VEX functions for procedural modeling

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29.05.2022

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VEX

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Houdini 19.0

1 Introduction

An ongoing collection of VEX functions useful for procedural modeling.

2 Smooth minimum

Calculates the smooth k minimum of two distances a and b.

function float smooth_min(float a, b, k){
    float h = max(k - abs(a - b), 0.0) / k;
    float d = min(a, b) - h * h * h * k * (1.0 / 6.0);
    return d;
}

3 Shape

Creates shapes based on u, eg. the curveu attribute from the resample node, or a component of the UV coordinates from the sweep node.
Recommended values: center 0.5; weight <= 0.5; power > 0.0; amount > 0.0.

float center = chf('center');
float weight = chf('weight');
float power = chf('power');
float amount = chf('amount');

function float shape(float c, w, p, a, u){
    u = abs(u - c);
    if(u > w) return 0.0;
    u /= w;
    float d = 1.0 - u * u * (3.0 - 2.0 * u);
    d = pow(d, p) * a;
    return d;
}

float u = vertexprimindex(0, i@vtxnum) / (primvertexcount(0, i@primnum) - 1.0);

v@P.y = shape(center, weight, power, amount, u);

4 Point density

Measures point density within a certain radius.

// POINT WRANGLE
int samples = chi('samples');
float radius = chf('radius');

int pts[] = pcfind(0, 'P', v@P, radius, samples);
float dens = len(pts) / float(samples);

f@dens = dens;

5 Curve U

Parametric U on curve:

float u = vertexcurveparam(0, i@vtxnum);

Symmetrical U:

float us = 1.0 - abs(u - 0.5) * 2.0;

6 Maximize UVs proportionally

// vertex wrangle
string geo_uv = geounwrap(0, 'uv');
vector bb = relbbox(geo_uv, v@uv);
vector size = getbbox_size(geo_uv);
float aspect = size.y / size.x;
bb.y *= aspect;
v@uv = bb;

7 Ping pong

float u = v@P.x;
v@P.y = 1.0 - abs((abs(u) % (2.0)) - 1.0);

8 Smoothly shorten curves

This will shorten and cubicly interpolate in-between point positions an open curve by a custom percentage.

float amount = chf('amount');

float u = vertexcurveparam(0, i@vtxnum);
float um = fit01(u, amount, 1.0 - amount);
vector uvw = set(um, 0.0, 0.0);

int pts[] = expandpointgroup(0, '*');
insert(pts, 0, 0);
append(pts, npoints(0)-1);

vector pos[];
foreach(int pt; pts){
    vector pos_pt = point(0, 'P', pt);
    append(pos, pos_pt);
}

v@P = spline('cubic', um, pos);

9 Accumulate point values

Assign random values @w to a line with several points. Use attribute promote set to promotion method "array of all" and type this into a point wrangle:

float s = sum(f[]@w_arr[0:i@ptnum+1]);
v@P.y = s;