Count Integral points inside a Triangle - GeeksforGeeks
Given three non-collinear integral points in XY plane, find the number of integral points inside the triangle formed by the three points. (A point in XY plane is said to be integral/lattice point if both its co-ordinates are integral).
Example:
Input: p = (0, 0), q = (0, 5) and r = (5,0) Output: 6 Explanation: The points (1,1) (1,2) (1,3) (2,1) (2,2) and (3,1) are the integral points inside the triangle. 
We can use the Pick's theorem, which states that the following equation holds true for a simple Polygon.
Pick's Theeorem: A = I + (B/2) -1 A ==> Area of Polygon B ==> Number of integral points on edges of polygon I ==> Number of integral points inside the polygon Using the above formula, we can deduce, I = (2A - B + 2) / 2
We can find A (area of triangle) using below Shoelace formula.
A = 1/2 * abs(x1(y2 - y3) + x2(y3 - y1) + x3(y1 - y2))
How to find B (number of integral points on edges of a triangle)?
We can find the number of integral points between any two vertex (V1, V2) of the triangle using the following algorithm.
1. If the edge formed by joining V1 and V2 is parallel to the X-axis, then the number of integral points between the vertices is : abs(V1.y - V2.y)-1 2. Similarly if edge is parallel to the Y-axis, then the number of integral points in between is : abs(V1.x - V2.y)-1 3. Else, we can find the integral points between the vertices using below formula: GCD(abs(V1.x-V2.x), abs(V1.y-V2.y)) - 1 The above formula is a well known fact and can be verified using simple geometry. (Hint: Shift the edge such that one of the vertex lies at the Origin.)
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