TYPE: POP
TITLE: AttribTransfer POP

AttribTransfer is used to sample point or vertex attributes from a reference
geometry.  Vertex attributes are changed to point attributes for the
particles.  Moreover, all of the vertices referencing a particular point
are considered to be a single sample and the value of the first one is
used as the sample value.

The attributes may be explicitly specified in the corresponding attribute text
field. Exclusion is specified by preceding an attribute name with the carat
character (^). All available attributes can be specified by leaving the field
blank or entering an asterisk (*) in the field. If the first attribute is
preceded by a carat then all attributes are assumed to be transferred except
where explicitly excluded. In all other cases, no attributes are assumed
except where explicitly included.

This operator can operate in two modes (controlled from the "Conditions"
folder).

Weighted Average Mode:
  Destination attributes are interpolated from the source attributes using
  a weighted proximity technique. Proximities used in attribute calculations
  are determined as the distance between the destination point and the
  candidate source points.

  There are several parameters that control how the source attributes are
  combined to give the resulting destination values.  The "Kernel Function"
  can be set to any available metaball kernel.  The kernels are parameterized
  by distance and typically give more weight to samples that are closer.
  The "Max Sample Count" parameter specifies the maximum number of samples
  to interpolate from.  A maximum sample count of 1 gives nearest neighbour
  interpolation.  The "Kernel Radius" specifies the drop-off radius for the
  kernel function.  As the kernel radius parameter approaches zero, the
  filter resembles nearest neighbour interpolation.  As the kernel radius
  parameter increases, the filter approaches uniform weighting.  The
  "Distance Threshold" parameter specifies the maximim distance source
  points must be to be considered.  Switching off this option indicates
  that no points/vertices will be excluded based on distance.  The
  "Initialize Attributes to Zero" means that the attributes will be set
  to zero if no samples are found, otherwise, the attribute will maintain
  the previous value.

Smoothed Particle Sampling Mode:
  This mode is based on the SPH (Smoothed Particle Hydrodynamics) model.
  Each attribute is sampled using the formula:
      A(r) = Sum(over all j) (m[j] * A[j]/p[j] * Wh(r-r[j])

      where:
        - r is a position in space,
        - Wh is a normalized smoothing kernel over a radius h,
        - m[j], A[j], p[j], r[j] are the mass, attribute value,
          density, and position of sample j.

  Several parameters control the computation of this sum.  The 
  SPH Kernel Function" specifies the normalized smoothing kernel to use.
  The "SPH Kernel Radius" specifies the maximum distance over which the kernel
  acts.  The actual radius h used above is computed based on this value for
  each kernel.  The "Coefficient Attribute" specifies the name of the point 
  attribute on the sample geometry to use as the m[j]/p[j] factor in the above
  formula.
	
Parameters:
    Activation                  - Turns the POP on and off            /activate
    Source Group                - Subset of points to act on
    Geometry Source             - Specifies where the POP should get the
                                  collision Object and SOP.
    SOP                         - The Surface Operator to use.
    SOP Group                   - A subset of the geometry in the SOP to use
    SOP Group Type              - How to interpret the SOP Group parameter

Attributes:
    Points                      - Source point attributes to transfer.
    Vertices                    - Source vertex attributes to transfer.

Conditions:
  Weighted Average:
    Kernel Function             - Interpolating metaball kernel.
    Kernel Radius               - Radial extent of the kernel function.
    Max Sample Count            - Number of closest samples to interpolate from.
    Distance Threshold          - Radial extent of the filter domain where the
                                  final attributes are determined from the
                                  source selection only.
    Initialize Attributes to Zero - Zero attributes when there are no samples.

  Smoothed Particle Sampling:
    SPH Kernel Function         - Normalized smoothing function.
    SPH Kernel Radius           - The extent over which the samples are
                                  smoothed.  Samples outside this radius
                                  do not contribute.
    Coefficient Attribute       - An attribute that is used as a coefficient
                                  of the term for a sample in the sum.

See also: Attribute
