This is a analytic algorithm that judge a point whether in a triangle. If true, get the outline of the triangle. The algorithm face to simple picture. The following content will demonstrate the algorithm. About this algorithm implementation, please refer to the src directory.
At the beginning, we've got a picture of a after binarization. This image is expressed by 2D array. Marked points value is 1. We want to test point is called the origin.
Traverse the pictures and find all the marked points. Push these points into array markPos.
In order to find all possible first edge after origin in the triangle. Find all the attachment that origin with all the elements in markpos. Maybe you will ask me, if we want to find first edge after origin, we only need to check the pointer that origin neighbors. The problem of the idea is you don't know which point is neighbor with origin. We discuss in two different conditions.
- Circumstance 1: The line we are looking for just the slope of 45°and 135. Now, the neighbors of
originalso is its neighbors on the pixel space. The number of such points only 8. - Others Circumstance: As we know, any two points can be together as a straight line. Consider the situation: 
And push these lines into array SLEP.
This operation will be repeated in the three stages of algorithm. Each process check point will be much more than the last one process. So there are two optimization method, to eliminate some point that needn’t to check.