Houdini VEX: How to Flatten or Align Points to a Master Reference Group

When building procedural modeling setups, ground alignment tools, or mechanical rig offsets in SideFX Houdini, you frequently need to snap or level geometry against a specific anchor point selected by an artist or upstream procedural rule.

Using an explicit point group (like master_sel) lets you decouple your VEX logic from hardcoded point numbers (@ptnum == 0). Even if upstream topology changes and point counts shuffle, the reference logic remains completely intact.

Here is an analysis of how expandpointgroup() works in a Point Wrangle, a major performance optimization to keep in mind, and how to expand the snippet into full 3D alignment.

The Core VEX Pattern

This snippet queries all points inside the master_sel group, takes the first available index as the master anchor, and blends each evaluated point’s vertical position ($Y$) toward that master point's height.

// Run over: Points
// Retrieve an array of all point numbers belonging to 'master_sel'
int master_pts[] = expandpointgroup(0, "master_sel");

// Guard against empty groups to prevent out-of-bounds indexing
if (len(master_pts) > 0) {
    // Select the first point in the group as our master anchor
    int master_pt = master_pts[0];
    
    // Extract the spatial coordinates of the master point
    vector master_pos = point(0, "P", master_pt);
    
    // User parameter to control blend strength (0.0 = untouched, 1.0 = fully flattened)
    float blend = chf("blend_amount");
    
    // Smoothly interpolate current Y elevation toward the master's elevation
    @P.y = lerp(@P.y, master_pos.y, blend);
}

⚠️ Performance Check: The Per-Point Evaluation Cost

While this script works cleanly on small models, there is an important performance consideration for large meshes:

A Point Wrangle runs its entire code block in parallel for every single point on your geometry. If your mesh has 500,000 points:

expandpointgroup(0, "master_sel") allocates an array and evaluates the group membership table 500,000 times.
point(0, "P", master_pt) queries the attribute dictionary 500,000 times for the exact same coordinate.

The High-Performance Alternative: Detail Pre-Pass

If you are working on dense meshes, extract the master coordinate once in a Detail Wrangle before your Point Wrangle, or store it as a detail attribute:

// STEP 1: Detail Wrangle (Run Over: Detail)
// Reads the master point once for the entire geometry
int master_pts[] = expandpointgroup(0, "master_sel");
if (len(master_pts) > 0) {
    setdetailattrib(0, "master_P", point(0, "P", master_pts[0]), "set");
}

// STEP 2: Downstream Point Wrangle (Run Over: Points)
// Pure parallel position update with zero array overhead
vector master_pos = detail(0, "master_P", 0);
float blend = chf("blend_amount");

@P.y = lerp(@P.y, master_pos.y, blend);

Expanding Beyond Height: Full 3D Snapping & Axis Masking

If you want to use this master reference point to align more than just elevation ($Y$), you can parameterize which axes get affected using a toggle vector in VEX:

// Run over: Points
int master_pts[] = expandpointgroup(0, "master_sel");

if (len(master_pts) > 0) {
    vector master_pos = point(0, "P", master_pts[0]);
    float blend = chf("blend_amount");
    
    // Vector toggles to independently choose axes to lock (e.g., {0, 1, 0} for Y only)
    vector axis_mask = chv("axis_mask"); 

    vector target_pos = @P;
    
    if (axis_mask.x > 0.5) target_pos.x = master_pos.x;
    if (axis_mask.y > 0.5) target_pos.y = master_pos.y;
    if (axis_mask.z > 0.5) target_pos.z = master_pos.z;

    @P = lerp(@P, target_pos, blend);
}

This setup provides an intuitive leveling and projection workflow: pick any vertex on your model, assign it to master_sel via a Group SOP, and instantly level, center, or align arbitrary mesh regions relative to that single reference anchor.