I need help coming up with an algorithm to determine if the user clicks on any point of a line. A line is given by two points.

So you have x1, y1 positions of the first point, and x2,y2 positions of the second point.

I need to determine in the mousePressed event, if the x and y positions of the mouse (evt.getX() and evt.getY()) fall on a point on the line between the two original points.

Any help on this would be greatly appreciated. Thanks

In the case that line is horizontal x1 == x2, if line is vertical y1 ==y2.
1) So what you do collect pointer position(a,b)
2) Find out if your line is horizontal or vertical
A) If horizontal a == x1 && ( b >= y1 && b =< y2)
B) if vertical (a >= x1 && a =< x2) && b == y1
That is just one of many approches, however if line is under certain angle this will not work and you will need serious arithmetic to work it out

yes, the line will not always (in fact, usually not) be exactly horizontal or vertical.

The program i am working on is similar to mapquest. I have a Map Editor which loads an image of a (somewhat small area) map. then i create points for locations. Then i connect the points to create a path. I have all of this working, and all the locations are stored in a Vector and all the paths are stored in another Vector. I need to have the option to 1) delete a path, and 2) choose whether it is directed (one-way) or not. In order to do both of these, I need to be able to click on any point of the line to select it. Quite complicated

I figured this out. and yes, it involves comparing a lot of numbers and taking square roots, etc. it works perfectly though.

Good job, would you mind to share with us your solution?

Yep- here it is.

The
((MapEdge)edge.elementAt(index)).get------
lines are just getting the X and Y coordinates of the two endpoints of the lines

``````public boolean pathSelected(int index, int x, int y)
{
int _x0 = ((MapEdge)edge.elementAt(index)).getFrom().x;
int _y0 = ((MapEdge)edge.elementAt(index)).getFrom().y;
int _x1 = ((MapEdge)edge.elementAt(index)).getTo().x;
int _y1 = ((MapEdge)edge.elementAt(index)).getTo().y;
int _dx = ((MapEdge)edge.elementAt(index)).getTo().x
- ((MapEdge)edge.elementAt(index)).getFrom().x;
int _dy = ((MapEdge)edge.elementAt(index)).getTo().y
- ((MapEdge)edge.elementAt(index)).getFrom().y;
double dx0 = _dx;
double dy0 = _dy;
double dx = x - _x0;
double dy = y - _y0;
double d = dy0 * dx - dx0 * dy;
double r = Math.sqrt(dx0*dx0 + dy0*dy0);
if(Math.abs(r) < 0.1)
{
r = 1;
}
d = Math.abs(d/r);
if(d < 3 &&
x >= getMinX(_x0, _dx) && x < getMaxX(_x0, _dx) &&
y >= getMinY(_y0, _dy) && y < getMaxY(_y0, _dy))
{
return true;
}
else
{
return false;
}
}

public int getMinX(int _x0, int _dx)
{
int x1 = _x0 + _dx;
if(_x0 < x1)
return _x0;
else
return x1;
}
public int getMaxX(int _x0, int _dx)
{
int x1 = _x0 + _dx;
if(_x0 > x1)
return _x0;
else
return x1;
}
public int getMinY(int _y0, int _dy)
{
int y1 = _y0 + _dy;
if(_y0 < y1)
return _y0;
else
return y1;
}
public int getMaxY(int _y0, int _dy)
{
int y1 = _y0 + _dy;
if(_y0 > y1)
return _y0;
else
return y1;
}``````
commented: Thank you for sharing the knowledge you learned +6
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