Strange, yes. But you may not realize that you've seen this effect before. Practically every light source and light beam in Uru does the same thing.
Look carefully at one of the standing lamps in the Cavern. This one has a lit-up yellow-and-pink design; but it also has a vague halo of yellow illumination around it.
The halo gives the effect of a sphere of glow. But it's not a sphere. It's a flat polygon with a circular glow pattern. And it always faces you. Shift sideways around the lamp; you can see it turn to follow you.
In the images above, you can actually see where the glow polygon intersects the pillar below it. I chose this lamp because it's above eye level; that means that the glow is tilted forward, so the bottom edge slices into the pillar. If you climb the stairs behind the lamp, then the glow will tilt back, and you won't see that sharp cut-off any more.
Why doesn't the glow slice through the lamp itself? Simple -- it's in front of the lamp. It doesn't just rotate in place; it turns to keep itself between the lamp and you. (Between the lamp and the game camera, actually.) It's like... er... like a fake moustache hanging on the eye-stalk of a Dalek who is watching you.
Okay, that metaphor kind of got away from me. But you see what I mean. The lamps in Relto do the same thing. So do the ones in the Pubs, and indeed throughout Uru.
It's not just lamps. A firemarble also has a glow polygon -- in fact, the glow polygon is the firemarble's entire (visible) presence. There's nothing else to see. As with the lamps, the glow hangs slightly in front of the marble's position, and always faces you. In these images, you can see the lower edge cutting into the floor. (Since the marble is below eye level, the glow plane is tilted upwards -- it "looks up" at the camera..)
Why do it this way? I've already said it: this trick lets the designer show a spherical glow effect, using a single flat polygon and one texture.
What about a glow that isn't spherical? Well, the examples above rotate freely in three dimensions. If you fixed the glow polygon to rotate only on a vertical axis -- like the Pub imagers -- you could create a vertical cylinder of light.
Shaft of light in the Kahlo Pub, Aegura
Okay, this one is a cone. But it's the same idea. These images are taken from a sharp upward angle; you can see the upper edge of the light polygon, which makes the rotation obvious. If you're not looking for it, you just see a shaft of light. If it didn't rotate, you could walk around and see it edge-on, which would look funny. And that's why it rotates.
Most of the light-shafts in Uru do this. Take a look at the vertical lamps in the Bevin stairway behind the linking room. Or the lamp above the Nexus terminal. (That has both a shaft and a sphere, in fact.) Or go to Kadish Tolesa, look upward, and run through the light beams that filter down through the trees.
(Actually, the Kahlo Pub shaft has an additional trick, which you can make out in the images above. It gets wider and narrower as it rotates. This allows it to look like a natural result of the ceiling crack, which is much longer than it is wide.)
(However, the changing width is the result of a stretching transform. That throws off the angle calculations. I won't try to explain the math here; but you can see that the polygon doesn't face the camera correctly. It turns too far as I slide left and right.)
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