User:KevinDietrich/Cycles Alembic Procedural

Alembic Procedural

To align with more production render engines, Cycles now support reading data from Alembic directly, using an Alembic procedural. This procedural will create objects in the Cycles scene from an Alembic archive, skipping the export process from Blender. When using the procedural, data from the Alembic archive is not read in Blender.

During viewport renders, the procedural can load the data from the entire animation in memory to accelerate data lookups and updates on frame change. It is not yet available for final renders.

Caching the animation allows for fast playback in the viewport, potentially real-time with the limiting factors being the compute power, scene complexity, and features used (subdivided meshes, and dynamic meshes coming from physics simulations will increase the cost of updating the scene).

Since data is loaded directly from Cycles instead of going through Blender, we are able to overcome some of Blender's limitations. For example, Blender does not generate subdivision from Alembic SubD meshes, it merely builds a mesh from the data and any subdivision control is lost. Arbitrary attributes can also be trivially read.

To enable the Cycles procedural, first switch on the experimental feature set for Cycles, and enable the procedural in the Mesh Sequence Cache by toggling "Use Render Engine Procedural". This setting is global for the CacheFile, so every object in the Scene imported from the CacheFile will use the Cycles procedural for rendering. Those objects will be displayed using bounding boxes in the viewport.

If the Alembic object pointed to by the path is a subdivision object, Cycles will only perform the subdivision if there is a Subdivision Surface modifier after the Mesh Sequence Cache modifier.

Features

• Support polygonal and subdivision meshes
• Limited support for Curve objects
• Instancing
• Arbitrary attributes (along with UVs, vertex colors)
• Basic cache controls (it is possible to load the entire animation with some memory limit, if the memory limit is reached, rendering is aborted)
• Layers : extra files to use for property override

Known Limitations

• Object level attributes are not supported
• Motion blur (transformation or vector based) is not supported
• N.U.R.B.S. and Point clouds cannot be rendered at the moment (Cycles cannot render point clouds at the moment)
• Curves are only expected to be acyclic
• Attributes are not read for Curves
• Cameras are ignored
• No support for file sequences
• No support for detecting invisible objects

Possible future settings

This is a list of possible features for the Cycles Alembic Procedural and the Blender Alembic Importer. This list excludes the limitations above, as it would be redundant to mention them again.

• FaceSet name scope, to decide what face sets should be considered for shader assignments
• Material attribute, to decide which attribute instead of face set should be considered for shader assignments
• Exclude transform, to not concatenate child transformations matrices to their parent's one
• Ignore velocity, to ignore computing motion blur data even if a velocity attribute has been detected
• Velocity attribute, to specify which attribute to use for computing motion blur data
• Velocity unit, to specify the unit of time used to differentiate velocity vectors
• Radius attribute, to specify which attribute to use for setting curves or points radiuses
• Expand hidden, to ignore the visibility attribute in the Alembic archive
• Frame step
• Replay speed, to control the speed of the playback (scales the FPS)
• Animation boundary conditions : to specify what should happen before or after the animation begins or ends
  • none : nothing is displayed, objects will appear suddenly when the animation starts, and disappear when it ends
  • freeze : simply hold the first or last frame
  • loop : repeat the animation from the beginning
  • ping pong : plays back and forth the animation (first foward, then backward, then forward again, etc.)
• Make instances, detect geometries with the same topology and make instances out of them, even if they are not set as instances in the archive