PhiFlow

Reading and Writing Simulation Data

This document describes how simulation data can be written and read using ΦFlow. The values format itself is described in the scene format specification.

Referencing a Scene Object

Scenes are represented by instances of Scene. There are two possibilities to reference existing scenes:

from phi.flow import *


scene = Scene.at('~/data/sim_000000')  # reference an existing scene by full path
scene = Scene.at('~/data', 0)  # reference an existing scene by directory and id
scenes = Scene.list('~/data')  # list all scenes in directory

New scenes can be created using the function Scene.create which creates a new folder in the directory. By default, it also copies the python script that created that scene into the included src folder.

Writing to a Scene

The simulation data are stored as in individual files, one for each quantity and frame. Frames are integers (up to 1000000) that typically represent time steps of a simulation. They can, however, be used for any purpose.

Instead of writing tensors directly, the scene writes instances of SampledField. These also encode physical size and extrapolation. The method write() stores a dictionary containing fields and their names.

scene.write({'smoke': smoke, 'velocity': staggered_velocity}, frame=0)

Subdirectories in the scene can be created using the method subpath().

The methods copy_calling_script() and copy_src can be used to copy source files into the src folder of a scene.

Reading from a Scene

Similar to write(), the method read() loads Field objects that were previously stored. It can be used to read single fields or multiple fields.

smoke = scene.read('smoke', frame=0, convert_to_backend=False)
smoke, velocity_staggered = scene.read(['density', 'velocity'], frame=0)

The convert_to_backend argument determines how the loaded data is stored. If True, the default backend is used to create the tensors, e.g. TensorFlow or PyTorch. If False, the loaded data will be held as NumPy arrays and future operations will also use NumPy functions unless converted manually.

Properties

Scenes may contain a property file named description.json that stores information about the simulation in an easy-to-parse format. For further documentation, see the scene format specification.

Properties can be set using put_property and read from scene.properties.

Batches of Scenes

As discussed in the performance optimization guide, it is recommended to combine data into large tensors whenever possible. For example, multiple smoke simulations can be simulated in a data-parallel way by stacking the tensors of the different simulation, assuming all have the same resolution.

To facilitate reading and writing batched data, scenes have a batch mode. A batch of scenes can be created using create with count>1.

scenes = Scene.create('~/data', count=batch_size, batch_dim='batch')

When writing data using scenes.write(), the fields are unstacked along batch_dim and written to the corresponding scenes.

Similarly, reading from a batch of scenes stacks the loaded fields along the batch dimension.