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This is a very powerful, low-level node that lets experts who are familiar with VEX tweak attributes using code.
This node corresponds to the Attribute VOP SOP, but uses a textual VEX snippet instead of a VOP network.
This node requires that you understand the vex language. It is very easy to write incorrect code using this node.
This node runs the snippet on the detail or every point/primitive/vertex (depending on the Class parameter) in the input geometry. The snippet can edit the input geometry by changing attributes. It can access information from other geometry using attributes and VEX functions.
Press on the node to see any error output from the snippet.
You can use the VEX function
chto evaluate parameters. The path is relative to this node (
ch("parm")will evaluate the parameter
parmon this node). This evaluation will be done at the current time.
Unlike the Attrib Create SOP, this does not use local variables. Further, all backtick expressions and
$Fvariables will be evaluated at frame 1, not the current time. Use
A subset of points in the input geometry to run the program on. Leave this blank to affect all points in the input.
What the group is made of.
What class of geometry attributes to modify. No attributes will be bound, except read-only detail attributes, when set to Numbers.
Iteration count when Run Over is set to Numbers.
@elemnum will be the iteration number from 0 (inclusive), to this number (exclusive),
@numelem will be this number.
No attributes will be bound when in this mode, except read-only detail attributes.
Thread Job Size
Maximum number of iterations run at a time in any thread when Run Over is set to Numbers. If this is greater than or equal to Number Count, the VEX code will run single-threaded. If each iteration will take a very large amount of time, set this to 1, but there is quite often a significant advantage to making this value larger, especially if each iteration will take a small amount of time, and if any work can avoid being duplicated by running multiple iterations at a time.
A snippet of VEX code that will manipulate the point attributes.
You can use
@variable_name syntax to access geometry attributes.
Attributes to Create
Only create attributes if their names match this pattern. The default pattern allows any attribute to be created.
You can restrict the created attributes by replacing the
* with a list of allowed names.
Bound attributes, such as
vtxnum, cannot be created and will be ignored.
Requires that you declare
@ bindings in snippets as prototypes before using them. This applies to both attributes (for example
@Cd) and "convenience" bindings such as
@Frame. For example:
// Declare bindings int @ptnum; float @Frame; vector @Cd; // Use bindings after declaration int pointnum = @ptnum; float red = @Cd / @Frame;
Automatic binding with the
@ syntax can be convenient, but as your scene becomes more complex there is the risk that a typo in an
@ binding will silently just bind a non-existent attribute.
Autobind by Name
Automatically bind attributes to parameters by name. If for some reason you need CVEX parameters to have different names than the corresponding attributes, turn this off and use the Number of bindings parameter to set up mappings between Attribute name and VEX parameter.
Integer attributes will bind to integer parameters. Float attributes will bind to float, vector, point, matrix, or matrix4 depending on their tuple size. String attributes will bind to strings.
Autobind Groups by Name
Automatically bind any groups to the integer parameter prefixed with
group_. If for some reason you need CVEX parameters to have different names than the corresponding groups, turn this off and use the Group Bindings parameter to set up mappings between Group Name and VEX Parameter.
Manually specifies the bindings of each group.
Evaluation Node Path
VEX functions like
ch() usually evaluate with respect to this node.
Enter a node path here to override where the path search starts from.
This is useful for embedding in a digital asset, where you want searches
to start from the asset root.
When a VEX parameter is exported, the bound attribute will be created if it doesn’t exist. This pattern can be used to override the export option on the VEX shader to avoid writing to or creating certain attributes. The pattern matches the VEX parameter, not the bound attribute. The attribute will still be bound for reading.
Update Normals If Displaced
If points are being run over, and the
P attribute is written to,
N attribute is not written to, any incoming normals will
become out of date. When this option is set, vertex and point
normals will be updated when this occurs.
Attribute To Match
Which attribute to use for matching. This attribute must be
present on both inputs for matching to be done. Otherwise matching
is done by element number (ie, point number when running over points).
The attribute should either be an integer or string attribute.
This controls how the
opinput#_ virtual bindings connect.
You can use
v@opinput1_Cd, for example, to read the second input’s
Compute Results In Place
When compiled the Attribute VOP is able to work on the incoming geometry in place without making a copy of it. This can speed up processing as one less copy is made, but requires that the VEX code doesn’t bind for writing any attributes that are read from the first input.
Output Selection Group
The name of a group to use as the output selection. When the highlight flag is enabled for this node, this group will be the output selection used by later modeling tools (if it exists).
VEX can evaluate at 32-bit or 64-bit precision. 64-bit provides higher accuracy, especially for transforms.
Incoming attributes will preserve their original precision, so using 64-bit VEX on 32-bit positions will convert them to 64-bit, apply the operation, then convert back to 32-bit when writing out.
The auto mode will switch between 32-bit and 64-bit depending on the preferred precision of the incoming geometry. When run in 64-bit precision, any created attributes will be 64-bit. When run in 32-bit any created attributes will be 32-bit. Use Attribute Cast to change the preferred precision.
This example shows you how to add a single new point using the Attribute Wrangle SOP and the addpoint() vex expression.
This example shows you how the primintrinsic method can be used to obtain the number of vertices for a primitive. The point corresponding to a vertex can be obtained by first translating a primitive-vertex offset pair to a linear vertex value then looking up the point referenced by the linear vertex value.