Carve polygon curves by attribute or expression

polycarve.py

node = hou.pwd()
geo  = node.geometry()

u_parm = []
for pr in geo.prims():    
    node.setCurPrim(pr)    
    u_parm.append(node.evalParm("u_carve_start"))

    
# Reference u parameter used as the carve parametric location on the curve
# u = hou.ch("../null2/parm")

# Create u attribute used to test against u parameter
vu = geo.addAttrib(hou.attribType.Vertex, "u", -1.0)

# Create storage for the original primitives
prims_orig = []

# Loop over the input primitives
for prim in geo.prims():
            
    # Store primitve 0 for each primitive instead of each primitive
    # This is due the primitive count updating as each primitive is deleted 
    prims_orig.append(geo.prims()[0])
    
    # Get the position(x,y,z) from the u location parameter on the curve
    u = u_parm[prim.number()]
    p_u = prim.positionAtInterior(u, 0, 0)

    # Set the vertex attribute vu - 0-1 over each primitive
    nv = prim.numVertices() - 1
    for v in prim.vertices():
        val = float(v.number()) / nv % nv
        v.setAttribValue(vu, val)

    # Store the points of each primitve above and below the u paramete(user defined)
    pts_below_u = []
    pts_above_u = []
    for v in prim.vertices():
        if u > v.attribValue(vu):
            pts_below_u.append(v.point())
        elif u < v.attribValue(vu):
            pts_above_u.append(v.point())            

    # For each prim create 2 new points - one for the above u prim and one for below prim
    # Set both point positio to the u parameter position
    new_pts = []
    for i in range(2):
        point = geo.createPoint()
        point.setPosition(p_u)
        
        new_pts.append(point)
        
    pts_below_u.append(new_pts[0])
    pts_above_u.insert(0, new_pts[1])
    
    # Now to create the new prims for above and below the u parameter based on the points stored
    # in pts_above_u and pts_below_u resp.
    new_polys = []
    for i in range(2): 
        poly = geo.createPolygon()
        poly.setIsClosed(False)
        
        new_polys.append(poly)

    for i in pts_below_u:
        new_polys[0].addVertex(i)
        
    for i in pts_above_u:
        new_polys[1].addVertex(i)

    # Refernce toggle parameters that define whether to keep primitives above or below 
    keep_above = hou.ch("keep_above")
    if keep_above == 0:
        geo.deletePrims([new_polys[0]])
    
    keep_below = hou.ch("keep_below")
    if keep_below == 0:
        geo.deletePrims([new_polys[1]])


# Delete the original primitves
for pr in prims_orig:
    geo.deletePrims([pr])