Rays and Parametric Lines

A ray tracer asks one question, millions of times a second: starting here, looking that way, what do I hit? The "here" and the "that way" together are a ray — the single atom every other raytracing idea is built from. Before we can intersect a ray with a sphere, a plane, or a triangle, we need a way to name every point along it with one tidy formula. That formula is the parametric line, and it is almost embarrassingly simple.

Building the ray, line by line

A ray has two ingredients: a place to start and a direction to walk. Call the start the origin O (a point) and the direction D (a vector). We want a single point-valued function of one knob, t, that slides along the ray as we turn it.

Step 1 — start at the origin. When the knob is at zero, we have not moved at all, so we are sitting on the origin:

P(0) = O.

Step 2 — take one step in the direction. Turning the knob to t = 1 should walk us exactly one copy of D forward from O. Point plus vector is a point:

P(1) = O + D.

Step 3 — scale the step by the knob. Half a turn, t = \tfrac12, should land us halfway; two turns, t = 2, twice as far. So we scale the direction by t before adding it. That single line is the whole idea:

P(t) = O + t\,D.

Every point on the ray is some value of t, and every value of t is a point on the ray. To find where you are, plug a number in; to find which number a point is, that is exactly what an intersection test solves for.

Step 4 — keep t \ge 0 to look forward. A negative t walks backwards, behind the camera — light that never reached the lens. A ray, unlike an infinite line, is a one-way street: we only ever want t \ge 0. Negative solutions to an intersection are discarded as "behind me".

Step 5 — make D a unit vector and read off distance. If we normalise the direction so \lVert D\rVert = 1, then walking t copies of a length-one step covers exactly t units of ground. The parameter is the distance travelled:

\lVert P(t) - O\rVert = \lVert t\,D\rVert = t\,\lVert D\rVert = t \qquad (\text{when } \lVert D\rVert = 1).

That is why t is the prized quantity in a ray tracer: with a unit direction it is the literal depth of whatever you hit, and the closest hit is simply the smallest positive t.

A ray with origin O and direction D is the set of points P(t) = O + t\,D.

Parametric form — each t names exactly one point; plug in t to get the point, solve for t to find a hit.
Forward only — a ray keeps t \ge 0; t < 0 is behind the origin and is discarded.
Unit direction ⇒ distance — if \lVert D\rVert = 1, then t is the actual distance from O to P(t).
One ray per pixel — a camera fires a ray through each pixel; the colour of that pixel is whatever the ray hits first (smallest positive t).

The same formula P(t) = O + tD describes three different objects; the only difference is which values of t you allow.

Line — t \in (-\infty, \infty). The full infinite line through O in both directions. Useful for "which side of this wall is the whole room on", but it has no notion of forward.
Ray — t \in [0, \infty). Starts at O and shoots off forever in one direction. This is the camera ray, the shadow ray, the bounce ray.
Segment — t \in [0, L] with a finite L. A ray with a stopping point — exactly what a shadow ray is: it travels only as far as the light, and any hit with 0 < t < L means something is in the way, so the point is in shadow.

One equation, three jobs, picked apart purely by the interval you let t roam over.