Vex and Geometry Attributes

Introduction to Vex and Attributes

• Vex is used for algorithmic design and procedural modeling, notably in Houdini.
• This includes applications like liquid simulations.
• Understanding geometry attributes is crucial for using Vex.

Types of Attributes

• Point, primitive, vertex, and detail are the four main attribute types.
• Each attribute type stores information for different geometric components.

Point Attributes

• Point attributes are linked to individual points in a geometry.
• Each point can possess unique attribute values.

Primitive Attributes

• Primitive attributes are linked to the faces (primitives) of a polygon.
• Edges or entire meshes can sometimes be treated as single primitives.
• Attributes can vary from face to face.

Vertex Attributes

• Vertex attributes are similar to point attributes but are stored per primitive.
• Each primitive has its own distinct set of vertices.
• They are often used for UV mapping and are related to the points used to construct the geometry

Detail Attributes

• Detail attributes are associated with the entire object.
• Only one detail attribute can exist per object.
• They store information applicable to the whole geometry.

Checking Attributes in Houdini UI

• The Geometry Spreadsheet in Houdini displays attribute information.
• It shows all of the attributes of a selected geometry.
• Icons in the spreadsheet represent point, vertex, primitive, and detail attributes.

Geometry Spreadsheet

• Displays all attributes within the selected geometry.
• Icons at the top represent attribute types: points, vertices, primitives, and detail.
• Clicking these icons reveals the attributes for each type.
• Point attributes are associated with each point in the geometry.
• Vertex attributes show point references within each primitive.

Sphere Geometry Attributes

• A sphere's point attributes include Px, Py, and Pz to indicate position.
• Numbers on the Geometry Spreadsheet correspond to point index numbers.
• Point numbers correspond to index numbers in the Geometry Spreadsheet window.

Vertex Tab

• It use a different index format compared to points.
• Its format is "primitive number column vertex number".
• Each vertex has a unique number specific to its primitive.

Primitive and Detail Tabs

• The primitive tab shows the index number of each face.
• The detail tab displays attributes for the entire object.

Attribute Wrangle Node

• It allows writing Vex code to modify attributes.
• Specific wrangle nodes exist for each attribute type: point, primitive, vertex, and detail.
• Point wrangle node modifies point attributes based on incoming geometry.
• Each wrangle node has a "run over" parameter.

Data Types in Vex

• Vex supports float, vector, integer, matrix, and string data types for attributes.
• The data type must be specified when creating an attribute.
  • Float (f@): Floating-point number.
  • Vector (v@): Three-component vector (x, y, z).
  • Integer (i@): Integer number.
  • String (s@): Text data.
  • Array (f[]@): List of a data type

Syntax for Accessing Attributes

• f@attribute_name is used for float attributes.
• v@attribute_name is used for vector attributes.
• The prefix indicates the data type which is followed by the attribute name.

Setting Attributes

• f@attribute_name = value; sets a float attribute for each point.
• i@attribute_name = value; sets an integer attribute.
• s@attribute_name = "text"; sets a string attribute.
• v@attribute_name = set(x, y, z); sets a vector attribute using the set function.

Array Attributes

• f[]@attribute_name = array(value1, value2, value3, ...); stores an array of values as an attribute which is enclosed by square brackets

Multi-Threading in Vex

• Point wrangle operates in a multi-threaded environment, executing code simultaneously on each point.
• Vex is faster than Python for geometry tasks due to its ability to process all points at once.

Changing Attribute Types

• Change the "run over" parameter in the wrangle node to modify attribute type.
• Options include point, primitive, vertex, or detail.
• Each primitive is calculated independently, enabling parallel processing.

Primitive and Vertex Wrangle Nodes

• Primitive and vertex wrangle nodes modify their respective attributes.
• They are similar to the point wrangle, with the "run over" parameter set accordingly.

Detail Wrangle Node

• Set "run over" to details to designate it as a detail wrangle.
• Or use one of the other wrangle nodes to set it to detail

Retrieving Attributes

• Established attributes can be transferred by connecting nodes.
• Access attributes using the same syntax as when setting them.
• This technique facilitates various operations, such as point movement.

Accessing Attributes That have been Set

• Use the value of an attribute to modify point position in the X direction with [email protected] = f@fval;.

Special Attributes

• These attributes are predefined by Houdini.
• @P (point position) does not require explicit data type definition.

Other Common Special Attributes

• ax (acceleration): For dynamics.
• cd (color): For point and primitive colors.
• N (normal): Directional value for points and primitives.
• scale: Point scale value
• force: Used in dynamics to determine force and direction of movement.

Accessing and Modifying Point Position

• @P directly accesses point positions.
• Use the rand function with @P to randomize point positions, such as demonstrated by @P = rand(@P);.

Color Attribute

• @cd accesses and modifies the color attribute, a vector type (red, green, blue).
• @cd = set(1, 0, 0); sets the color to red.
• Random colors are generated using @cd = rand(@ptnum).

Point Number Attribute

• Corresponds to the index number of each point, starting from 0.

Point Number Access in VEX

• Each point has a unique number, accessed in VEX with ptnum.
• Point numbers begin at 0 and increment for each point.
• Point numbers can be used to modify specific points.

Other Number Attributes

• primnum: Represents the primitive number.
• vtxnum: Represents the vertex number.
• The availability depends on the context.

Point Wrangle Context

• Primarily handles point attributes when "Run Over" is set to "Point".
• ptnum accurately retrieves the point number.
• primnum in a Point Wrangle may return a primitive number associated with the point but isn't unique for the point.
• A point can be associated to multiple primitives, but primnum will only retrieve one of said primitives.
• Avoid using primnum inside a point wrangle if you want to retrieve the point number

Primitive Wrangle Context

• "Run Over" set to "Primitive" changes the context to primitive attributes.
• primnum accurately represents the primitive number in a Primitive Wrangle.
• Special Attributes for primitives include Color (@cd).
• "Run over" set to "primitive" enables access to primitive values, allowing each face to have a different value.
• @P in a Primitive Wrangle retrieves the center position of the primitive, not individual point positions, and cannot be used to modify the point position.

Attribute Compatibility

• Attribute usage depends on the node's "Run Over" setting.
• Some attributes are specific to certain geometry types.
• It is important to understand attribute behavior in each context.
• Details are available in the Houdini documentation.

Global Attributes

• Accessible from any attribute type
• @Frame represents the current frame number.
• @Time represents the current time in seconds.
• Accessed using the @ symbol (e.g., @Frame, @Time).
• Custom attributes, like "fr," can store the frame number.

Detail Wrangle

• Operates on the entire geometry once, not per-point or per-primitive.
• The code inside runs only once.
• Useful for global attributes.
• Unlike Point Wrangle which is multi-threaded, Detail node only processes one operation.
• When working with time values, it is better to store as one value on the whole setup instead of at each point, primitive or vertex.