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  • Calculating land impact

    Jeremy Linden

    By Jeremy Linden (edited)

    Land impact is Second Life's mechanism for calculating the computational weight of an object against land usage limits. All mesh objects, all objects that contain mesh content, and all objects with a physics shape type other than Prim use an algorithm for determining land impact that is not dependent upon the number of prims in the object. 

    By using an algorithm that considers each object's impact on Second Life's performance, we make sure that mesh objects and traditional prim objects receive fair shares of Viewer and server resources, encouraging content creators to continue designing performance-efficient objects even if they're working with uploaded meshes.

    Tip: The terms land impact and land capacity replace prim count and prims parcel supports, but the numerical values remain the same for legacy objects made only of standard prims.  Therefore, a linked object composed of 42 normal prims with their physics type set to Prim has a land impact of 42.

    How it works

    For each object in the Second Life world, Second Life compares three important performance factors: download weight, physics weight, and server weight. It then chooses the highest of these weights and assigns it to the object as that object's land impact rating.

    Here's a very quick overview of the different weights; for more information on each, follow the links below:

    • Download weight: Calculated by determining how much bandwidth is required to download and view the object. Larger and more visually complex objects have a higher download weight. You can reduce the download weight of complex objects by generating or uploading less complex meshes for differing levels of detail when you upload a model.
    • Physics weight: Calculated by determining the complexity of the object's physics model. You can reduce the complexity of a mesh's physics model by using the analysis and simplification tools in the Upload Model window, by uploading your own less-detailed physics model, or by choosing a different physics shape type, such as Convex Hull, on the Features tab of the Build Tools window. Vehicles must have a physics weight of 32 or lower, but may have higher download or server weights.
    • Server weight: Measures the impact an object has on Second Life's server resources. Objects that are composed of many prims and have physics enabled and/or contain scripts tend to have high server weights.

    When is an object's land impact calculated using the new algorithm?

    Legacy prim objects have a land impact rating equal to the number of prims they contain.  However, any object's land impact is calculated using download, server, and physics weights if it meets any of the following conditions:

    • The object is an uploaded mesh.
    • The object is linked to an uploaded mesh.
    • The object, or any part of the object, has a physics shape type other than Prim.  You can change this on the Features tab of the Build Tools window.
    • The object has a normal or specular map applied to it.

    How to find an object's land impact rating

    When an object is rezzed inworld, you can find its land impact rating by editing it and viewing the Build Tools window. Next to the number of objects you have selected, a number labeled "land impact" indicates the land impact rating.

    You can view detailed information about your selection, including prim count, weights, and land impact, by clicking the More info link in the Build Tools window.

    Land impact.png

    Edited by Jeremy Linden

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    @Perrie Juran

    The software we use to publish articles sometimes doesn't interpret HTML correctly and inserts unexpected characters.  Thanks for noticing!  I've fixed it.
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    Jenni Darkwatch

    Would it be possible to expand this article to at least link to a KB article explaining physics shapes? Also, download weight can vary for different object sizes. That might be a good point to add.

    The debug option "ShowAdvancedBuilderOptions" does not seem to exist, at least not on the LL release viewer (Second Life 3.2.4 (246439)).

    When a script moves an object (and possibly otherwise manipulates an object) it's impossible to get the advanced floater to display the actual weights, it perpetually loads. I'm not sure if that's a bug or not, but stopping the scripts in the object fixes it. Easy to test with the LL premium boat gift. If it's intentional it might be worth mentioning that caveat.

    When inspecting individual parts of a linkset it is not possible to determine the individual LI of the components. I've got a vague suspicion why that's the case. Might also be worth mentioning :)
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