Mesh/Mesh physics
This article explains the physics behavior of imported mesh objects and, to a lesser extent, triangle-based prims under new accounting
Mesh Assets: Physics Shapes and Decompositions
A valid mesh asset will contain:
- A single convex hull with no more than 256 vertices
- All assets will contain a single convex hull with no more than 256 verts that will be used:
- If the physics shape type is set to "Convex Hull"
- If there is no decomposition and the prim is in motion of any kind
- All assets will contain a single convex hull with no more than 256 verts that will be used:
- (Optional) An additional convex decomposition containing up to 256 vertices per hull and up to 256 hulls
- (Optional) A mesh designated specifically for use in the physics engine
Mesh Asset Header Identifiers
- "physics_shape" - optional, reference to mesh to be used in place of visible mesh for collisions
- "decomposition" - required, reference to one or more convex hulls of no more than 256 vertices each to be used in place of physical mesh
Decomposition Data Block Members
- "Min"/"Max" - required, LLVector3, floating point domain to decode binary U16 data to
- "HullList" - optional, binary U8, list of point counts of hulls stored in "Position". '0' is special cased to indicate 256 points.
- "Position" - optional, binary U16, list of positions of points in hulls that make up the multi-hull decomposition of this mesh
- "Hull" - required, binary U16, list of positions in a single convex hull of no more than 256 verts that wraps entire physical mesh
- "Hull" is generated automatically at upload time by feeding the physics shape, mid lod, or high lod into LLConvexDecomposition:
- Stage 1: Mode set to "Preview"
- Stage 2: Simplify mode set to "Retain%", retain percentage set to zero.
- "Hull" is generated automatically at upload time by feeding the physics shape, mid lod, or high lod into LLConvexDecomposition:
Physics Representation for Rezzed Prims
Exceptions:
- A non-physical prim which is moving (e.g., via llSetVelocity) is treated as dynamic until all motion has ceased for several frames
Notes:
- Meshes should initially rez with shape type "Convex Hull" as the physics resource cost (see below) for detailed physics meshes may be very high and depends on the prim's scale
- A mesh object uploaded without an explicit physics shape or decomposition cannot be changed to shape type prim and will always be represented as a single convex hull bounding the mesh.
- At any time prior to release to Agni, we may choose to require a physics mesh be provided
- No linkset or prim with a pure physics resource cost exceeding 32.0 may be set physical
Physics Resource Cost
There will be a new physics-based resource cost (PRC) computation. It will be used directly in determining whether objects can be made physical and will be a factor in determining total prim resource cost for a parcel or region.
Opt-In
A prim or linkset will have a physics resource cost (PRC) computed if and only if it contains:
- A mesh asset
OR
- A prim with a physics shape type other than "Prim".
Formula
Notes:
NOTE: The third term in the above equation (the one governing triangle mesh cost) is no longer correct. The actual math is now substantially more complex but the result is easier to understand.
Triangle Mesh Overview Briefly, the cost of a physics triangle mesh now depends on two things: (1) The number of triangles and (2) The "average width" of those triangles at the mesh's current scale. The width of a triangle is defined as the width (smaller side) of the tightest rectangle you can draw around the triangle. The average is computed "harmonically" (1/avg = 1/a + 1/b + ... ), which was done to favor weight smaller triangles more heavily. This average is then clamped to be between 0.001m and 20m (right now) and then used in the simple formula:
C = MAX( constant * num_triangles / triangle_width, minimum_mesh_cost )
The value of "constant" has been selected so that for a flat mesh with 0.5m wide triangles, each triangle costs 0.1 units with a minimum total mesh cost of 0.5 units. However, these values are subject to change.
Please note that the computed triangle_width is only an APPROXIMATION of the exact triangle width in most cases.
IMPORTANT: For triangle-based shapes, the cost grows inversely with the scale of the prim! More specifically, it varies linearly with triangle density. This is important as triangle density has a huge impact on physics performance. Also note that for large meshes, the density multiplier may be less than unity, resulting in a discounting effect.
The resulting number may not be an integer. A prim with physics shape type "none" does not contribute to the pure PRC!
Simulator resource cost (SRC)
Simulator resource cost (SRC) reflects the cost of "managing" a linkset or prim. For example, this includes things like sending out updates about the object's state to relevant viewers.
SRC = simulation_factor * active_cost * ( num_objects + (linkset_factor/num_objects) )
- simulation_factor and linkset_factor are constants for tuning the overall cost.
- active_cost is always either active_cost_high or active_cost_low (two additional tuning constants). The high value is used if the object is scripted, physics-enabled, or both.
What this means is that to minimize SRC, you should:
- Group your prims into as few linksets as possible
- Minimize the number of linksets you need to script or make physical
- Minimize the number of unlinked or small linksets
- If an object is physical or scripted, keep its linkset as small as possible (but don't create multiple scripted unlinked prims instead!)
Region resource cost (RRC)
Regions and parcel limits for prims/linksets will use the region resource cost which is the greater of the SRC and the bandwidth resource cost (also known as streaming resource cost, described elsewhere).