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This does what a Point SOP with a
chopci() function can
do, but is much faster.
There are two different ways of importing attribute values, controlled by the Method parameter. Each method requires that the CHOP channels are set up in a specific way.
The Static method is used for modeling a surface without any animation.
A single channel is required for each attribute component, and its length must
be the number of points in the geometry or specified point group(s).
P in a geometry with 100 points, three channels with 100 samples
would be required.
The second method, Animated, provides animation for the attribute.
There must be one set of channels for each attribute component, and this set
must be indexed from 0 to the number of points minus one, by adding a numeric
suffix. For the attributes
Cd Alpha with 3 points, you would need
r0 g0 b0 a0 r1 g1 b1 a1 r2 g2 b2 a2. The length of the channels determines
the animation length.
The Channel SOP can also update portions of the geometry. By using point groups from the incoming op, the channels can be inserted only into the groups' points. To modify only some components of a vector, use brackets to specify the component index (P(1) refers to the Y value of the point position).
Modify only the points within this point group. If blank, all points are modified.
Retrieves the sample data from this CHOP.
The sample data fetch method.
Uses one channel for each attribute, and all points use this channel (the first point resides at index 0, the next at 1, and so on). The length of the channels should be at least the number of points modified.
Uses one channel per attribute per point. The channels show
the animation of each point’s position/attribute values. The channels
must have a numeric suffix starting at zero, with at least one channel
for each point. For vector attributes, there should be one channel per
component, per point (eg. for the
P attribute, the channels must
follow the pattern
tx0 ty0 tz0 tx1 ty1 tz1 tx2 ty2 tz2...).
The names of the CHOP channels that will modify the attributes. There must
be at least one channel name per attribute. Vector attributes may require
multiple channels for all components specified in Attribute Scope
or Matrix Attribute Scope.
(eg. an attribute scope of
P should have a channel scope with three
A string list of attributes to modify. For only one component of
an array, such as the Y value of the point position, specify the
number of elements in brackets (in this case,
P(1) would be
The common attributes are:
|P||Point position (X, Y, Z), 3 values|
|Pw||Point weight, 1 value|
|Cd||Point color (red, green, blue), 3 values|
|Alpha||Point alpha, 1 value|
|N||Point normal value (X, Y, Z), 3 values|
|uv||Point texture coordinates (U,V,W), 3 values|
Matrix Attribute Scope
A string list of 3×3 and 4×4 matrix attributes to modify. These matrices are computed using translation, rotation, scale, and shear channels. If Use Shear Channels is disabled, then shear channels will not be used to compute the matrices.
3×3 matrices are computed using up to 9 channels: xyz channels for each of the matrices' rotation, scale, and shear components in that order.
4×4 matrices are computed using up to 12 channels: xyz channels for each of the matrices' translation, rotation, scale, and shear components in that order.
The order in which to apply the euler rotation angles when recomputing all matrices that are specified within Matrix Attribute Scope.
Use Shear Channels
If this option is selected, then shear channels will be required to compute all matrices specified within Matrix Attribute Scope.
Organize by Attribute
(Applies to 'Animated Method' only.)
Will reorganize the fetched channels by the value of this
attribute. A common example is the
id attribute found in
particles. A channel is built for each unique id since the
number of points may vary.
This is a simple example of how to utilize the Channel SOP to bring information from CHOPs into SOPs and apply it to geometry.
We use an animated sphere and create a lag in the animation of selected areas of the sphere.
In a CHOP network, the Geometry CHOP brings in point position data of the sphere geometry and runs it through a Lag CHOP for the delaying effect. The Channel SOP then references the Lag CHOP and applies the point data back to the selected areas of the original NURBS sphere.