Alpha Masked Sculpt = 2LI, Alpha blend it = 1LI

[Callum Meriman]

Is it expected that LI on a sculpt object would double if one sets it to alpha masking as opposed to alpha blending?

I noticed it when delagging some Kidd lavender, the LI jumped from 1 to 2. Tried the same on a 30LI Heart oak and it went from 30 LI to 60 LI.

It was my impression that masking was a better technique to reduce lag as the blending calculations could be skipped. But if it doubles the LI of everthing it's done to that seems a bit counter productive.

Anyone have any ideas on this, is it expected, are there any workarounds?

[Whirly Fizzle]

Yes, It's expected.

When you set a texture to alpha mask or emissive mask mode or add a material texture to a legacy prim/sculpty, it will use the "new" mesh LI accounting instead.

It's generally not a good idea to add materials or use alpha mask/emissive mask on sculpted prims because of the huge LI increase.  You are lucky your LI only doubled. If the sculpt was more complex or legacy prim was very tortured, you can end up with the LI increasing by many thousands.

 

MATBUG-13 - Alpha masking can dramatically inflate Physics Land Impact

[Callum Meriman]

Thanks Whirly! I will leave the prims in laggy mode.

I can just imagine doing that, ending up with a huge LI and half the sim being returned O_O hehe.

[Whirly Fizzle]

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Callum Meriman wrote:

 

I can just imagine doing that, ending up with a huge LI and half the sim being returned O_O hehe.

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lol yep!  Been there, done that, had to get a region rollback  :smileywink:

[Theresa Tennyson]

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Callum Meriman wrote:

Is it expected that LI on a sculpt object would double if one sets it to alpha masking as opposed to alpha blending?

I noticed it when delagging some Kidd lavender, the LI jumped from 1 to 2. Tried the same on a 30LI Heart oak and it went from 30 LI to 60 LI.

It was my impression that masking was a better technique to reduce lag as the blending calculations could be skipped. But if it doubles the LI of everthing it's done to that seems a bit counter productive.

Anyone have any ideas on this, is it expected, are there any workarounds?

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With the new accounting there are three separate factors that make up land impact and any given object's land impact will be the highest of the three for that object. The factors are:

Download weight: based on the number of vertexes the object has. With the new accounting most sculpted prims will have a download weight of 2 per prim. This is the increase you're seeing with those objects. (Incidentally, sculpts are capped at 2 for download weight - the land impact of a sculpted item will at most double.)

Physics weight: how hard the physics engine needs to work to simulate collisions with that object. The big factor here is whether the object has holes through it that the physics engine needs to worry about. This is why a torus will have a high land impact under the new math, and why tortured prims have spectacularly high land impacts. However, this can be avoided by going to "Features" and changing their "Physics Shape Type" to "Convex Hull" (basically shrink-wrapping that object to fill in holes) or "None" (causing the physics engine to completely ignore it.)

Server weight: The number of pieces the servers need to keep track of. Generally an object's server weight is 0.5 per prim plus a small additional amount for each script. With simple objects this number will therefore be lower than the basic prim count.

You can determine which of these factors is controlling an object's land impact by editing it and selecting "More info" after where the land impact is listed.

The trick to optimizing land impact is to set linksets up so that an object that's high in one factor but low in another is compensated by objects controlled by other factors. For instance, sculpted prims are high in download weight but low in server weight. Ordinary prim builds usually have very low download weights and are controlled by server weight, which is still lower than their prim counts. If you link your lavender, even at its new land impact of 2, to 5 conventional prims (very little download weight, server weight of 2.5) the total land impact of the linkset will only be three and controlled by the server weight because the higher download weight of the sculpt is "absorbed" by the low download weight of the prims.