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# How to Check if a Line Segment Intersects a Rectangle?

Last modified: January 26, 2023

## 1. Introduction

In this tutorial, we’ll show how to check if a line segment intersects a rectangle.

## 2. Intersection

Let’s say that the segment’s endpoints are and . Similarly, let , , , and be the rectangle’s corners.

We say that the segment intersects the rectangle if has at least one common point with at least one rectangle’s side. For example:

So, the idea is to check if intersects , , , or . If we find an intersection with any side, we don’t need to check the rest.

## 3. The Intersection of Two Segments

So, let’s see how we can check if intersects . The steps will be the same for the three remaining sides.

If intersects , there will be at least one point that belongs to both and :

Focusing on the triangle and treating , , , , and as vectors directed from the origin, we see that the following must hold:

**The condition ensures that belongs to both segments.**

### 3.1. Solving a Linear System

If , , , and , we get a system of two linear equations:

(1)

The next step is to solve it for and .

### 3.2. Interpreting the Solution

**If the system has no solution, doesn’t intersect .**

If there’s a unique solution such that and , then they intersect at a single point .

If there’s more than one solution, we’ll get a range of values for and . If the ranges include values from , and coincide at least partially. In particular, if the ranges are completely inside , we have , , or both.

### 3.2. Algorithm

Here’s our algorithm:

So, we start with one side and check all the others iteratively until we either find an intersection or test all of them without success.

Since we deal with a fixed number of dimensions, solving systems takes time, so **the complexity of the entire algorithm is also **.

### 3.3. What If The Segment Is Inside The Rectangle?

We set out the problem and formulated our algorithm to test only if has a mutual point with the rectangle’s boundary.

On the other hand, **if we want to cover the rectangle’s interior, we need to extend our algorithm.** The already formulated pseudocode deals with all the cases except when lies inside the interior of .

To handle it, we note that is inside the rectangle only if and are linear combinations of the rectangle’s directed non-parallel sides such that all the coefficients are between 0 and 1:

**We can take any two adjacent sides as the vectors as long as we direct them so that the interior points have positive coefficients.**

To check that, we solve two linear systems we derive from:

and

If both have single solutions and , is in the interior of .

## 4. Conclusion

In this article, we talked about how to check if a line segment intersects with a rectangle. We differentiated between two cases: in the first one, we considered if the segment intersects the rectangle’s boundary, and in the second one, the case where we covered the intersections with the interior as well.