Houdini 17.0 Nodes VOP nodes

Point Instance Procedural VOP node

The underlying procedural when using Fast Point Instancing with the instance render parameters.

The Point Instance Procedural takes a set of objects and distributes them over a given number of points at render time. Each point can assign a material to the object it instances, independent of the object itself. The main benefit is that at render time, unlike the Copy SOP, the only memory overhead is from the small set of source objects. So instancing a single object 1000 times doesn’t cost much more memory-wise than having a single copy of that object.

It is useful for several scenarios, particularly where a small set of geometries, each with a different material variation, will be distributed over large number of points. For example, forests and vegetation, crowds, constructing cities, asteroid fields, rocks and rubble, swarm of flies, birds, bats, etc.

Parameters

Main

Point Object

The object that contains the points to instantiate from.

Instance Object

The object to instantiate on each point on the point object. This can be overridden, on a per-point basis, by the instance attribute on the point object. In addition, it’s possible to do a delayed load of geometries, using the instancefile attributes. This is a string attribute that should contain an absolute path of any geometry file that can be loaded by Houdini. If motion blur is required, the companion attribute, instanceblurfile can be used. In that case, the instancefile attribute would be applied to shutter open, and this second attribute to shutter close. If the geometry file uses the v attribute for velocity blur, it is very likely that some clipping will occur at render time, due to the object’s bounding box not including the velocity values. In this case a float attribute, instancefile_maxv, can be added to the point object. This attribute specifies the absolute maximum velocity value of all the velocity vector components, and inflates the object’s bounding box accordingly.

Instance Transform

Typically, the transform on the instanced object is combined with the object it’s being instanced by. This option allows you to turn off this behavior.

Point Motion Blur

Specifies the motion blur behavior when rendering. Each point will have an instance of geometry. The transform of this instance is determined by the point’s position. Transformation motion blur on the instance is determined by the motion of the point. The motion of the points is controlled by this parameter.

No Motion Blur

Do not perform motion blur.

Compute sub-frame geometry (deformation)

Compute the point’s position by evaluating the geometry at sub-frame intervals. This can be problematic if the point count is changing frame to frame.

Use point velocity attribute

The point’s vector attribute v is used to extrapolate the motion of the point. If using the delayed load attributes, velocity blur will be automatically assumed on the delay load geometry, if the instanaceblurfile attribute is not given. In that case it is the responsibility of the user to tell the procedural the maximum length of the velocity vectors through the instancefile_maxv float attribute. Otherwise clipping artifacts may occur.

Render Geometry as Bounding Boxes Only

Don’t render the actual geometries, only their bounding boxes. This is useful for scene debugging and rendering quick tests.

CVEX Operator

An optional path to a CVEX SHOP node. The point instancing will run this CVEX shader on each instance before rendering it. This allows you to set the transform for each instance procedurally.

The point instancing CVEX shader has a single output, a transform (xform) matrix.

The point instancing CVEX shader can use the following globals.

Name

VEX Type

Description

P

vector

World position of the point being instanced on

id

integer

ID of the point being instanced on. If the 'id' attribute already exists on the source object, it will be used. Otherwise the points will be numbered in sequence.

t

float

Shutter time (0-1)

obj_xform

matrix

Instance object’s transform

cam_xform

matrix

Camera’s transform

inst_bbox_min and inst_bbox_max

vector

The minimum and maximum corners of the bounding box of the instance geometry

inst_xform

matrix

The transform of the instance geometry

You can use dditional attributes of the points being instanced on. The available attributes are controlled by the CVEX Attribute Mask parameter.

CVEX Attribute Mask

A glob to indicate which point and detail attributes should be copied into the CVEX context, if needed.

IFD Bounds

Bounding Box

The bounding box controls let you specify a bounding box for the referenced geometry. Only when this bounding box is "on screen" will mantra load the referenced geometry. You can set Bounding Box to No bounding box required, in which case mantra will always load the references geometry regardless. You can also specify the bounding box by referencing the SOP’s bounding box in the SOP Bounds parameter.

SOP Bounds

The geometry whose bounding box you want to reference.

Min Bounds

The minimum bounding box dimensions, when setting explicit bounds.

Max Bounds

The maximum bounding box dimensions, when setting explicit bounds.

VOP nodes