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Alembic_Geometric_Types  
Basic properties of geometric types in Alembic
Updated Dec 16, 2013 by miller.lucas

Geometric types proposed in Alembic 1.0:

Transform

The AbcGeom Transform class has these attributes:

  • ops - One or more operations of translation, scale, rotation, or 4x4 Rotation is angle in degrees and axis of rotation. Operations can optionally indicate a hint (e.g. for Maya to record hinting for pivot behavior).
  • inherits - Does this transform concatenate or ignore the transforms of its parents
  • childBounds - (optional) The bounding volume, including all subobjects
Alembic uses a right handed coordinate system.

Points

The AbcGeom Points class has these attributes:

  • P - the positions of each point
  • ids - for each P a 64bit unsigned integer that can be used to identify a Point
  • selfBounds - The bounding volume for just this object
  • childBounds - The bounding volume, including all subobjects
  • velocities - (optional) per point value is in units/second

Curves

The AbcGeom Curves class has these attributes:

  • type - linear or cubic (for all curves in object)
  • wrap - periodic or non-periodic (for all curves in object)
  • P - vertices
  • nVertices - for each curve the number of vertics in the curve
  • width (optional) - can be constant or varying, expressed as a diameter
  • N - (optional) Normals
  • uv - (optional) u-v coordinates
  • velocities - (optional) per point value is in units/second
  • selfBounds - The bounding volume for just this object
  • childBounds - (optional) The bounding volume, including all subobjects

PolyMesh

The Abc PolyMesh class has these attributes:

  • faceCounts - for each face the count of vertices that make up the face
  • faceIndices - for each vertex of each face the slot in P for the vertex
  • P - vertices
  • N - (optional) Normals
  • uv - (optional) u-v coordinates
  • selfBounds - The bounding volume for just this object
  • childBounds - (optional) The bounding volume, including all subobjects
  • facesets - (optional) named sets of faces from this mesh
  • velocities - (optional) per point value is in units/second

SubD

The Abc SubD class has these attributes:

  • faceCounts - for each face, the count of vertices that make up the face
  • faceIndices - for each vertex the slot in P for the vertex's coordinates
  • P - vertex coordinates
  • creases - (optional) creased edges
  • corners - (optional) vertexes that are corners
  • holes - (optional) regions to remove from the surface
  • uv - (optional) u-v coordinates
  • selfBounds - The bounding volume for just this object
  • childBounds - (optional) The bounding volume, including all subobjects
  • facesets - (optional) named sets of faces from this subd
  • velocities - (optional) value is in units/second

FaceSet

The Abc FaceSet class can be created as part of a SubD or PolyMesh and has these attributes:

  • faceNumbers - the faces from the SubD or PolyMesh that are in this faceset

NuPatch

The Abc NuPatch class has these attributes:

  • nucvs - the number of CVs in the u direction
  • nvcvs - the number of CVs in the v direction
  • uorder - the order of the patch in the u direction
  • vorder - the order of the patch in the v direction
  • uknot - floating point array containing the number of knots in the u direction
  • vknot - floating point array containing the number of knots in the v direction
  • Parameter lists (N, P, uv, arbparams)
  • selfBounds - The bounding volume for just this object
  • childBounds - (optional) The bounding volume, including all subobjects
  • velocities - (optional) per point value is in units/second
Additionally, the NURBS objects will contain TrimCurves which may contain zero or more trim curves. The path is composed of segments in the u direction and then the v direction.

Camera

The Abc Camera class has these attributes:

  • focalLength (mm)
  • horizontalAperture/verticalAperture dimensions (cm)
  • horizontalFilmOffset/verticalFilmOffset (cm)
  • lenseSqueezeRatio: to handle anamorphic lenses, among other things. (width / height)
  • 2D transform stack (translate/scale only) for expressing additional 2D translate/scale filmback operations
  • overscan stored as 4 percentage values for independent left/right/top/bottom values (to support fractional extension of screen window)
  • non-symmetrical overscan, which we've come across)
  • fStop (focal length divided by "effective" lens diameter)
  • focusDistance
  • shutterOpen/shutterClose: these would be frame-relative values, in seconds (secs.)
  • near/far clipping planes

Light

The attributes of the Abc Light class are to be expressed via an attached AbcMaterial. The other optional attributes on the light include:

  • childBounds - (optional) The bounding volume, including all subobjects
  • camera - (optional) An Abc Camera.

Comment by syoyofuj...@gmail.com, Oct 16, 2011

Please include "Alembic uses Y-up coordinate" somewhere.


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