes the octant by noticing the signs of .alpha. and .beta., the change of octant can be continuously performed until the signs of .alpha. and .beta. become different, and, therefore, a sharp curve in which a plurality of octant changes are continuously occurring can easily be drawn.

In addition, double lines that never cross with each other can easily be drawn by first drawing a line approximate to F (x, y)=0 in a region of F.gtoreq.0, and then drawing a line approximate to F=0 in the region of F&lt;0.

As seen from the foregoing description, the invention reduces the number of parameters, simplifies the operation, and makes realization in hardware easy by selecting a new point close to F (x, y)=0 in only one of either region of F (x, y).gtoreq.0 or F (x, y)&lt;0 for generating signals representing F (x, y)=0.

Claims

1. A method for generating signals representing a line approximate to a quadratic curve

F(x, y)=ax.sup.2 +bxy+cy.sup.2 +dx+ey+f=0

by repeating a step selecting a new point close to F(x, y)=0 from among eight points (x+1, y+1), (x+1, y), (x+1, y-1), (x, y-1), (x-1, y-1), (x-1, y), (x-1, y+1) and (x, y+1) adjacent to a current point (x, y) in a Cartesian coordinates system, characterized in that said step selecting one of said eight points consists of a step selecting a new point close to F (x, y)=0 in only one of either the region of F (x, y).gtoreq.0 or the region F (x, y)&lt;0, said step selecting a new point close to F (x, y)=0 comprising:

an octant selecting step selecting one octant from among the first octant in which point (x+1, y+1) or (x+1, y) can be selected, the second octant in which point (x+1, y) or (x+1, y-1) can be selected, the third octant in which point (x+1, y-1) or (x, y-1) can be selected, the fourth octant in which point (x, y-1) or (x-1, y-1) can be selected, the fifth octant in which point (x-1, y-1) or (x-1, y) can be selected, the sixth octant in which point (x-1, y) or (x-1, y+1) can be selected, the seventh octant in which point (x-1, y+1) or (x, y+1) can be selected, the eighth octant in which point (x, y+1) or (x+1, y+1) can be selected, and

selecting a point close to F(x, y)=0 in either one region of F (x, y).gtoreq.0 or F (x, y)&lt;0 from two selectable points in the octant selected by said octant selecting step.

2. A method for generating quadratic curve signals as claimed in claim 1, wherein said octant selecting step selects an octant having .alpha. and .beta. values with different signs, when assuming that .alpha. and .beta. are:

in the first octant,

.alpha.=F(x+1, y+1)-F(x, y)

.beta.=F(x+1, y)-F(x, y)

in the second octant,

.alpha.=F(x+1, y-1)-F(x, y)

.beta.=F(x+1, y)-F(x, y)

in the third octant,

.alpha.=F(x+1, y-1)-F(x, y)

.beta.=F(x, y-1)-F(x, y)

in the fourth octant,

.alpha.=F(x-1, y-1)-F(x, y)

.beta.=F(x, y-1)-F(x, y)

in the fifth octant,

.alpha.=F(x-1, y-1)-F(x, y)

.beta.=F(x-1, y)-F(x, y)

in the sixth octant,

.alpha.=F(x-1, y+1)-F(x, y)

.beta.=F(x-1, y)-F(x, y)

in the seventh octant,

.alpha.=F(x-1, y+1)-F(x, y)

.beta.=F(x, y+1)-F(x, y), and

in the eighth octant,

.alpha.=F(x+1, y+1)-F(x, y)

.beta.=F(x, y+1)-F(x, y).

3. A method for generating quadratic curve signals as claim in claim 2, wherein said point selecting step includes the steps of:

(a) comparing the sign of F (x, y) with that of .alpha. at the point (x, y),

(b) comparing the sign of F (x, y) with that of F (x, y)+.beta. when the signs of F (x, y) and .alpha. are the same in the comparison of step (a),

(c) comparing the sign of F (x, y) with that of F (x, y)+.alpha. when the signs of F (x, y) and .alpha. are different in the comparison of step (a),

(d) selecting a point that displaces by (+1) or (-1) in the X direction and by (+1) or (-1) in the Y direction from the point (x, y) when the signs are judged to be the same in the step (b), or when the signs are judged to be different in the step (c), and

(e) selecting a point that displaces by (+1) or (-1) in the X direction and by (+1) or (-1) in the Y direction from the point (x, y) when the signs are judged to be different in the step (b), or when the signs are judged to be the same in the step (c).

4. A method for generating quadratic curve signals as claimed in claim 2, wherein, when F (x, y).gtoreq.0, said point selecting step includes the steps of:

(f) checking the sign of .alpha. or .beta.,

(G) checking the sign of F (x, y)+.beta. when it is judged that the sign of .alpha. is positive, or that the sign of .beta. is negative in the step (f),

(h) checking the sign of F (x, y)+.alpha. when the sign of .alpha. is judged to be negative, or the sign of .beta. is judged to be positive in the step (f),

(i) selecting a point that displaces by (+1) or (-1) in the X direction and by (+1) or (-1) in the Y direction from the point (x, y), when the sign of F (x, y)+.beta. is judged to be positive in the step (g), or when the sign of F (x, y)+.alpha. is judged to be negative in the step (h), and

(j) selecting a point that displaces by (+1) or (-1) in X direction and by (+1) or (-1) in Y direction from the point (x, y), when the sign of F (x, y)+.beta. is judged to be negative in the step (h).

5. A method for generating quadratic curve signals as claimed in claim 2, wherein, when F (x, y)&lt;0, said point selecting step includes the steps of:

(k) checking the sign of .alpha. or .beta.,

(l) checking the sign of F (x, y)+.alpha. when it is judged that the sign of .alpha. is positive, or that the sign of .beta. is negative in the step (k),

(m) checking the sign of F (x, y)+.beta. when the sign of .alpha. is judged to be negative, or the sign of .beta. is judged to be positive in the step (k),

(n) selecting a point that displaces by (+1) or (-1) in the X direction and by (+1) or (-1) in the Y direction from the point (x, y), when the sign of F (x, y)+.alpha. is judged to be positive in the step (l), or when the sign of F (x, y)+.beta. is judged to be negative in the step (m), and

(o) selecting a point that displaces by (+1) or (-1) in the X direction and by (+1) or (-1) in the Y direction from the point (x, y), when the sign of F (x, y)+.alpha. is judged to be negative in the step (l), or when the sign of F (x, y)+.beta. is judged to be positive in the step (m).

6. A method for generating quadratic curve signals as claimed in claim 3, 4 or 5, wherein said point selecting step further comprises the steps of:

(p) updating the values of F (x, y), .alpha. and .beta. after selecting a point which displaces by (+1) or (-1) in the X direction and by (+1) or (-1) in the Y direction from the point (x, y), according to the following equations:

F(x,y)=F(x, y)+.beta.

.alpha.=.alpha.+T2

.beta.=.beta.+T1

wherein, T1 is:

in the first and second octant, 2a (=.beta.(x+1, y)-.beta.(x, y)),

in the third and fourth octant, 2c(=.beta.(x, y-1)-.beta.(x, y)),

in the fifth and sixth octant, 2a(=.beta.(x-1, y)-.beta.(x, y)),

in the seventh and eighth octant, 2c(=.beta.(x, y+1)-.beta.(x, y)), and

T2 is:

in the first octant, 2a+b(=.alpha.(x+1, y)-.alpha.(x, y))

in the second octant, 2a-b(=.alpha.(x'1, y)-.alpha.(x, y))

in the third octant, 2c-b(=.alpha.(x, y-1)-.alpha.(x, y))

in the fourth octant, 2c+b(=.alpha.(x, y-1)-.alpha.(x, y))

in the fifth octant, 2a+b(=.alpha.(x-1, y)-.alpha.(x, y)),

in the sixth octant, 2a-b(=.alpha.(x-1, y)-.alpha.(x, y)),

in the seventh octant, 2c-b(=.alpha.(x, y+1)-.alpha.(x, y)), and

in the eighth octant, 2c+b(=.alpha.(x, y+1)-.alpha.(x, y)), and

(q) updating the values of F (x, y), .alpha. and .beta. after selecting a point that displaces by (+1) or (-1) in the X direction and by (+1) or (-1) in the Y direction from the point (x, y), according to the following equations:

F(x,y)=F(x, y)+.alpha.

.alpha.=.alpha.+T3

.beta.=.beta.+T2

wherein, T2 is:

in the first octant, 2a+b(=.beta.(x+1, y+1)-.beta.(x, y)),

in the second octant, 2a-b(=.beta.(x+1, y-1)-.beta.(x, y)),

in the third octant, 2c+b(=.beta.(x+1, y-1)-.beta.(x, y)),

in the fourth octant, 2c+b(=.beta.(x-1, y-1)-.beta.(x, y)),

in the fifth octant, 2a+b(=.beta.(x-1, y+1)-.beta.(x, y)),

in the sixth octant, 2a-b(=.beta.(x-1, y+1)-.beta.(x, y)),

in the seventh octant, 2c-b(=.beta.(x-1, y+1)-.beta.(x, y)), and

in the eighth octant, 2c+b(=.beta.(x+1, y+1)-.beta.(x, y)); and

T3 is:

in the first octant, 2a+2c+2b(=.alpha.(x+1, y+1)-.alpha.(x, y))

in the second octant and third octant, 2a+2c-2b(=.alpha.(x+1, y-1)-.alpha.(x, y)),

in the fourth and fifth octant, 2a+2c+2b(=.alpha.(x-1, y-1)-.alpha.(x, y))

in the sixth and seventh octant, 2a+2c-2b(=.alpha.(x-1, y+1)-.alpha.(x, y)), and

in the eighth octant, 2a+2c+2b(=.alpha.(x+1, y+1)-.alpha.(x, y)).

7. A method for generating quadratic curve signals as claimed in claim 6, wherein said method further comprises the steps of:

(r) checking the signs of .alpha. and .beta. updated in said step (p) or (q),

(s) changing the octant to an octant in which the signs of .alpha. and .beta. are different when the signs of .alpha. and .beta. are judged to be the same in said step (r).