[JAVA2D] Finding intersection points

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Anonymous

I have two queries. The first is, I want to be able to draw lines
connecting circles, but I can't seem to find the point of intersection
between the lines and circles. I want to have the lines begin at the
point of intersection of the circle, that is, not have any trace of the
line inside the circle. I have tried using intersecting areas, but I
don't think this idea works with lines and shapes. I also tried using
composite rules (DstOver, SrcOut) but the lines don't appear when drawn.
Right now the lines are being drawn from the center of the circles,
which poses my next query. This one is a bit complicated.

I have tried and tried to see how to do this, but to no avail. As I
said, I currently have lines drawn from one center of a circle to
another. The desired outcome is to be able to have two lines drawn from
one circle to another, with a given offset from the line connecting the
centers. Well I have tried to use geometric equations to find out how to
calculate the points of the other lines, but the constraint of having it
worked for any sloped line is giving me troubles.

Walter Shirey

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Walter H Shirey

Yes, thank you for this information. Let me ask you this: how do you
determine the intersection of a line starting inside a circle and that
circle without knowing the distance from the end of the line to the
point on the circle? What I want to do is to be able to have two lines
connecting the circle, offset a given distance from this center line,
without regard to the angle.

Walter Shirey

-----Original Message-----
From: Bill Casselman [mailto:cass@math.ubc.ca]
Sent: Sat 1/24/2004 10:38 PM
To: Walter H Shirey
Cc: david@merganser.math.gvsu.edu; cass@math.ubc.ca
Subject: Re: [JAVA2D] Finding intersection points

One of the most useful simple calculations in
all of graphics is a point interpolation function.
It has two points P = (xP, yP) and Q = (xQ, yQ)
and a real number generally between 0 and 1 as arguments,
and returns the weighted average

(1-t)P + tQ = ( (1-t)*xP + t*xQ, (1-t)*yP + t*yQ )

which lies t of the way from P to Q.
Thus if t = 1/2 this average lies halfway between the two
points.
In your case you apply it twice, both with P = first center,
Q = second center; t = R/D for the starting point of your line;
t = (D-R)/D for its end. Where D = distance from P to
Q, or D = sqrt((xQ-xP)*(xQ-xP) + (yQ-yP)*(yQ-yP)).

Helps?

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