Analytical Geometry

How To Check If A Point Is Inside A Rectangle?

The Area Method

  1. Form a triangle between the point and every pair of consecutive verticies on the rectangle.
  2. Calculate the area of this triangle
  3. Sum up the areas of all of these triangles.
  4. If the sum of these areas is greater than the area of the rectangle, then the point is outside the rectangle, else the sum of the areas must equal the area of the rectangle and the point is either inside or on the edge of the rectangle.
  5. To differentiate between a point that is on the edge of the rectangle vs. one that is inside it, look at all the triangle areas. If any are 0, then the point is on the edge of the rectangle, otherwise it is inside the rectangle.

This method works not just for a rectangle but for any polygon!

The Angle Method

  1. Travel around the verticies of the rectangle in a clockwise fashion.
  2. For each line segment, calculate the angle between this line and a line formed from the point and the first verticy which makes up the line.
  3. If all the angles are positive (incl. 0), then the point is inside the rectangle. If all the angles are positive and one or more is 0, then the point is on the edge of the rectangle (if one is 0, then it is on the edge, if two are 0, then the point is on one of the rectangle’s corners).
  4. If one or more angles is negative, then the point is outside the rectangle.

The Product Of Vectors Method

This uses vector notation:

$$ (0 < AM \cdot AB < AB \cdot AB) \wedge (0 < AM \cdot AD < AD \cdot AD) $$

\(A\), \(B\) and \(B\) are 3 adjacent vertices of the rectangle
\(M\) is the vector representation of the point that could be inside the rectangle
\(<\) is the logical less-than operator
\(\wedge\) is the logical AND operator

Notice that this method does not use point C at all! It projects the point M onto the line AB and AD and makes sure that these projections lie somewhere on the line.

How To Find The Distance Between A Line And A Point In 3D

Given two points \(x_1\) and \(x_2\) which form a line in 3D space, and a third point \(x_0\) in 3D space, the shortest distance \( d \) between the point \(x_0\) and the line is given by:

$$ d = \frac{|(x_0 – x_1) \times (x_0 – x_2)|}{|x_2 – x_1|} $$

\( \times \) is the cross-product
\( | vector | \) is the magnitude of the vector

Posted: November 3rd, 2017 at 8:18 am
Last Updated on: November 9th, 2017 at 5:36 am