Will changing the physics shape type from prim to convex hull reduce my buildings land impact?

[WanderleiMMA]

I was under the impression that changing an objects physic shape type from prim to convex hull would reduce the number of prims in an object thus reduce the land impact.  That was until just a few minutes ago when I linked all the objects in my building together.  I changed the buildings physic shape type from prim to convex hull right before linking all the peices, yet when I view it's prim count, it reads 24....  Shouldn't the prim count be reduced to zero?

[Theresa Tennyson]

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WanderleiMMA wrote:

I was under the impression that changing an objects physic shape type from prim to convex hull would reduce the number of prims in an object thus reduce the land impact.  That was until just a few minutes ago when I linked all the objects in my building together.  I changed the buildings physic shape type from prim to convex hull right before linking all the peices, yet when I view it's prim count, it reads 24....  Shouldn't the prim count be reduced to zero?

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Prim counts will never be reduced to zero. "Prim" is used as shorthand for an object that can't be broken down into smaller objects - if the linkset was made from 24 sub-objects it will always say that as the prim count. However, the land impact score can be lower (or higher) than the prim count, and the land impact score is now used instead of prim count to determine the number and complexity of objects a lot can hold.

Depending on the build, setting the physics type to convex hull can reduce the land impact, but there are other factors that can occur as well - for instance, with the new accounting that came in with convex hulls, conventional solid shapes (the legacy "prims") usually score lower but a sculpt usually scores higher.

 

[Madelaine McMasters]

Hi Wanderlei,

I've never quite figured out exactly how it all works, but setting the root prim of a linkset can cause the Land Impact to explode, particularly if one or more of the children are tori. You must be careful to set the physics shape for such child prims to "none". And, if I recall correctly, setting the physics type of box (and perhaps other simple shape) prims to "none" prevents them from being reduced below one in land impact. So, linkset of four box prims would have a land impact of two if the root prim physics type is set to "convex hull" and the other three boxes are set to "prim". Setting any of the child boxes to "none" would increase the land impact towards the full four. Changing one of the boxes to a torus with physics type "prim" would shoot the land impact up by 67 or so (as I recall). The torus would have to be set to "none".

 

[Lindal Kidd]

As the others say, changing the physics shape type of a linkset can reduce the land impact, often by half, or nearly.  It can also INCREASE the land impact, depending on what prim shapes are in the linkset.  The easiest way to find out which is just to try it and see.

The other thing about using the Convex Hull physics shape is that it can make a building unusable.  If there is a door formed by hollowing out a prim, that hollow area will be "closed off" by the change in physics shape, which treats a hollow object as having no hollow part.  If you make a wall or a fence by hollowing out a cubic shape, you can walk right over the top of it as if it were a solid object.

Whenever you use Convex Hull to save on land impact, you must be careful which prims you include in the linkset!  And test your build by walking all through it, looking for places where you are unexpectedly blocked.