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Trimming land impact costs on prim builds

Having escaped Blender’s clutches for a while, I’ve been mucking about a bit with the physics  options for prim-based objects.    The new settings designed for mesh can also be applied to ‘ordinary’ prims, with handy results for land impact and physics complexity.  Recent chats suggest that most Babbagers haven’t tried these settings, so I thought I’d make a brief post.

Firstly, a warning: settings other than ‘prim’ use the new land impact system, which assigns a higher cost to complex shapes (eg torus, tortured prim, etc), large prims, and prims containing scripts. Selecting ‘none’ or ‘convex hull’ for part of a complex build can cause the object’s land cost to go through the roof, catapaulting the build into your lost and found. Go gently with the dropdown on existing builds.

 The settings can also be used to improve and simplify builds, reducing physics load and land impact costs … just be careful where you click ;)


When the new land impact system is used, size and complexity of prims is taken into account. A large hollow torus has four times the land impact cost of a small solid torus. Equally, an object composed of several cubes can have a much lower land impact than single complex prim. Since buildings are often based on cubes, this can work in our favour :)

For example, this wall with cutout windows consists of eight normal prims and four mesh objects (you could probably mesh it out completely, but in this case we have a pile of prims.) Linked up with the wall set to convex hull it has a land impact of six. You can use the new settings without involving uploaded mesh at all though.


There are currently four physics options for objects/prims:

Prim: the physics shape follows the prim, and each prim is treated as having a land impact of 1 whether complex or simple.  It’s possible to have low ‘cost’ items that are actually quite physics heavy compared to simpler items with more prims.  Basically SL as we have known it for years.

None: the prim is ignored by the physics engine and purely decorative. Child prims within an object can be set to ‘none’, reducing physics costs. A root prim cannot be set to ‘none.’

Convex Hull: produces a solid physics shape that fills in holes and results in a simpler object for the physics engine to handle.

Convex hull or None are helpful settings for simplifying certain types of build, and getting a handy reduction in land impact costs. For example:

This wall object with cutout windows has eight prims, but it can be set to Convex Hull so that it is treated as a single shape. The land impact cost drops to 4.

This shopfront has prim detail, but can be set to ‘convex hull’,  reducing cost from 8 to 4.  There is a cut overhang at the top, however, so the new physics shape might extend slightly outside the object. (The fit seems to vary a bit – I have seen wonky shapes with a good fit.)


Wall and poster with four prims. The posters can be set to  ‘None’, reducing land impact to 2.



A hollowed cube set to convex hull.  An av can’t pass through because the new physics shape fills in the hole in the middle.

An object comprising one small sculpt object and one cube.   The root cube can remain physical while the sculpt’s physics shape is set to None. In this case, the linked cube and phantom prim have a land impact cost of 1. (Larger sculpts generally cost more when linked in this way – sometimes a lot more! Same with other complex single prims.)

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  1. Blackberry Harvey Blackberry Harvey April 3, 2012

    Ooh, I didn’t know sculpts could be set to None. Thanks!

  2. Elilka Sieyes Elilka Sieyes April 3, 2012

    Yep, though for anything other than small sculpts the land impact will tend to go up. Useful for adding clutter though, when there’s really no need for it to be part of the physics shape (and sometimes has a bothersome bounding shape anyway.)

  3. Avariel Falcon Avariel Falcon April 3, 2012

    So with some care the windowed walls of the power station could have their effective prim count reduced? I will test this on a copy on my build grid later.

  4. Mr Tenk Mr Tenk April 3, 2012

    we can has more posters!

    but how is remaining capacity a number more than 15,000? this is becoming strange calculus.


  5. Mr Tenk Mr Tenk April 3, 2012

    interesting! mileage does vary, but i managed to free up a few prims. this is very valuable on small single prim lots. i wasn’t able to get the reduction effect on sculpted windows, but it works great for flat glass inserts.



    • Blackberry Harvey Blackberry Harvey April 3, 2012

      Yep, when I have a wall of many windows, I can make the wall sections and the window pane Convex Hull.


      Be very careful not to link Convex Hull prims to Prim-physics prims, especially sculpts and scripted prims.  Always keep them as separate linksets.  Otherwise you could fill up a sim allowance with just a few prims.

    • Elilka Sieyes Elilka Sieyes April 4, 2012

      Yep, it is encouraging me to switch to mesh for window detail etc…getting there…

  6. Mr Salazar Jack Mr Salazar Jack April 3, 2012

    Thank you for the documentation!

  7. Professor Parx Professor Parx April 4, 2012

    This will come in very useful for things like lab equipment. Thank you for pointing this out.

  8. Avariel Falcon Avariel Falcon April 4, 2012

    A quick test shows it is possible to reduce the land impact of the power station walls to half the original level!

    Time for a refresh! ^_^

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