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Geometry attributes

Describes how Houdini represents geometry using details, primitives, points, vertices, and attributes.

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Attributes

Attributes are named values stored on vertices, points, primitives, and objects. Point color, position, UV coordinates , spline weight (W), and normal, for example, are stored as point attributes.

  • Houdini sets some point attributes that you can use in expressions. For example, you could set the per-point color based on the vertical position (PY) of the point.

  • Certain attributes you can set manually and if they're present they will be used by Houdini. For example, the dynamics solvers and rendering engines will often use certain attributes on the geometry if they exist.

  • You can also set your own custom attributes to be used by your node expressions, scripts, exporters, and so on.

The surface nodes in the Attributes group of the tab menu set different types of attributes. You can also set or create arbitrary attributes using the Attribute SOP.

To see the attributes on the geometry in a node, press on the node’s tile in the network editor and choose Spreadsheet to open the geometry spreadsheet for the node.

Particles also have attributes, and attributes are very important when geometry and particle systems interact. For example, particles emitted from geometry inherit (by default) the attributes of the point from which they are emitted, and you can create interesting effects by transferring attributes between surfaces and particles using the Attribute Transfer POP.

Geometry components

You can create/set attributes using the generic Attribute Create surface node, or the Vertex, Point, Primitive nodes. You can attach attributes to vertices, points, primitives (e.g. polygon faces), or the entire piece of geometry (called the "detail" level).

Note

When you merge two or more branches in a geometry network, if a detail attribute with the same name exists in multiple branches, Houdini uses the value in the first branch. If the attribute is a string array, Houdini merges the arrays.

Attribute precedence

When two components in the same geometry have an attribute with the same name, the attribute on the "lower level" of geometry is used, so:

  • Vertex attributes override

    • Point attributes override

      • Primitive attributes override

        • Detail (whole geometry) attributes

Applying nodes to components based on attribute values

Most surface nodes have a Group field that lets you enter a named group or a pattern to select which components the node should apply to. This field recognizes special syntax that lets you select components by the value of an attribute. For example, in a Point node's Group field, @foo>5 selects all points whose foo attribute is greater than 5.

See the section on manually specifying groups for more information.

Intrinsic attributes

Intrinsic attributes are computed values derived from the geometry that you can access as if they were attributes. For example:

Examples of intrinsic attributes

Detail

intrinsic:pointcount

Number of points in the geometry

Detail/primitive

intrinsic:bounds

Bounding box of the geometry

Primitive

intrinsic:measuredarea

Surface area of the geometry.

Primitive

intrinsic:memoryusage

Amount of memory used by the primitive.
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See what intrinsic attributes are available

  • The command line utility ginfo -I prints a list of all known intrinsic attributes.

  • You can show intrinsic attributes in the geometry spreadsheet. By default they are not shown.

    If you are viewing geometry at a component level that has intrinsic attributes, you can use the Intrinsics menu to show all intrinsics or choose which intrinsics to show in the spreadsheet.

  • The Inspect tool can show which intrinsic attributes are available on the inspected geometry.

Use intrinsic attributes

  • In HScript expressions and VEX, use the attribute functions to read intrinsic attributes in expressions and VEX, for example:

    prim("../volume1", 0, "intrinsic:bounds", 0)

    (You can’t use the @ attribute shortcut to read an intrinsic in VEX wrangle snippets. However, you can use primintrinsic and detailintrinisc() to read the intrinsic directly instead of using the intrinsic: prefix magic.)

  • In Python, you can access intrinsic attributes using hou.Geometry.intrinsicNames(), .intrinsicValue(), and .intrinsicValueDict().

  • In the group syntax, you can group based on intrinsic values. For example, @intrinsic:indexorder<100

Tip

Despite the name, some "intrinsic" attributes can actually be written to, causing Houdini or Mantra to change some internal setting. For example, you can set the intrinsic:unexpandedfilename intrinsic attribute on a packed primitive.

Higher and lower precision storage

You can choose the numeric precision of attributes you create with the Attribute Create node, and/or convert attributes to higher precision types (at the cost of more memory) or lower precision types (to save memory) using the Attrib Cast surface node.

See the help for Attrib Cast for information on the available storage types and limitations.

Common attributes

The following attributes are commonly set by Houdini and/or used by Houdini if set by the user.

Geometry

P

 vec3

Point position. The viewport uses this to lay out the points of the model in 3D space.

Default (0,0,0) On point

N

 vec3

Surface or curve normal direction. Houdini will compute the normal if this attribute does not exist.

v

 vec3

Velocity (used for rendering blur).

uv

 vec3

UV coordinates for this point/vertex.

On point, vertex

lod

 float

Level of detail.

On detail, prim

rest

 vec3

Rest position of this point.

On point

Shading

shop_materialpath

 string

The path to a material to use to shade the surface when rendering. Overrides the object’s material is present on primitives.

On detail, prim

shop_oglpath

 string

The path to an OpenGL shader to use to shade the surface in the viewport.

rishade

 string

Detail/primitive. The name of a RenderMan shader.

On detail, prim

Shader overrides

Most shaders will use the override attributes to change how elements with the attributes are shaded.

Cd

 vec3

Diffuse color override. The viewport uses this to color OpenGL geometry.

Alpha

 float

Alpha transparency override. The viewport uses this to set the alpha of OpenGL geometry.

Cs

 vec3

Specular color override.

Cr

 vec3

Reflect color override.

Ct

 vec3

Transmit color override.

Ce

 vec3

Emission color override.

rough

 float

Roughness override.

fresnel

 float

Fresnel coefficient override.

shadow

 float

Shadow intensity override.

sbias

 float

Shadow bias override.

Point and line display

The viewport uses the pscale attribute to scale sprites and points. The Disc scale display options lets you further scale discs.

spriteshop

 string

Path to a material node for shading sprites.

On point

spritescale

 vec3

Additional sprite scaling.

Default (1, 1, 1) On point

spriterot

 float

Sprite rotation (around camera angle) in degrees.

Default 0 On point

width

 float

Line width (in world space units), for drawing open curves. If this attribute does not exist, the curve is drawn as a thin line instead of as a ribbon with thickness.

On point

Viewport

gl_wireframe

 int

If this detail attribute is 1, the geometry always appears as wireframe in the viewport.

If this detail attribute is -1, the geometry always appears as shaded in the viewport.

Default 0 On detail

gl_lit

 int

If this detail attribute is not 0, the geometry always appears without lighting.

Default 0 On detail

gl_showallpoints

 int

If this detail attribute is not 0, all points will be rendered as sprites, even if they are connected to geometry. Normally only unconnected points are rendered.

Default 0 On detail

vm_cuspangle

 float

Controls the cusp angle used for generating normals when the geometry doesn’t have any. 180 will produce entirely smooth point normals.

Default 60 On detail

See also

Geometry

Understanding

Modeling

Next steps

Guru level