# In Ray-Sphere intersection is $b=2*(O-C) \cdot dirv$?

In Ray-Sphere intersection is $b=2*(O-C) \cdot dirv$?

Where $dirv$ is the Ray direction vector. $O$ is origin and $C$ is center of the sphere.

I've also seen this without the $2$, i.e. $b=(O-C)\cdot dirv$.

Particularly here it's calculated like:

// Compute:
//    d = L - E ; // Direction vector of ray, from start to end
//    f = E - C ; // Vector from center sphere to ray start
const Vector &d = ray.direction ;
Vector f = ray.startPos - this->pos ;
real a = d • d ;  // a is non-negative, this is actually always 1.0
real b = 2*(f • d) ;

real discriminant = b*b-4*a*c;


TL;DR $b = 2(f \cdot d)$

I'll try to derive it and then we can see what $b$ should be. If the sphere is centered at point $a$ with radius $r$, its equation is $|p - a| = r$, where $p$ is a point on the sphere. $|p - a|$ is the distance from a point $p$ to the centre of the sphere, and if that is equal to the radius, then it's a point on the sphere.

We'll assume the centre of the sphere is at the origin, so $a = 0$ and we have $|\vec{p}| = r$. Squaring both sides gives $|\vec{p}|^2 = r^2$ which I'm going to write as $$\vec{p}^2 = r^2 \tag{1}$$

Ray has equation $$x + td\tag{2}$$ where $x$ is the endpoint, $d$ is the direction, and $t$ is the parameter.

If the ray doesn't intersect the sphere, we will not be able to find a $t$ satisfying the ray equation. In the quadratic equation the discriminant (the bit under the square root) will be $< 0$. If it grazes the sphere (i.e. tangent) then we'll have a single point of intersection and a single $t$ and the discriminant will be $0$. If it goes through the sphere, we'll get a $t$ for the sphere entry, and a distinct $t$ for the sphere exit and the discriminant will be $> 0$. We need to plug $(2)$ into $(1)$ and solve for $t$:

\begin{alignat}{2} &&(x + td)^2 &= r^2\\ &\Rightarrow\quad &(x + td)(x + td) &= r^2\\ &\Rightarrow\quad &x \cdot x + 2t(x \cdot d) + t^2(d \cdot d) &= r^2\\ &\Rightarrow\quad &(d \cdot d)t^2 + 2(x \cdot d)t + (x \cdot x - r^2) &= 0 \tag{3}\\ \end{alignat}

Equation $3$ is a quadratic expression in $t$ (i.e. $at^2 + bt + c = 0$) where $$a = (d \cdot d)$$ $$b = 2(x \cdot d)$$ $$c = (x \cdot x - r^2)$$

This matches the code you posted, but the code is allowing spheres to be centered at an arbitrary point C. The code then has this line:

// f = E - C ; // Vector from center sphere to ray start


which effectively translates the endpoint of the ray E to a new endpoint f so that we can now consider the sphere to be centered at the origin. The ray direction is unchanged.

• I wonder what the $b$ without $2$ is then. I've seen it used somewhere, but cannot find it now. Mar 13 '17 at 16:56