Hello,

Since H19.0 we have the Attribute Combine SOP [www.sidefx.com]. It combines float attributes by layering different float operations (add, subtract, multiply...). It works with float vectors, but it doesn't support vector operations such as cross product, vector projection/rejection, or rotation around a vector/axis.

This gave me the idea to make an HDA called Attribute Combine Vector focused on combining vector operations.

In this very first prototype, the HDA applys a single operation from this list :

• Cross : cross product between destination and source vectors
  
• Project : projection of destination attribute to the direction of source attribute (it makes vectors aligned in same direction)
  
• Reject : rejection of destination attribute from source attribute (it makes them orthogonal)
  
• Rotate : rotates destination attribute around source attribute with a given angle
  

Examples of where this HDA can be usefull :

• (Rotate) Twisting curves, by rotating @out vector around @tangent vector, and updating positions with rotated @out vector
  
• (Reject) Displacing curves in orthogonal direction (electric strand), by rejecting displacement direction from tangent vector.
  
• (Project) Inherite a vector direction and keep magnitude
  

For next versions, I will be working on these features :

• Layering multiple operations
  
• Adjust/process source attrib before applying operation
  
• Support all attribute classes (it only works on point attribs for now)
  
• Apply this node on a given group only
  
• Creation of xform and/or orient attributes as part of post-processing
  

v0.2

• All attribute classes are supported (point/primitive/vertex/detail)
  
• Multiple combines can be done
  
• Choice between a vector attribute or a constant vector parm to be combined
  
• Added new operations : Copy, Add, Subtract
  
• Added post-processing : normalize, and overal scale
  

Still TBD :

• Adjust/process combined attrib before applying operation
  
• Apply this node on a given group only
  

Creation of xform and/or orient attributes may come in another HDA, rather than being part of post-processing of this HDA.

v0.5 : available to dwonload and test

• Fixed a couple of bugs and performance issues, and reworked the structure of the VEX script so it's easier to maintain
  
• Added a menu to select an adjustment operation on the combine vector (normalize or scale)
  
• Added group parm so the combine operations apply only on selected group
  

Attached a hip file demonstrating how to bend and twist curves using this tool.

v0.6

• Added a blend mode to select between spherical (houdini 20) [www.sidefx.com] and linear [www.sidefx.com] interpolation
  
• Added a toggle to keep magnitude when performing "Project" or "Reject" operation.
  

And here's a hip to demonstrate vector operations by visualizing three vectors (operands and result).