I did say trust me. But you have not. So I must show you. Unfortunately with math.

So the vector from any point A to any other point B is B - A. In particular, given that points are 3-dimenstional tuples, this is Bx - Ax, By - Ay, Bz - Az.

Clearly if we move A, this changes.

Clearly if we move B, this changes.

Suppose A is your shading point.

Suppose B is your light position.

The vector from surface to light is B - A.

And if you move either this will change.

Or will it?

Imagine you are looking at a small light bulb, and standing next to you is another person a few feet away, also looking at the light bulb. If the bulb is near you two, it is likely that you are looking in different directions.

For example, suppose your bulb is at 0, 0, 0. You are standing 5 feet west, looking east. Your friend is standing 5 feet east, looking west. You two are looking in totally opposite directions, to see that bulb, right?

Now imaging the bulb moves 10 feet north. You'd both be looking at it, and the arrow from your eyeballs to the bulb would be a lot more parallel than they were.

Now move the bulb a mile away. Your eyeball-arrows are nearly parallel.

Move the bulb a hundred miles away. Your eyeball-arrows are so parallel I can hardly measure any difference. The direction to the light is nearly the same, even though you two are standing ten feet apart.

Now move the bulb a hundred million billion trillion miles away. What happens to your eyeball arrows?

Now move the bulb a googleplex (10 to the 100th power, i.e. a 1 followed by 100 zeros) miles away. You two are looking in the same direction, for all intents and purposes. In fact, you two could move around anywhere on the face of the earth and you'd still be looking in the same direction.

Now move the bulb infinitely far away. (Trust me) You are now looking in the same direction, by definition, no matter where you stand.

That is why the light is called an "infinite" light. It is infinitely far away. It has no position. But it has a very clear and specific direction.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)