How do I find intersection of "GeneralPath" objects

[Ravi Kotha]

Hi,

Is there any method in java.awt to find the intersection of two "GeneralPath" objects? I couldnt find any method in the class API. Can somebody tell me how to find the intersection region of two path objects?

Iam thinking of a way like, getting the path iterator for one "GeneralPath" object and checking each point in this, whether it lies in the another "GeneralPath" object using "contains" method. Is there any other best way to do this...?

Appreciate any good response..... Thanks,

[Campbell Ritchie]

Don't know; I looked up GeneralPath and found it described as a legacy class; the recommend the java.awt.geom.Path2D subclasses (Float and Double). There are intersects methods, but they seem to be similar to contains. I think you are right that you will have to count the points in pathA which are inside pathB and the points in pathB which are inside pathA, put them in a List and produce a new Path.

Beware: Be sure that the two original Paths wind in the same direction, and also that they represent continuous lines without gaps. There appears to be a new getPathIterator method which will probably help you.

[Ravi Kotha]

Thank you very much for immediate reply....

I tried to follow the winding rule, but wasn't clear. What is the main purpose of this winding rule? Also you mentioned like "to check whether all the paths are continuous straight lines without gaps", is there any mechanism to check the path is continuous? My assumption is like, I will be cautious when creating the path without any gaps.

Also, I can not make use of java.awt.geom.Path class, as I am using 1.4.2 API.

Thank you very much frnd,

[Campbell Ritchie]

I have checked and GeneralPath is considered up-to-date programming in J1.4.2. The "winding rules" are means the GeneralPath object uses to work out whether a particular point is inside or outside the area of the object. I think non zero is easier to use.
You have to check that the Path is closed, and that it doesn't have any "moveTo" operations, otherwise you have a gap in the line drawn.

Try getting a PathIterator for pathA, if its Points are inside pathB if(pathB.contains(p)). . . put "p" in a List, then repeat for the other way round.

Possible easier way I have just found. Look up the Area class in J2SE1.4.2. You can use a Shape in its constructor,

Use pathA and pathB in the constructors to set up areaA and areaB.

Set up a third Area (temp) same shape as areaA. Don't say "temp = areaA"

subtract areaA from the temp Area,

and subtract what remains of the temp Area from areaB.

Now re-create the GeneralPath from what remains of areaB.

[ March 27, 2007: Message edited by: Campbell Ritchie ]

[Ravi Kotha]

Thank you very much Ritchie.. hope the Area class will do for me. Thanks once again...

[Campbell Ritchie]

. . . Only too pleased to help. But I think I have said areaA when I meant areaB and vice versa.

Sorry

[Ravi Kotha]

yup, I got the point.

Here is a doubt in case of GeneralPath.

GeneralPath gPath = new GeneralPath();
for(int i=0; i<size; i++)
{
 
// add points to the gPath
gPath.add(i);
}

if we say

gPath.getPathIterator(),

will it gives the iterator of boundary region points or for all the points...?

[Campbell Ritchie]

Don't know. Difficult to tell from the PathIterator API page. I would have thought all the points, but I think you will have to try it and see. Put a while loop in:-

PathIterator pit = gPath.getPathIterator();
int i = 0;
while(!pit.isDone())
{
   Point p = pit.next();
   System.out.println("Next point: x=" + p.getX() + ", y = " + p.getY() + ". This point is in other path: " + gPath2.contains(p));
   i++;
}
System.out.println("Number of points counted = " + i);

That will allow you to test the point counting.

[Campbell Ritchie]

Have made a mistake.
You can't get the Point of an iterator, only an int which represents the locations of a segment.
So you can't get Points out of the PathIterator.

The Area method does work; I have tried it.

[edit]Changed float[] which is the parameter type to int which is the return type of the PathIterator method.[/edit]
[ April 02, 2007: Message edited by: Campbell Ritchie ]

[Campbell Ritchie]

Actually you might be able to get the coordinates out of the PathIterator after all. The float[] array passed will contain the values of the points. Read the )]PathIterator page in the API.

It is a rather odd way to pass a parameter, rather like the C method of passing a pointer as an argument; you get the result in the array passed, and the number of points can be guessed from the return type.

[Moshe Elisha]

For people who are looking for an answer
http://forums.sun.com/thread.jspa?threadID=5222850&tstart=1108

[Rob Spoor]

Please Don't Wake The Zombies. The last post was over 18 months old.

[Martijn Verburg]

Hi Moshe and welcome to Javaranch!

I appreciate the enthusiasm to help, but we tryo not to WakeTheZombies around here