Houdini 20.5 Nodes Geometry nodes

Channel geometry node

Reads sample data from a chop and converts it into point positions and point attributes.

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This node is complementary to the Geometry CHOP. The Channels created by the Geometry CHOP can be modified and then re-inserted into the op network via a Channel SOP.

This does what a Point SOP with a chopci() function can do, but is much faster.

There are two different ways of importing attribute values, controlled by the Method parameter. Each method requires that the CHOP channels are set up in a specific way.

The Static method is used for modeling a surface without any animation. A single channel is required for each attribute component, and its length must be the number of points in the geometry or specified point group(s). To modify P in a geometry with 100 points, three channels with 100 samples would be required.

The second method, Animated, provides animation for the attribute. There must be one set of channels for each attribute component, and this set must be indexed from 0 to the number of points minus one, by adding a numeric suffix. For the attributes Cd Alpha with 3 points, you would need r0 g0 b0 a0 r1 g1 b1 a1 r2 g2 b2 a2. The length of the channels determines the animation length.

The Channel SOP can also update portions of the geometry. By using point groups from the incoming op, the channels can be inserted only into the groups' points. To modify only some components of a vector, use brackets to specify the component index (P(1) refers to the Y value of the point position).

Parameters

Group

Modify only the points within this point group. If blank, all points are modified.

CHOP

Retrieves the sample data from this CHOP.

Method

The sample data fetch method.

Static

Uses one channel for each attribute, and all points use this channel (the first point resides at index 0, the next at 1, and so on). The length of the channels should be at least the number of points modified.

Animated

Uses one channel per attribute per point. The channels show the animation of each point’s position/attribute values. The channels must have a numeric suffix starting at zero, with at least one channel for each point. For vector attributes, there should be one channel per component, per point (eg. for the P attribute, the channels must follow the pattern tx0 ty0 tz0 tx1 ty1 tz1 tx2 ty2 tz2...).

Channel Scope

The names of the CHOP channels that will modify the attributes. There must be at least one channel name per attribute. Vector attributes may require multiple channels for all components specified in Attribute Scope or Matrix Attribute Scope. (eg. an attribute scope of P should have a channel scope with three

Attribute Scope

A string list of attributes to modify. For only one component of an array, such as the Y value of the point position, specify the number of elements in brackets (in this case, P(1) would be used).

The common attributes are:

P Point position (X, Y, Z), 3 values
Pw Point weight, 1 value
Cd Point color (red, green, blue), 3 values
Alpha Point alpha, 1 value
N Point normal value (X, Y, Z), 3 values
uv Point texture coordinates (U,V,W), 3 values

Matrix Attribute Scope

A string list of 3×3 and 4×4 matrix attributes to modify. These matrices are computed using translation, rotation, scale, and shear channels. If Use Shear Channels is disabled, then shear channels will not be used to compute the matrices.

3×3 matrices are computed using up to 9 channels: xyz channels for each of the matrices' rotation, scale, and shear components in that order.

4×4 matrices are computed using up to 12 channels: xyz channels for each of the matrices' translation, rotation, scale, and shear components in that order.

Rotation Order

The order in which to apply the euler rotation angles when recomputing all matrices that are specified within Matrix Attribute Scope.

Use Shear Channels

If this option is selected, then shear channels will be required to compute all matrices specified within Matrix Attribute Scope.

Organize by Attribute

(Applies to 'Animated Method' only.) Will reorganize the fetched channels by the value of this attribute. A common example is the id attribute found in particles. A channel is built for each unique id since the number of points may vary.

Examples

BlobbySphere Example for Channel geometry node

This is a simple example of how to utilize the Channel SOP to bring information from CHOPs into SOPs and apply it to geometry.

We use an animated sphere and create a lag in the animation of selected areas of the sphere.

In a CHOP network, the Geometry CHOP brings in point position data of the sphere geometry and runs it through a Lag CHOP for the delaying effect. The Channel SOP then references the Lag CHOP and applies the point data back to the selected areas of the original NURBS sphere.

ChannelSOPColorExample Example for Channel geometry node

This example demonstrates using CHOPs to drive geometry color values via the Channel SOP.

Geometry nodes