Houdini 21.0 Copying and instancing

Copying and instancing point attributes

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When you copy or instance geometry onto points, Houdini looks for specific attributes on the destination points to customize each copy/instance.

Attributes

Name

Type

Description

orient

float4 (quaternion)

Orientation of the copy

pscale

float

Uniform scale

scale

float3

Non-uniform scale

N

vector

Normal (+Z axis of the copy, if no orient)

up

vector

Up vector of the copy (+Y axis of the copy, if no orient)

v

vector

Velocity of the copy (motion blur, and used as +Z axis of the copy if no orient or N)

rot

float4 (quaternion)

Additional rotation (applied after the orientation attributes above)

P

vector

Translation of the copy

trans

vector

Translation of the copy, in addition to P

pivot

vector

Local pivot point for the copy

transform

3×3 or 4×4 matrix

Transformation matrix overriding everything except translations from P, pivot, and trans.

shop_materialpath

string

Path to a material. You can add this attribute to points using the Material surface node. When you instance an object onto a point with this attribute, the instanced object uses the given material.

Not supported in the viewport.

material_override

string

A serialized Python dictionary mapping parameter names to values. You can add this attribute to points using the “override” controls on the Material surface node. When you instance an object onto a point with this attribute, the instanced object applies the given overrides to its material.

Not supported in the viewport.

Priorities

  • If pivot exists, use it as the local transformation of the copy/instance.

  • If the transform attribute exists:

    • Use it as a 3×3 or 4×4 matrix to transform the copy/instance.

  • If the transform attribute does not exist:

    • If the orient attribute exists:

      • Use it to orient the copy/instance.

    • If the orient attributes does not exist:

      • Orient the copy/instance using N as the +Z axis and up as +Y axis.

      • If N does not exist, use v (velocity) if it exists.

    • If the rot attribute exists, apply it after the above.

    • If pscale exists, use it to scale the to scale the copy/instance (multiplied by scale if it exists).

    • If scale exists, use it to scale the copy/instance (multiplied by pscale if it exists).

  • If trans exists, use it and P to move the copy/instance.

Or, put another way:

Key: 
X = pivot matrix (translate by -pivot)
O = orient matrix 
S = scale matrix (scale * pscale) 
L = alignment matrix (*) 
R = rot matrix 
T = trans matrix (trans + P) 
M = transform matrix

(*) The alignment matrix (L) is defined by N or v and up. 

IF N exists AND up exists and isn't {0,0,0}: 
   L = mlookatup(N,0,up) 
ELSE IF N exists: 
   L = dihedral({0,0,1},N) 
ELSE IF v exists AND up exists and isn't {0,0,0}: 
   L = mlookatup(v,0,up) 
ELSE IF v exists: 
   L = dihedral({0,0,1},v) 

IF transform exists:
   Transform = X*M*T
ELSE IF orient exists: 
   Transform = X*S*(O*R)*T
ELSE: 
   Transform = X*S*L*R*T

Instancing and motion blur

Velocity Blur

The shutter open transform is based off the Axis Alignment and Positioning stated above. For the shutter close transform the shutter open transform is used plus the vector v/$FPS * shutter_speed. In the cases where that you have N and a v attribute, N is used to align but v is still used to blur.

Note

v is independent of the transform constructed for the Axis Alignment and Positioning. You can rotate, scale and skew all you want but v = (0,0,1) then your object will be blurred in the z direction.

Transform Blur

Same as Velocity Blur but the Object level transform is also taken into account.

Deformation Blur

The Axis Alignment and Positioning transform is constructed for both shutter open and close, the object level transform is also taken into account. This method is the most accurate.

Examples

(The image in the lower right corner of each frame is the geometry if it were instanced without any of the attributes.)

Copying and instancing

Getting started

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