Tuesday, 26 June 2012

Too much information

Too much information running through my brain, too much information driving me insane. - The Police.
Level Of Detail (LOD) seems to vex us and yet it can be quite hard to find out exactly what it is. In previous entries we established that in Second Life there are 4 LOD levels, this applies to all objects, prim, sculpt or mesh though the standard prims are very "LOD resistant." LOD resistant is a term used to convey the ability of an object to remain visibly intact at lower LOD levels. In sculpt maps this was achieved by making sure that key "structural" vertices were reinforced by ensuring that vertex with those coordinates appeared in each of the reduced sculpt map images. With Mesh we have to control this ourselves as shown in the last blog.

But when does one LOD get shown as opposed to another?
The best answer I have found to this is still asserted to be an approximation but it's close enough to work with.
The LOD boundaries can be approximated using the following equations.

Medium LOD = r/0.24
Low LOD = r/0.06
Lowest LOD = r/0.03 

where r is the "radius," determined as being the distance from the centre of a prim to one corner of the bounding box of the prim.
(Bounding box is a theoretical box of the same dimensions as the object.)

Thus if for a given object r is 10m then it will display the high LOD until the viewer is about 10/0.24 or ~42m away. After this, the medium LOD will appear and then the low LOD when the camera has withdrawn to 167m. Finally, the lowest LOD will appear at 333m. Which the observant will note is a fair way across the next door sim.

Clearly, a smaller object with a value of r of say 1m will have a different set of LOD boundaries. (4, 17m and 33m respectively). 
Ignoring mega prims, for now, the largest object we can have is 64mx64mx64m. This would have an r value of ~45m. The high LOD for this would be visible all the way out as far as 187m.

The fact that the highly detailed mesh is visible across most of the sim is the basic reason why larger mesh objects have a higher Land Impact penalty applied to them.

To close this post and draw a line under the LOD discussions for now, here are a series of images comparing the carefully LOD controlled mesh lamp with the standard sculpted lamp. To be fair to sculpt aficionados, though, there is not attempt in the sculpt to harden it against LOD.
High LOD - Mesh on Left (LI=1) , sculpt+prims on right (LI=5)

Medium LOD - Mesh on Left (LI=1) , sculpt+prims on right (LI=5)

Low LOD - Mesh on Left (LI=1) , sculpt+prims on right (LI=5)

Lowest LOD - Mesh on Left (LI=1) , sculpt+prims on right (LI=5)


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