# Reflection of a point about a line in C++

Let’s first consider a general case where the line is nothing but the X-Axis. We can now definitely say that the conjugate of a point is the reflection of the point about X-Axis.

Now, using the methods of translation and rotation of coordinate axes we will find out the reflection of a point about the generic line.

The idea of translation was described in the previous post. Here we describe the idea of rotation. **What is Rotation?**

In Euclidean geometry, a rotation of axes in two dimensions is a mapping from an xy-Cartesian coordinate system to an x’y’-Cartesian coordinate system in which the origin is kept fixed and the x’ and y’ axes are obtained by rotating the x and y axes through an angle θ. **How to Perform Rotation?**

Rotation can be interpreted as multiplying (rotating in anticlockwise direction) or dividing (rotating in clockwise direction) every point of the coordinate system by a constant vector.

Note here that if we want to rotate a point by θ in the anticlockwise direction about the origin, we multiply it by polar (1.0, θ) as discussed in SET 1. Similarly, we divide by polar (1.0, θ) to rotate the point by θ in the clockwise direction.

After the rotation, required computations are performed and rotation is nullified by dividing or multiplying every point by the constant vector respectively.

So, we have to reflect a point P about a line specified by points A and B denoted as AB. Since, we know that the conjugate of a point is the reflection of the point about X-Axis. In order to be able to use this fact, we will first perform translation (making A as the origin in the new system) and then rotating the coordinate axes in such a way that the line becomes the X-Axis in the new coordinate system.

Now we can simply apply the formula for reflection about X-Axis and then nullify the effects of rotation and translation to get the final result.

These steps can be described as under:

**1.Translation (Shifting origin at A):** Subtract A from all points.

P_{t}= P – A B_{t}= B – A A_{t}is origin

**2.Rotation (Shifting B _{t}A_{t} to the X-Axis):** Divide all points by B

_{t}(dividing means rotating in clockwise direction which is the requirement here to bring on X-Axis).

P_{r}= P_{t}/B_{t}

**3.Reflection of P _{r} about B_{r}A_{r} (which is nothing but the X-Axis):** Simply take the conjugate of the point.

P_{r}reflected = conj(P_{r})

**4.Restoring back from Rotation:** Multiply all points by Bt.

P_{t}reflected= conj(P_{r})*B_{t}

**5.Restoring back from Translation:** Add A to all points.

P reflected = conj(P_{r})*B_{t} + A

Thus,

return conj(P_{r})*B_{t}+ A where, B_{t}= B – A P_{t}= P – A P_{r}= P_{t}/B_{t}

## CPP

`// CPP example to illustrate the` `// reflection of a point about a line` `#include <iostream>` `#include <complex>` `using` `namespace` `std;` `typedef` `complex<` `double` `> point;` `#define x real()` `#define y imag()` `// Constant PI for providing angles in radians` `#define PI 3.1415926535897932384626` `// Function used to display X and Y coordinates of a point` `void` `displayPoint(point P)` `{` ` ` `cout << ` `"("` `<< P.x << ` `", "` `<< P.y << ` `")"` `<< endl;` `}` `// Function for Reflection of P about line AB` `point reflect(point P, point A, point B)` `{` ` ` `// Performing translation and shifting origin at A` ` ` `point Pt = P-A;` ` ` `point Bt = B-A;` ` ` `// Performing rotation in clockwise direction` ` ` `// BtAt becomes the X-Axis in the new coordinate system` ` ` `point Pr = Pt/Bt;` ` ` `// Reflection of Pr about the new X-Axis` ` ` `// Followed by restoring from rotation` ` ` `// Followed by restoring from translation` ` ` `return` `conj(Pr)*Bt + A;` `}` `int` `main()` `{` ` ` `// Rotate P about line AB` ` ` `point P(4.0, 7.0);` ` ` `point A(1.0, 1.0);` ` ` `point B(3.0, 3.0);` ` ` ` ` `point P_reflected = reflect(P, A, B);` ` ` `cout << ` `"The point P on reflecting about AB becomes:"` `;` ` ` `cout << ` `"P_reflected"` `; displayPoint(P_reflected);` ` ` `return` `0;` `}` |

Output:

The point P on reflecting about AB becomes: P_reflected(7, 4)

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