What's up with this cylinder?

[Rick Nightingale]

I was PBRing my house and noticed this. I don't think it's anything to do with PBR though:

It's a simple, cylindrical prim top to a railing. The railing is the same thickness, but the tops... well, you can see what is happening. The tops are identical, in fact B was made by shift-dragging A. What's going on?

This was in Firestorm, latest beta. I thought it might be a bug in that, but it does the same in the release FS and in the official LL viewer, both tops look like A.

While the smooth shading hides the fewer faces on A at a distance, the edge of the top of the railing is visible where it shouldn't be, and where the top goes into a post in the corner (not visible in the picture) you can clearly see the flat faces of the top of A.

Edit to add:

Even stranger... If I cam close to the centre of A (centre of its 20m length) it's a proper cylinder. As I cam closer to the end of A, I can see the number of faces dropping as I get closer to the end until it becomes as shown in the picture. It's like the LODs are going in reverse, but only as I cam towards the end of it.

Edited March 31 by Rick Nightingale

[ChinRey]

8 hours ago, Rick Nightingale said:

The tops are identical, in fact B was made by shift-dragging A. What's going on?

Are they the same length too? A is expected behaviour for a long, thin cylinder, B is not.

There is a quirk in how SL handles LOD for cylinders. For all other objects the swap distances are determined (among other factors) by the size along all three axises. But for cylinders the z size is ignored and since the distance are calculated from the center of the object, you can sometimes get LOD swaps even before you reach the end of it.

I once asked Grumpity Linden about it and she said "it's amazing how many tris we save this way." She may have a point; it's been this way for more than 20 years and it's amazing how few people have noticed.

[Rick Nightingale]

Thank you ChinRey.

No, A is 20.1m, B is about 6m; saying they were identical wasn't accurate.

I'm amongst those who have never noticed in all these years. I might have to mesh them because it's irritating me now, just knowing about it even if I don't look

It is quite visible at normal distance (close enough to see, but not deliberately trying to look closely) because of the gap it leaves along the edge of the railing's flat top. It catches the light.