Houdini VEX Programming

Write high-performance VEX code for geometry manipulation, simulation, and shading in Houdini.

Quick Start

Basic Point Wrangle pattern:

// Access and modify point position
@P += @N * 0.1;

// Set color based on position
@Cd = normalize(@P);

Critical Concepts

1. @ Syntax: Use @ to access attributes (@P, @Cd, @N)
2. Type Prefixes: Specify types with prefixes (v@, i@, s@, f@)
3. Context Matters: Code runs per-point, per-prim, or per-detail depending on wrangle type
4. No Recursion: Functions are inlined by compiler
5. Multi-threaded: Code runs in parallel automatically

Attribute Access

// Automatic type detection for known attributes
@P         // vector - position
@N         // vector - normal  
@Cd        // vector - color
@pscale    // float - point scale

// Manual type specification for custom attributes
v@velocity  // vector
i@id        // integer
s@name      // string
f@custom    // float

Common Patterns

See examples/PATTERNS.md for ready-to-use code:

• Point manipulation
• Neighbor operations
• Group creation
• Attribute transfer
• Noise displacement
• Particle systems

VEX Contexts

Point Wrangle: Runs per-point (most common)

• Access: @ptnum, @numpt
• Use: Deform, scatter, color

Primitive Wrangle: Runs per-primitive

• Access: @primnum, @numprim
• Use: Primitive operations, extrusions

Detail Wrangle: Runs once for entire geometry

• Access: Detail attributes only
• Use: Global calculations, metadata

Vertex Wrangle: Runs per-vertex

• Access: @vtxnum, @numvtx
• Use: UV manipulation, vertex colors

Essential Functions

// Geometry queries
int nearpoints(geometry; vector pos; float radius)
int neighbours(geometry; int ptnum)
vector point(geometry; string attr; int ptnum)

// Attribute creation
addpoint(geometry; vector pos)
removepoint(geometry; int ptnum)
setpointattrib(geometry; string name; int ptnum; value)

// Noise
float noise(vector pos)
vector curlnoise(vector pos)
vnoise(vector pos)

// Math
normalize(vector)
distance(vector, vector)
dot(vector, vector)
cross(vector, vector)

Response Format

When providing VEX code, always include:

1. VEX Code with comments explaining logic
2. Wrangle Type (Point/Prim/Detail/Vertex)
3. Required Inputs if any
4. Expected Attributes that must exist
5. Parameters to create if using ch() functions

Common Functions

See reference/FUNCTIONS.md for complete API including:

• Geometry manipulation
• Attribute operations
• Math and vectors
• Noise and randomness
• Groups and selection

Best Practices

See reference/BEST-PRACTICES.md for:

• Performance optimization
• Memory efficiency
• Code organization
• Debugging strategies

Troubleshooting

See reference/TROUBLESHOOTING.md for solutions to:

• Type mismatch errors
• Attribute not found
• Performance issues
• Common syntax mistakes

Additional Resources

• examples/COMPLETE.md - Full procedural systems
• templates/ - Starter code for common tasks
• reference/CONTEXTS.md - Deep dive into each context

Key Differences from Other Languages

From Python:

• No recursion
• Strongly typed
• Runs per-element (parallel)
• Use @ for attributes, not variable names

From C/C++:

• Context-specific entry points
• No pointers
• Built-in vector/matrix types
• Automatic parallelization

From GLSL:

• Geometry manipulation focus
• Different attribute system
• More procedural modeling functions
• Different execution model