CODEWITHSUNDEEP
java
tonybs23
tonybs23

Reputation: 23

How do i find if a set of coordinates are inside a 2D triangle?

Does anybody know how to find out if a set of coordinates are within a triangle for which you have the coordinates for. i know how to work out length of sides, area and perimeter, but i have no idea where to begin working out the whereabouts within the triangle of other points. Any advice would be appreciated

Upvotes: 2

Views: 161

Answers (3)

ROT13
ROT13

Reputation: 375

For a solution without using the Polygon-class:

Assume that you have giving three points A,B,C the vertices of your polygon. Let P be the point you want to check. First calculate the vectors representing the edges of your triangle. Let us call them AB, BC, CA. Also calculate the three vectors PA, PB, PC.

Now calculate the cross product between the first two of the vectors from above.

The cross product of the first pair gives you the sin(alpha), where alpha is the angle between AB and PA, multiplied with a vector pendenpicular to AB and PA. Ignore this vector because we are interested in the angle and take a look at the sine (in the case of 2D vectors you can imagine it as the vector standing perpendicular to your screen).

The sine can take values between (let's say for the ease) betwenn 0 and 2*Pi. It's 0 exactly at 0 and Pi. For every value in between the sine is positive and for every value between Pi and 2*Pi it's negative.

So let's say your Point p is on the left hand side of AB, so the sine would be positive.

By taking the cross product of each pair from above, you could easily guess that the point P is on the left hand side of each edge from the triangle. This just means that it has to be inside the triangle.

Of course this method can even be used from calculating whether a point P is in a polygon. Be aware of the fact, that this method only works if the sides of the polygon are directed.

Upvotes: 0

Hugo Sousa
Hugo Sousa

Reputation: 1926

You can create a Polygon object.

Polygon triangle = new Polygon();

Add the vertexes of your triangle with the addPoint(int x, int y) method.

And then, you just need to check if the set of coordinates is inside your triangle using contains(double x, double y) method.

Upvotes: 8

Guy Bouallet
Guy Bouallet

Reputation: 2125

Use the contains method of the Polygon class as documented here.

Upvotes: 0

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