Suppose you have a ray of light that hits a flat surface such as a mirror and is reflected. What is the

direction of reflection? This is a problem that arises in areas such as optics and computer graphics. To

solve it, we need to know the original direction of the light ray, and the orientation of the surface. Let

the direction be given by a vector v = (vx , vy, vz), and let the surface orientation be specified by an

outward normal vector n = (nx , ny , nz) of the surface1.

To compute the reflection vector we first check if v is heading towards the surface. If it is not, then it

will not reflect off the surface. This check is done by using the dot product

v n = vxnx + vyny + vznz .

Your first task in this part is to write a function:

double dotProduct(double vx, double vy, double vz, double nx, double ny, double nz)

that computes the dot product. If the dot product is greater than or equal to zero, the ray of light is

headed away from (or parallel to) the surface, and so no reflection occurs. So we only compute the

reflection vector if the dot product is less than zero. In this case, the next step is to normalize the

surface normal vector by dividing each component by the vectors length2; this ensures the resulting

vector has unit length. Write a function:

void normalize(double& wx, double& wy, double& wz)

that, given any vector (w x , w y , w z ), computes its length, divides each component by that length,

and then uses call by reference to return those components. The final step is to use the formula

r = v 2(v . n)n

to compute the reflection vector. Here n is the normalized vector from the last step. Write a function:

1 A normal of a flat surface is a vector perpendicular to the surface. An outward normal is any such vector pointing

outward from the surface object rather than inward.

2 Recall the length of a 3D vector w is given by

void reflectionVector(double vx, double vy, double vz,

double nx, double ny, double nz,

double& rx, double& ry, double& rz)

that does the following: (i) takes the original surface normal vector n and normalizes it using the

normalize function, (ii) computes the reflection vector r = (rx, ry, rz ) (note to do this the formula above

needs to be applied three times: once for each of the x, y, and z components; also note that computing

the reflection vector requires use of the dotProduct function), and (iii) uses call by reference to return

the reflection vector components. Finally, write a main driver program that does the following:

1. Asks the user to input the original direction, and then input the surface outward normal.

2. Checks if reflection occurs. To do this it should use the dotProduct function.

3. If reflection does not occur, your main program should print out an appropriate message. If

reflection does occur, your main program should call the reflectionVector function to

compute the reflection vector, and should print out that vector.

4. In either case, your main program should then ask the user if they wish to continue. If they do,

the program should allow the user to continue this process as many times as they wish.

Here is some example input and output.

Input the original direction vx vy vz: 0 -1 0

Input the surface outward normal nx ny nz: 1 1 0

Reflection vector: 1 -2.22045e-16 0

Continue (y/n)? y

Input the original direction vx vy vz: 1.5 2.5 3.5

Input the surface outward normal nx ny nz: 1 1 1

No reflection

Continue (y/n)? y

Input the original direction vx vy vz: 1.5 2.5 3.5

Input the surface outward normal nx ny nz: -1 -1 1

Reflection vector: 1.16667 2.16667 3.83333