Good LOD Guide?

[Pamela Galli]

I need to make a lot of LODs for windows and doors but want to make sure the ones I make meet all the requirements re bounding box, materials, etc. I have been searching and can't find a good reference or guide. Anyone know one?

[Nalates Urriah]

The absolute best information is here in this forum. It is the most detailed and complete. Also, the most technical.

I suggest you look for Drongle's posts. I believe he has done more testing and experimenting with Land Impact Costs and mesh than anyone else. He gets pretty geeky from time to time, but is generally understandable.

There is a mesh best practices in the WIKI. It gives good general information and should be enough for good building.

A number of bloggers have information that can help to. I have a reference to Ciaran Laval's article on reducing prim count. Google will get you the most popular of the bloggers. YouTube has a load of stuff, but it is a mix of the good and the bad.

Give up the idea of their being a BEST WAY or even a RIGHT WAY. Each model is a work of art. The least amount of prims that will do the job is what you are trying for. If the item will be physical or collideable then the physics model becomes important, in which case tiny triangles are deadly driving up cost.

The best LoD models are those with as few tryangles as possible that will maintain the outline of the object. In some models a square with a picture of the object is used to reduce the poly count to the absolute minimum.

GC Cookie has good tutorials and all their design principals apply to Second Life.

[Pamela Galli]

Thank you Nalates.  I did read over the forum threads I found in search. So far I know that the bounding box has to be the same, and the number of materials  has to be the same. I need to set Rotation so that will be the same as the high LOD.

I am adding a hidden face on the high LOD called Low Lod, to be used with the low LOD, which I want to be a plane or cube with a picture of the window on it. I did this on the last house I made and it worked pretty well but that was a while ago and I want to make sure I am doing everything right. 

[Drongle McMahon]

Sounds like you have the main points alright. Windows are the things that work best of all with this technique. The lowest LOD is just two planes, one for each direction, unless you are certain it will never be seen from the inside at the low LODs (even through another window?). Don't use a cube - the edges are wasted triangles. I use picture al lowest LOD, then add solid outer frame at the next step, and add the remainder of the frame at the next step. Which step is which depends on the overall size. In my gallery, I also had several windows on the same mesh, to get the LODs switching where I wanted them to.

You still have to have the same number of materials, with the same names, at each LOD, as they never fixed the bug with subsets.So you have to have triangles at the low LODs to hold the high LOD textures as well as the other way round. To try to save materials, and therefore the triangles to hold them at the lowest LOD, I have tried using the same alpha texture with the frame at all the LODs, making sure that  solid frames completely covered the right parts of the picture, so that at the high LOD, only the glass parts are visible. That was quite hard to get right. Examples are the skylights on the roof of the gallery in Mesh HQ 3. (You can see how they work by dialling down renderVolumeLODFactor while standing next to them. They aren't completely optimised for LI, as they were made before the weighting was finalised.)

The least satisfactory part of the method is that the lighting effects can't be reproduced on the low LOD picture. In particular, if you use shiny on the solid frame, you can't use it on the alpha texture with the low LOD picture. That can make very abrupt changes to percieved colour under some lighting conditions. I think the materials project will help to alleviate that problem.

 

[Pamela Galli]

Thanks so much Drongle. Some questions:

 

Why would I need a back side for the LOD, if no one inside the house will be far away enough from the window to see any LOD degradation?

What I want to do is use the picture for the lowest two LODs, and a stripped down mesh for the 2nd one. How does that sound?

 

I could not fine Mesh HQ 3.

 

 

[Drongle McMahon]

"Why would I need a back side for the LOD, if no one inside the house will be far away enough from the window to see any LOD degradation?"

(a) Because someone outside might be able to see through one window into the back of another, (b) if the building is large enough. It all depends on the size of the window and the size and geometry of the building. (and on people's settings for renderVolume LODFactor). It's only two more triangles to have both sides, and then it will do for all puposes. Mesh HQ 3 is on Aditi. You have to teleport in at height to avoid getting trapped under lanscape meshes.

"What I want to do is use the picture for the lowest two LODs, and a stripped down mesh for the 2nd one."

Sounds good for most purposes. Again, it all depends on exectly what you are making, how big it is, and what you want to achieve.

[Pamela Galli]

Ah okay I see. I think.  Thank you!

 

 

[Kwakkelde Kwak]

If you are considering people looking through the building to the back of a window, you also need to keep the entire interior visible from all distances. Personally I'd say that's a huge waste of geometry and therefor performance. From 150 meters away nobody will look through a tiny window. Therefor I like to make the window opaque at the lower LoDs and remove the entire interior at lower LoDs. If you're making a greenhouse that won't work, but on your average build it will.

[Drongle McMahon]

Oh dear. I was setting up a demo to show you the effect of disappearing your windows for people with default settings (it's not about looking through them or not; it's about sudden changes of appearance), when I found an effect that ruins the appearance anyway, rendering the question moot. It seems that there is an effect where things get culled at a certain distance unrelated to renderVolumeLODFactor. Look at the picture here. The upper shot is with the camera 149m from the stepped height boxes (mesh girder, prim box). The lower is from 151m. The stepped boxes are from 4-9m tall in 1m steps, 1x1 section. . You can see the window disappear too. Draw distance is set to 512. What's going on here? Seems a disaster to me. Is this new, or have I been missing it for ages?

ETA: by the way, my point is that even a very large window has download weight much lower than 1 if you use two planes - so what is the point of reducing it further, especially as only one will get rendered, depending on which side you are on?

[Pamela Galli]

Thanks Kwakkelde and Drongle, appreciate the info, and esp you setting up the demo, Drongle. 

 

Drongle please explain what is unexpected about the demo and why it is a disaster. (I am already dealing with a broken animation disaster, I can only handle one at a time!)

[Drongle McMahon]

The things that disappear! They are within the draw distance, and it has nothing to do with LODs. The meshes have the same mesh at low and lowest LOD, and you can see that the 4m to the 8m have already switched (with the lighting effect I mentioned). The lowest LOD the blue prims is still a box, and it shouldn't disappear at until they go beyond the draw distance. This was with rVLF=2. It was exaclty the same with it at 0.2 orv 0.  Things disappearing when they shouldn't is, in my view, a disaster. Note that the bigger object to the left of the pillars also disappeared. This is a linkset of the smaller hecagonal mesh. So a whole linkset made of small pieces can disappear before it is supposed to.

I didn't finish setting up the window part of it because this effect pre-empted that. I could do it with very low rVLF, but that wouldn;t be a realistic situation.

[Kwakkelde Kwak]

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Drongle McMahon wrote:

 

ETA: by the way, my point is that even a very large window has download weight much lower than 1 if you use two planes - so what is the point of reducing it further, especially as only one will get rendered, depending on which side you are on?

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The point is if you model the backside of a window, the "inner" side, because then you can see it when you look through a building as you said, you also need all the other stuff inside the building to show. Otherwise you'd be looking at just a window in a void. It's not about the single extra plane, it's about how you set up the entire build and with small windows you do not need an interior rendered when you're far away. Even if people notice the windows going opaque when they look for it, it is easily outweighed by the fact you don't have to render as much, lowering landimpact and improving performance. The arguement  that removing the one plane doesn't affect any weights is a bit of a confusing one, you don't aim for 1.4999999999999999999999 for every build just because that gives you a LI of 1. If you don't need geometry, don't use it.

I haven't noticed objects disappearing too soon, I have noticed big objects staying rezzed beyond the draw distance. The things might be related.

btw, I always combine a couple of windows into one object, that's less versatile, costs more to stream than a bunch of instances, but results in a lower server weight. Whether this weight is bogus or not I do not know. It would certainly prevent the window from disappearing like it does in your example.

 

[Drongle McMahon]

I agree. As I said, it all depends on what you are trying to achieve. Everyone will balance things differently. For example, combining windows increases their LOD distance (and this disappearing effect), as you point out, but it reduces the versatlility of the uploaded mesh. So if you want to make windows that can be used by anyone, anywhere, that's completely different from making windows that will only be part of one specific building. Same with stuff inside that could be used in quite different environments. Another factor is that different people have very different sensitivities to visual accuracy. I guess I am pretty intolerant in that  regard. The disappearing window in the picture I showed is a major irritant to me. Others will not even notice it, as you say.

[Pamela Galli]

Well, turns out there is no reason to make a picture-LOD because with the last two LOD levels set to 0 I can pull back 256m and still see the windows fine. I made a medium LOD which improves the look a bit but does nothing to bring down the LI, which is my main problem. Nice detailed windows but man they are going to cost me.

[Drongle McMahon]

Are you talking about a free-standing window? What was your renderVolumeLODFactor? Or is it part of a larger mesh? If it's free-standing, then it must be quite large  to stay at medium LOD at 256. That implies at least one dimension of about 20m for a thin object (at default rvlf=1.125; at rvlf=4, that would need to be be only 5-6m).

On the other hand, if the window is part of a big mesh (same mes, not linkset), then its LOD behaviour would be determined by the whole building size, That might explain what you are saying, that even a medium LOD doesn't affect LI. When the "radius" (half bb diagonal) gets near 40m, even the medoium LOD has little or no effect. In that case, you are missing an opportunity for saving LI that you could get if you made the window(s) a separate mesh. That way they would switch LOD earlier and consequently the lower LOs would then contribute to lowering the overall LI.

[Pamela Galli]

I have my LOD set to 3 -- at 2 it disappears, but at an acceptable distance.  It is not part of a larger mesh.

[Pamela Galli]

I ran into every possible error trying to get this window uploaded with medium LOD and 0 for the last two LODs, but finally got it uploaded and cut the LI by almost 2/3, and looks good from a distance, both sides.  Since I have three of these windows, it was actually worth it. :-) 

 

Many thanks for the advice!