Why does 'convex hull' increase land impact?

[SamsonMcgarrett]

I apologize in advance if this question is too noobish. I've been attempting to animate a large ship that currently sits about 86 prims. So, I've been researching how to get the physics weight below 32 and have been playing with converting prims to "convex hull". However, when I do so, it always increases the land impact. This is someone else's build but it seems to me that they are all simple prims, not sculpties and certainly not mesh. 

So, when I try to edit an individual prim in the linkset, for some reason the land impact increases by about 20. Any ideas as to why this is happening?

 

Thanks!

[Amethyst Jetaime]

Changing a box prim or other simple prim to convex hull does save LI.  However if curved surfaces are involved in the shape of the prim, particularly if it has been tortured it may actually increase it because it takes more triangles to make a curved surface that a flat one.  You just have to experiment changing on prim at a time to convex hull to see if it saves any LI.  If not, return it to prim physics.

[Drongle McMahon]

By default, traditional prims use the legacy LI accounting. This means their LI is always 1. For example, if you rez a torus, its LI is 1, but if you use the edit dialog "More Info" link, you will see that its physics weight is 35. That is the triangle-based "Prim" type physics weight of the torus' mesh, but it isn't used by the LI sytem because this is a legacy prim. If you now go to the texture tab, click "Bumpiness" and select the blank normal map, you will see the LI jump to 35. This is because using any non-legacy feature switches the prim to use the new LI accounting. This includes materials (normal and/or specular maps), any alpha blending mode other than "Alpha blending", and any physics shape type other than "Prim".

If you reset the "Bumpiness" to None, the LI goes back to 1. Now change the physics shape type to "Convex Hull". The LI goes up to 2, not 35. If you click the More Info link, you will see the physics weight is now 1.8, much less for the convex hull than for the triangle-based "Prim". It's just that this isn't obvious because the "Prim" weight doesn't get used by default. Thus if the new accounting is in effect, the "Convex hull" has lower LI han "Prim". If it isn't, the "Prim" has lower LI than the "Convex hull". That's for the torus. It will most often be true, but the numbers will depend in which kind of prim it is and which torture parameters have been applied.

Unfortunately, the LI is calculated for a whole linkset at a time. This means that in linksets containing several legacy prims, like your ship, changes that make ANY of the linked prims use the new accounting make ALL of them use it! If you rez a cube and link it to the torus (after setting its shape back type to "Prim"), then use "Edit linked" to switch just the cube to shape type "Convex hull*", the LI of the linkset will jump to 35 because the linked torus is forced to use the new accounting too, although none of its own settings would make it do so. Unlink them and the LI of each will go back to 1. Relink and it will be 35.

So the moral of all that is that setting the physics shape types of one or more prims in the ship linkset to "Convex hull" may reduce the physics weight IF it is already using new accounting. If not, it is likely to increase it because it will invoke new accounting. So will setting any prim to physics type "None" (except the root, which cannot be made "None"). However, those set to "None" will contribute no physics weight at all. So the best way of reducing the physics weight may be to set a lot of the constituent prims to "none".

You should probably also look at this wiki page to read about the size-dependent "penalized" physics weight increase that happens when you make something physical. These wiki pages are often out of date. So it may well be that someone expert in vehicle making can add more information here.

ETA *corrected - I had put "Prim" (legacy accounting) when it should say "Convex hull" (new accounting).

 

[SamsonMcgarrett]

Thanks so much for both of your answers! This is incredibly helpful information.

 

 

[Aethelwine]

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SamsonMcgarrett wrote:

I apologize in advance if this question is too noobish. I've been attempting to animate a large ship that currently sits about 86 prims. So, I've been researching how to get the physics weight below 32 and have been playing with converting prims to "convex hull". However, when I do so, it always increases the land impact. This is someone else's build but it seems to me that they are all simple prims, not sculpties and certainly not mesh. 

So, when I try to edit an individual prim in the linkset, for some reason the land impact increases by about 20. Any ideas as to why this is happening?

 

Thanks!

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I think the reason why convex hull can sometimes increase land impact has already been explained.

But your underlying problem seems to be animating something over 32 prims. I don't know how that works in any technical detail. I have always used Karyn's over 32 prim script. It works out what properties to make each prim in the linkset. Any prims you want to be physical, eg tires or you might just put a box around your whole vehicle should be labeled solid, the root prim will be solid anyway. Drop the script in and it has always worked for me like magic regardless of the vehicle scripts I am using. It will I think allow anything up to 255 land impact to operate as a vehicle (with additional vehicle scripts).

[SamsonMcgarrett]

Oh wow. That's a great script. It didn't even occur to me to go searching for something like that. Thanks so much!