Hi,
Try this:
procedure DrawCurve;
var
p1, p2, p3 : TPoint;
mp1, mp2 : TPoint; // chord midpoints
c : TPoint; // centre of circle
rad : double; // radius of circle
r1, r2 : TPoint; // bounding rectangle (square)
m1, m2 : double; // slope of 2 chords
x : double; // temp
angrot, angp1p2, angp2p3 : double;
begin
SetPenAndBrush;
DrawingCanvas.Pen.Style := psDash; // only works if pen width = 1
p1 := MakePoint(Data.tbEntity.FieldByName('Pt1x').AsFloat*Option.Factor,
Data.tbEntity.FieldByName('Pt1y').AsFloat*Option.Factor);
p2 := MakePoint(Data.tbEntity.FieldByName('Pt2x').AsFloat*Option.Factor,
Data.tbEntity.FieldByName('Pt2y').AsFloat*Option.Factor);
p3 := MakePoint(Data.tbEntity.FieldByName('Pt3x').AsFloat*Option.Factor,
Data.tbEntity.FieldByName('Pt3y').AsFloat*Option.Factor);
if ((p1.x = 0) and (p1.y = 0)) and ((p2.x = 0) and (p2.y = 0)) and
((p3.x = 0) and (p3.y = 0)) then begin
exit;
end;
// sanity check - 3 different points
if ((p1.x = p2.x) and (p1.y = p2.y)) or ((p1.x = p3.x) and (p1.y =
p3.y)) or ((p3.x = p2.x) and (p3.y = p2.y)) then begin
ErrorMsg('All 3 points on a curve must be different.');
Data.tbEntity.delete;
exit;
end;
// determine direction of curve rotation
try
angp1p2 := math.arctan2((p2.y-p1.y), (p2.X-p1.X));
angp2p3 := math.arctan2((p3.y-p2.y), (p3.X-p2.X));
angrot := angp2p3 - angp1p2;
except
angrot := 0; // straight line
end;
// normalise the angle to between -pi..pi
if angrot > pi then
angrot := angrot - 2*pi
else if angrot < (-pi) then
angrot := angrot + 2*pi;
if abs(angrot) < 0.05 then begin
// Cannot draw a curve through points in a straight line.
// draw a straight line instead
DrawingCanvas.MoveTo(p1.x, p1.y);
DrawingCanvas.LineTo(p3.x, p3.y);
exit;
end;
if angrot > 0 then begin
// clockwise, so reverse start and end points
mp1.x := p1.x;
mp1.y := p1.y;
p1.x := p3.x;
p1.y := p3.y;
p3.x := mp1.x;
p3.y := mp1.y;
end;
// chord midpoint between p1 and p2
mp1.X := (p1.X + p2.X) div 2;
mp1.Y := (p1.Y + p2.Y) div 2;
// chord midpoint between p2 and p3
mp2.X := (p3.X + p2.X) div 2;
mp2.Y := (p3.Y + p2.Y) div 2;
// slope of lines 1 and 2
try
m1 := 1.0*(p2.y-p1.y)/(p2.x-p1.x);
except
m1 := maxint; // for now (vertical line)
end;
try
m2 := 1.0*(p3.y-p2.y)/(p3.x-p2.x);
except
m2 := maxint; // for now (vertical line)
end;
// don't let these slopes quite get to zero
if (m1 < 0.01) and (m1 > 0) then m1 := 0.01;
if (m1 > -0.01) and (m1 < 0) then m1 := -0.01;
if (m2 < 0.01) and (m2 > 0) then m2 := 0.01;
if (m2 > -0.01) and (m2 < 0) then m2 := -0.01;
// slopes of perpendicular lines, pointing to centre of circle
try
m1 := -1/m1;
except
m1 := maxint*iif(p3.Y<p2.Y,-1,1);
end;
try
m2 := -1/m2;
except
m2 := maxint*iif(p1.Y<p2.Y,-1,1);
end;
// x of circle centre
try
x := (mp2.y-mp1.y-(m2*mp2.x)+(m1*mp1.x))/(m1-m2);
except
// divide by zero, lines are parallel, should never happen but just in
case:
// draw a straight line instead
DrawingCanvas.MoveTo(p1.x, p1.y);
DrawingCanvas.LineTo(p3.x, p3.y);
exit;
end;
// y of circle centre
c.y := round(mp1.y+m1*(x-mp1.x));
c.X := round(x);
// radius
rad := sqrt(sqr(p2.X-c.x)+sqr(p2.Y-c.Y));
// bounding square and circle
r1.x := c.X - round(rad);
r2.x := c.X + round(rad);
r1.y := c.y - round(rad);
r2.y := c.y + round(rad);
DrawingCanvas.Arc(r1.X, r1.y, r2.x, r2.y, p1.x,p1.y,p3.x,p3.y);
end;
HTH,
Steve
_____
From: Marshland Engineering [mailto:marshl...@marshland.co.nz]
Sent: Tuesday, 18 January 2011 9:03 p.m.
To: delphi@delphi.org.nz
Subject: [DUG] Arc's
Is there an easy way of drawing an arc ? I have center, start and end
co-ordinates as XY.
>From what I can see, I need to look at least 16 combinations to work out the
perimeter size depending in which quadrant the start and ends are in.
procedure Arc(X1, Y1, X2, Y2, X3, Y3, X4, Y4: Integer);
Use Arc to draw an elliptically curved line with the current Pen. The arc
traverses the perimeter of an ellipse that is bounded by the points (X1,Y1)
and (X2,Y2). The arc is drawn following the perimeter of the ellipse,
counterclockwise, from the starting point to the ending point. The starting
point is defined by the intersection of the ellipse and a line defined by
the center of the ellipse and (X3,Y3). The ending point is defined by the
intersection of the ellipse and a line defined by the center of the ellipse
and (X4, Y4).
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