Alpha Mode vs. Land Impact

[Vulpinus]

I've just had this odd thing happen...

In my cellar is a wall with a doorway. It is simply two linked prims, each with a hollow, textured to give an archway with cell grating. See the picture:

I was tidying it up a litte, and changed the alpha mode from blending to masking, because I had noticed the occasional stacking issue and, I thought, to make life easier for rendering. It works visually perfectly with masking, but...

...suddenly, it got returned to my inventory along with a few other bits because my parcel filled up!

On investigation, I discovered that using Alpha Masking on a 'complex' prim (one with a hollow) changes it to use (I guess) mesh LI rules. My 2 prim (and 1 LI) doorway had jumped up to 55 LI.

I've confirmed this with a newly created cube, set with a transparent face. With no hollow, I can set either Alpha mode and it stays 1 LI. If I make it hollow, it jumps to 4 LI if I use Masking.

Weird - I thought Masking was supposed to be simpler to render.

I can sort of solve the issue if I change the linked wall to Convex Hull. It comes back down to 1 LI, but then I can't walk through the doorway. Set to prim, it goes back to 55 LI.

Oh well, looks like I'm stuck with Blending.

Just saying... comments are welcome.

[Drongle McMahon]

"if I change the linked wall to Convex Hull. It comes back down to 1 LI"

That confirms that the raised LI is due to the physics weight. I would guess that in your arches, it's because the geometry used for the physics includes the thin edges, which are very expensive in weight of triangle-based physics shapes. If you were making the equivalent thing from mesh, you would either leave out those edges or use a hull-based shape for the physics instead. For the prim, you don't get those choices. So you are stuck with the high weight.

You are right about masking being more efficient, and it also avoids the alpha-layering problem. However, the use of resource for texturing is (perhaps unfortunately) not included in the LI calculation. So you are left with the perverse choice because masjing changes the LI calculation, while the legacy blending does not.

There is a way around this, although it will still cost a bit of LI. First, set it all to convex hull, to avoid the return. Then add three thin box prims to the linkset to make the wall+hole, using one of them as the root. Set the new prims texture to default transparent. Use edit selected and set the physics shape types of the arches both to "None". Then change the relevant texture to masked. That should leave you with a physics weight of 0.3, although the LI will go up to between 2.5 and 5.

ETA: Better alternative: just get rid of the hollow square arch and replace it with three boxes with the grille texture nicely aligned. Link with one of these as the root, set the curved archn type to "None", and the three boxes to "Convex Hull". Now change the textures to masked. That should be quite a bit cheaper.

[Theresa Tennyson]

Try this:

"Edit linked" and then set the "stone arch" to physics type "None." Set the "grating" to physics type "prim." Note that you can't set a root prim to physics type "none." That should help some. If that doesn't help enough try making the grating into three separate prims in that configuration, link them together with the arch, set the linkset to "convex hull" and individually set the arch to physics type "none." The entire linkset should have a land impact of 2 that way.

The problem is that calculating physics for hollow shapes is calculation-intensive and always has been. The new land impact accounting reflects that and using any non-legacy tricks (such as masking as an alpha mode) will trigger the new accounting.

[ChinRey]

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Vulpinus wrote:

Weird - I thought Masking was supposed to be simpler to render.

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It is, but rendering is done by the client, that is by our computers, and land impact is all about server load.

 

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Vulpinus wrote:

On investigation, I discovered that using Alpha Masking on a 'complex' prim (one with a hollow) changes it to use (I guess) mesh LI rules.

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The term "mesh LI rules" is rather misleading and has caused lots of misunderstandings. The land impact system was indeed introduced with mesh but it's not only about mesh. I have a feeling LL would eventually have been forced to introduce it anyway since the old prim count system simply had become far to inaccurate as builders used more and more sculpts and heavily twisted prims.

The only reason the old prim count system is still used at all is that some older builds would break if their LI was recalculated the new way and the system is only used for builds that could have been made before the new system was introduced. Any feature introduced after LI wil cause the entire linkset to be calculated the modern way. That includes mesh, the new physics shape type, new materials, some script functions etc. (Alpha masking is a bit special there since it's actually always existed in SL but wasn't made available to us builders until recently.)

The switch from prim count to land impact calculations can give builders unfamiliar with the new system quite a lot of surprises. Fortunately they're usually pleasant ones and when they aren't, such as in this case, there is nearly always some way to fix it.

Drongle and Theresa have already given you some good methods to fix the issue. If you can use Theresa's first solution - grating as "prim" and arch as "none" - you may actually get the LI down to 1, lower than the build with have with the old prim count system.

[Vulpinus]

Thank you all for the informative comments and suggestions. I'll give them a try when I get back to my main PC tomorrow.

ETA:

Got back today...

Making the grating from three prims, linked with with the surrounding arch which is set to physics=none, works nicely. Same 2 LI as I started with (I incorrectly said it was 1 LI in my OP), but with alpha masking. Great!

Thanks for the education

[Penny Patton]

Basically, Linden Lab has tied newer features like materials, alpha masking, keyframing, pathfinding, etcetera to the new, more realistic "land impact" calculations. If you want to use any of these features, SL will stop using prim count and start using land impact for the LI cost.

The problem is, alpha masking is a vastly more optimal way of displaying alpha textures and does nothing except reduce the resource load. It should not have been tied to the new LI costs.

 Those other features? Yes, absolutely. Want new shiny? Then you have to be more realistic with resource use.

 But alpha masking? LL is basically discouraging people from optimizing their content.