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# Image map Polygon Algorithm

Posted on 1999-01-22
Medium Priority
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I'm trying to write the function which tells me whether
an (x,y) point is in a certain polygon?

i already written ones for rectangle & circle but stuck on
the Poligon - the function looks like that:
pointInPolygon(File i_mapFile , double x, double y)
{}
i dont need you to write a code just a way or an algorithm
to find that (x,y) point is in or not the polygon.

a polygon is at least 3 points of lines that are connected togther (0,0)(100,50)(200,30) the first point should be the last and if its not so im adding another connecting point
but thats for another session!

Thanks,
0
Question by:cs4ez
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LVL 7

Expert Comment

ID: 1866798
a simple way is to floodfill the region that contains the point you want with a specific color, and then check the color of a point your are sure it is inside the polygon , if it has the same color you floodfilled , then the point is in the polygon , and vice versa

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Expert Comment

ID: 1866799
or count the number of times a segment of the polygon crosses a ray from the point to infinity
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Expert Comment

ID: 1866800
yes ozo , i thought of that , but i found it so difficult , especialy if the polygon is drawn by giving the coordinations of the heads , and also the problems of a point that lies on the line , and also and most important the problems of rounding , which will have a great effect considering that we are using pixels

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Author Comment

ID: 1866801
coloring the polygon is a bigger problem!
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Expert Comment

ID: 1866802
ok , try ozo solution , it is difficult somehow , but it may help you

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Expert Comment

ID: 1866803
I think the easiest way to deal with boundary ambiguities is to treat the lines as lieing between pixels, rather than on them.
Rounding problems can usually be dealt with by doing everything with integers.
A flood fill would have all the same problems, but would have to deal with them for each pixel in the region.
Still, it may be advantagous to do that once, then use the resulting color map to test points many times.
If you have any difficulty, just ask for furthur help.
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Author Comment

ID: 1866804
in java.awt.Polygon

All answers from now on will be rejected!!!

Thanks ozo & Inteqam for all your efforts

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Accepted Solution

web_crusher earned 400 total points
ID: 1866805
look at java.awt.Polygon
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Expert Comment

ID: 1866806
do not forget who told you about it - cs4ez
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Expert Comment

ID: 1866807
the algorithm used by the contains method in java.awt.Polygon class is the counting one
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Expert Comment

ID: 1866808
i don't have the java.awt.polygon

can you pass it?

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