Copy SOP and Instancing
When you are instancing geometry onto points, (via the Copy SOP or during IFD generation on the Object's Render Tab), Houdini looks for specific attributes in order to build a unique transform for each piece of geometry being instanced.
Supported Attributes:
This is a list of the point attributes that are used with the Copy SOP and point instancing.
Attrib - Data Type
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orient - float4 (quaternion)
pscale - float
scale - float3
N - vector
up - vector
v - vector
rot - float4 (quaternion)
trans - vector
Axis Alignment and Positioning:
- If the orient attribute exists, N, v, and up are ignored.
- If the orient attribute does not exist:
- The geometry's +z-axis is aligned to point down N or v.
- If both N and v exist, v is ignored and N is used.
- If the up attribute exists then the geo's +y-axis is aligned to it.
Construction of the transform matrix is as follows:
Key:
O = orient matrix
S = scale matrix (scale * pscale)
L = alignment matrix (*)
R = rot matrix
T = trans matrix (trans + P)
(*) 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)
END
IF orient exists:
Transform = O*R*S*T
ELSE:
Transform = S*L*R*T
END
Motion Blur:
No Blur- No 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’.
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