Mesh/Calculating prim equivalent weight

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Revision as of 12:18, 15 July 2011 by Rand Linden (Talk | contribs)

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Prim equivalence is Second Life's mechanism for calculating the computational weight of an object in terms of traditional prims. All mesh objects and all objects with a physics shape type other than Prim have a prim-equivalent weight for the purposes of consuming a land parcel or region's prim capacity. Therefore, an object with a prim equivalent weight of 42 is equal to a linked object composed of 42 normal prims.

By using prim equivalence, we make sure that mesh objects and traditional prim objects receive fair shares of Viewer and server resources, encouraging content creators to continue designing prim-efficient objects even if they're working with uploaded meshes.

How it works

For each object in the Second Life world, Second Life compares three important performance factors: streaming weight, physics weight, and simulation weight. It then chooses the highest of these weights and assigns it to the mesh as the mesh's prim-equivalent value.

Normally, only the resulting prim-equivalent value is visible in the Viewer. To view physics weight, set ShowAdvancedBuilderOptions to TRUE in the DEBUG SETTINGS window.

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

  • Streaming weight: Calculated by determining how much bandwidth is required to download and view the object. Larger and more visually complex objects have a higher streaming weight. You can reduce the streaming 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 streaming or simulation weights.
  • Simulation 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 simulation weights.

How to find an object's prim-equivalent value

When an object is rezzed inworld, you can find its prim-equivalent value by editing it and viewing the Build Tools window. Next to the number of objects and prims you have selected, a third number labeled prim equivs indicates the prim-equivalent value.

Prim equivalent weight.png