YTEP-0006: Periodicity

Periodicity needs to be dealt with in an explicit, rather than implicit, fashion.


  • Created: January 10, 2013
  • Modified: January 29, 2013
  • Author: Matthew Turk, Nathan Goldbaum

Periodicity in yt has been handled poorly in the past. Some objects and fields have been set to be periodic by default, but not all. This YTEP aims to define a mechanism by which fields and objects can query periodicity information and use it correctly.


In progress. See pull request #410:

Detailed Description

Periodicity is a tricky business. By volume, the majority of simulations analyzed with yt are cosmology simulations, which are exclusively periodic. So objects such as ellipsoids, fields such as radius, and so on have all evolved to select data or regard data as wrapping around the edges of simulation boundaries.

However, all of these should only be periodic when it makes sense. This means that we need to have a method of marking a simulation as periodic and a method for applying this periodicity. For those situations where periodicity makes sense, it should either always be applied if the simulation is periodic, or never applied if it is not. I believe we should allow simulations to be periodic in any one of the three axes, but not necessarily all simultaneously; this may be overly complex. We explicitly do not support any type of domain-wrapping or boundary conditions more complex than simply wrapping around.

Affected Regions

Region of the Code Type of Periodicity Change? Status?
Light cone Implicit No N/A
Halo finding Implicit No N/A
Light ray Implicit No N/A
EnzoFOF Implicit No N/A
FOF Implicit No N/A
Halo objects Implicit No N/A
Fixed Res Buffers Explicit Yes Not done yet
Multi-halo profiler Implicit No N/A
Radial column density Implicit Yes See PR #410
Periodic regions Explicit Yes Not done yet
Spheres Implicit Yes Not done yet
Ellipsoids Implicit Yes Not done yet
Two-point functions Implicit Yes Not done yet
Clumps Implicit Yes Not done yet
Boolean regions Implicit Yes Not done yet
AMR kD Tree Explicit Yes Not done yet
Domain decomp Implicit Yes Not done yet
Radius Implicit Yes Finished: PR 410
ParticleRadius Implicit Yes Finished: PR 410
Covering grids Implicit Yes Not done yet

New Method

Two types of changes will be made. The first is to remove implicit periodicity and replace it with a check on the periodicity of the simulation. The second is to remove multiple definitions of objects or functions that operate either in periodic or non-periodic methods, and instead provide only one that self-distinguished. Some operations, such as anything that operates on cosmological simulations (which I reluctantly consider halo finding to do) can assume periodicity.

We need to take account of the following types of checks:

  • Distance between two points
  • Shortest path between two points (uncommon, can be special cased)
  • Object inclusion/collision
  • Selection of points

We will make the following changes:

  • Create a periodicity property on all StaticOutput objects. This will be a tuple of three booleans, indicating whether or not the simulations are periodic (and if they are, they must be periodic by one domain width). This has been implemented for most of the frontends in PR #410.
  • Remove all locally-defined periodicity functions in favor of the function periodic_dist in yt/utilities/ and checking the periodicity attribute. For situations where a purely euclidean distance is required, we also supply euclidean_dist, which calculates the distance between two points without considering the domain boundaries. These two functions were finalized in PR #410 and currently live in
  • Anything that applies periodic shifts to data for checks of inclusion should apply them exclusively through the periodicity attribute. For data selectors, we will have a two-step process: the data object will need to implement a check_periodicity function.
  • Everything that relies on periodic_region should instead rely on region which will include an option (default = True, which actually means) to check periodicity.
  • The periodic_region data object will, in 2.X, become a wrapper around the basic region object.

Backwards Compatibility

All operations that relied on implicit periodicity for datasets that cannot be identified as periodic will have different results.

Old results for non-periodic datasets that were incorrect will become correct.