Houdini VEX sink

· Houdini MOC · Houdini VEX · Houdini VEX reference · #note/sink · #code/vex

TOC

• Data types
  • Vectors
  • Specifying VEX Data Types
  • Type casting
• Operators
  • Dot operator
  • Comparisons
  • Operator type interactions
  • Pragmas
• Channel Shortcut Syntax
• VEX Snippets
  • Sampling functions
  • Sample Direction Uniform
    • Sample Hemisphere
    • Sample Direction Cone
    • Sample Hypersphere Uniform
  • Remapping attribs
  • Orientation

Data types

Vectors

@v = {1, 1, 1}
v@v = set(x, y, z)

Specifying VEX Data Types

The following characters are used to cast to the corresponding data type.

float       f@name    // Floating point scalar values.
vector2     u@name    // Two floating point values. Could be used to store 2D positions.
vector3     v@name    // Three floating point values. Usually positions, directions, normals, UVW or colors.
vector4     p@name    // Four floating point values. Usually rotation quaternions, or color and alpha (RGBA).
int         i@name    // Integer values (VEX uses 32 bit integers).
matrix2     2@name    // Four floating point values representing a 2D rotation matrix.
matrix3     3@name    // Nine floating point values representing a 3D rotation matrix or 2D transform matrix.
matrix      4@name    // Sixteen floating point values representing a 3D transform matrix.
string      s@name    // A string of characters.

1/100 results in integer, 1/100.0 results in float. See #Operator type interactions

Type casting

int a, b;
float c;
c = (float)a / (float)b; // float(a) is unnecessary

// type cast a function
float n;
n = noise(noise(P));
n = noise(vector(noise(P)));

Operators

Dot operator

// vector
v = set(v.x, v.y, v.z);
v = set(v.r, v.g, v.b);

// vector2
v2 = set(v2.x, v2.y);
v2 = set(v2.u, v2.v);

// vector4 forth element
f = v4.a;
f = v4.w;

// matricies
m[0][0] = m.xx;
m[2][2] = m.zz;
m[3][0] = m.ax;

//components
v.zyx   = set(v.z,  v.y,  v.x);
v4.bgab = set(v4.b, v4.g, v4.a, v4.b);

Comparisons

The logical (&&, ||, and !) and bitwise (& |, ^, and ~) operators are only defined for integers
All bool operations are casted to integers!

Operator type interactions

float * int = float
int * float = int

vector * scalar = vector

v2 * v4 = v4
**Important!!!**
The “missing” component(s) on the smaller vector are filled in as {0.0, 0.0, 0.0, 1.0}

Pragmas

User interface pragmas

#pragma opname   noise_surf
#pragma oplabel  "Noisy Surface"

#pragma label    clr          "Color"
#pragma label    frequency    "Frequency"

#pragma hint     clr          color
#pragma range    frequency    0.1 10

surface noise_surf(vector clr = {1,1,1}; float frequency = 1; export vector nml = {0,0,0})
{
    Cf = clr * (float(noise(frequency * P)) + 0.5) * diffuse(normalize(N));
    nml = normalize(N)*0.5 + 0.5;
}

Channel Shortcut Syntax

This is used to hint at the data type of auto generated wrangle parameters.

ch('flt1');             // Float
chf('flt2');            // Float
chi('int');             // Integer
chv('vecparm');         // Vector 3
chp('quat');            // Vector 4 / Quaternion
ch3('m3');              // 3x3 Matrix
ch4('m4');              // 4x4 Matrix
chs('str');             // String
chramp('r', x);         // Spline Ramp
vector(chramp('c', x)); // RGB Ramp

VEX Snippets

Sampling functions

Random Directed Vectors
Source

Sample Direction Uniform

Sample Direction Uniform
This is a great way of getting “random” vectors but you need the vectors to be “clamped” to a certain range of “spread”.
To illustrate this, we’ll use a buncha points on a grid and “shoot” (aka move) them out “randomly” using sample functions.

vector2 u = rand(@ptnum);
@P = sample_direction_uniform(u);

Sample Hemisphere

Sample Hemisphere

This function takes a center and a bias. Think of “center” in sample functions as “direction” – so {0, 1, 0} is up, {0, -1, 0} is down – and bias is simply the “angle of spread”.

vector2 uv = rand(@ptnum);
float bias = @Frame;
@P = sample_hemisphere({0, 0, 0}, bias, uv);

Sample Direction Cone

Sample Direction Cone

This is probably the most useful, instead of bias we just use angle instead to achieve something similar. This time, we’ll scatter upwards using {0, 1, 0} as the center.

vector2 uv = rand(@ptnum);
float angle = 90; // the "spread" of the cone @P = sample_direction_cone({0, 1, 0}, radians(angle), uv);

We could imagine using this to shoot out random particles, so instead of @P we could assign it to @v as initial velocity to a DOP – while constraining how far they spread.
But what happens if we want the magnitudes to be the same? Right now, we have some vectors that are ahead of others because it’s a dome-like spread.
We achieve this by only varying v in the uv.

vector2 uv = set(1, rand(@ptnum));
@P = sample_direction_cone({1,0,0}, radians(angle), uv);

Now the values are “clamped” along whichever axis the center is on e.g. all the points have the same @P.x value – since we are scattering in {1, 0, 0}.
Another way of thinking about this, if we have particles getting shot out in +x direction, they can only vary/wiggle up-down (y) or left-right (z) but how far they go (x) is always the same across all points.
At radians(90) the vectors are perpendicular to the projection plane. We can use this to randomly scatter lines that are perfectly flat on the ground but point in different directions:

// Before a Copy to Points SOP
@N = sample_direction_cone({0, 1, 0}, radians(90), set(1, rand(@ptnum)));

Sample Hypersphere Uniform

Sample Hypersphere Uniform

This is a quick way to randomly rotate a bunch of girls around in a Copy to Points e.g. get a random @orient.

@orient = sample_hypersphere_uniform(rand(@ptnum));

Remapping attribs

parm*=rand(@id+38+ch("seed")); 

//random number, linear spread
parm=fit01(rand(@id+31+ch("seed")),ch("min"),ch("max")); 

//particle age -> Ease in - Ease out
parm*=smooth(ch("start"), ch("end"),@nage); 

//parameter change in a certain time range
parm=fit(@Frame, ch("start"), ch("end"), ch("start_value"), ch("end_value"));

//connect params
parm=fit(@P.y, ch("low"), ch("high"), ch("low_value"), ch("high_value"));

//param from age by multiplier
parm*=chramp("parm",@nage);

//param from a particle speed
parm=fit(length(@v),ch("speedmin"),ch("speedmax"), ch("valuemin"), ch("valuemax"));

Orientation

// orient your points towards your velocity vector:
@orient = dihedral({0,0,1} ,@v);