There are many applications that require to know if a given point is inside or outside the polygon. For an example, if we need to select a polygon by clicking on it then we will need a way to know whether the user click inside or outside the polygon. Ray casting algorithm is one popular way to do it. In this trick I included an implementation of raycasting algorithm for a polygon selection in a canvas.
In this section I briefly explain how the ray casting algorithm can be used for check whether a point is inside or outside the polygon. When a point is given then we virtually draw a line from a point far away outside from the polygon to the given point. then we calculate the number of intersection of the virtual line with the edges of the polygon. if the number of intersections is odd then according to the ray casting theory we can conclude that the point is inside the polygon. Otherwise the point is outside the polygon. You can find more about ray casting algorithm from here.
Using the code
In this code there are two main functionalities. One is drawing of a polygon inside a usercontrol and second is select of deselect the polygon. I am not going to explain the drawing part of the polygon since the every line in the code is explained with a comment.
The following code is used for check the intersections
if (((polyK.Y > currentPoint.Y) != (polyJ.Y > currentPoint.Y)) &&
(currentPoint.X < (polyJ.X - polyK.X) * (currentPoint.Y - polyK.Y) / (polyJ.Y - polyK.Y) + polyK.X))
polyK and polyJ are adjacent points from the polygon.
According to the algorithm all what we need to know is whether the number of intersections is odd or even hence we use a flag that switches between odd and even as in the following code line.
oddNodes = !oddNodes;
After testing the each and every edge of the polygon for the intersection we can check whether the number of intersection is odd or even. If it is odd then the point should be inside the polygon so we flag it as selected as in the following code.
isSelected = true;
isSelected = false;