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enzo_movie - description of the Enzo movie format |
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The Enzo movie format is one of the data formats supported by the Enzo cosmology code. Enzo uses Adaptive Mesh Refinement (AMR) techniques in its computation and outputs. The enzo movie output format was developed by John Wise at Stanford University, and his documentation for the format can be found at <http://www.slac.stanford.edu/~jwise/professional/newMovie/newMovie.html> Unfortunately, even with only two Enzo users producing output in the new format, the format has already diverged into two variants. Using either ’flavor’ still requires a little hand-twiddling. This man page gives a brief overview of the format and describes the changes necessary so that enzo movie output can be used with the libc_amr library (which was developed at NCSA). For more in-depth information on the format, including the exact binary details of how volume and particle data is stored, see John Wise’s website. |
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A movie produced by Enzo produces several output files, each of which are one of four types: header,index, field data or particle data. Each movie has exactly one header file, but has several index and data files. |
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movieHeader.dat |
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This file is a plain text file containing global information about the simulation, including the data types, root grid resolution, and data file locations. This file is the only one which will require tweaking. |
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Index (.idx) files |
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These files contain meta-information about each subgrid,
including the origin, extent and spacing of the grid, as
well as fields indicating whether any field or particle data
for this grid was written out. The filenames of the index
files are all of the form:
<movie_name><num>.idx_<cpu_id>. |
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Movie data (.mdat) files |
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These files contain the volume data for a particular
scalar field of the movie. There is a one-to-one
correspondence between the index files and the movie files,
and within each movie data file, the scalar data for the
grids appears in the same order as the grids appear in the
index file. The filenames are of the form:
<movie_name><num>.mdat.<field-id>_<cpu_id>. |
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Particle data (.part) files |
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These files contain particle data for the simulation.
Particles are stored by grouping them together by which AMR
grid contains them, and then writing them out in grid order.
Similarly to the .mdat files, these groups of particles are
written out in the same order that they appear in the index
file. Not all simulations produce particle data. The
filenames are of the form:
<movie_stem><num>.part_<cpu_id>. |
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A note on the file naming scheme: Besides the cpu id portion of the filename, there is also a ’file number’ part ( <num> in the patterns above). To avoid generating huge files, Enzo places a limit on the number of grid volumes that can be written to the mdat file. Thus, when this limit is reached, Enzo names the new .idx, .mdat, and .part (if applicable) files by incrementing the file number. |
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The movie header file (usually named movieHeader.dat) is a text file. Each line contains a parameter name-value pair with the syntax: |
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Name = Value |
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Here are the valid parameter names: |
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MovieVersion(required) |
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Version of the enzo movie format used. This document describes version 1.4. |
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Endianness(required) |
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Byte order of the simulation output data. Valid values are BIG or LITTLE. |
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CoordFloatSize(required) |
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The size, in bytes, of the data used for the time and space coordinates of the grids (as described in the index file). Valid values are 4 for 32-bit floats, or 8 for 64-bit floats. |
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DtFloatSize(optional) |
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Index files produced by the Stanford variant of Enzo always use a 32-bit float to store grid lifetimes (delta_t), whereas the in the UCSD variant, delta_t is stored with the same precision as the other coordinates. Valid values are 4 or 8. If this keyword is omitted, then the precision defaults to the same value as CoordFloatSize. |
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DataFloatSize(required) |
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The size, in bytes, of the data used for storing scalar fields. Valid values are 4 for 32-bit floats, or 8 for 64-bit floats. |
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RecordSize(required) |
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The number of bytes per grid record in the index file. This is mainly a sanity check, as it can be calculated from the formula: |
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32 + 6*CoordFloatSize + DtFloatSize |
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RootReso(required) |
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An integer giving the resolution of the root grid. This is expressed as a single integer, so that a value of "128" means that the root grid has 128^3 cells. |
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FileStem(optional) |
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A stem filename for the movie index and data files. Movie files are expected to have the format <file_stem><num>.<type>_<cpu_id>, where <num> is a file number. The file stem may include a path prefix, such as /data/amr/MoviePack. The default is MoviePack. |
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IndexFilePattern(optional) |
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This is an alternative to MovieStem to specify the names
of the index files. It is more flexible. It uses
python-style format strings (which are much like c-style
formatting strings, but have named parameters) to specify
the files. You can use %(FILENUM)d and %(CPU)d
to specify parameters in the pattern. These also accept
precision arguments: %04(CPU)d means to use
"%04d" printf format specifier for the CPU id. |
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NumCPUs(required) |
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The number of CPUs used to run the simulation. This is important for finding all of the various data and index files. |
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MaxFilenum(required) |
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Maximum file number to look for in this dataset. |
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MinFilenum(optional) |
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Minimum file number to look for in this dataset. This defaults to 0, which normally suffices. However, after an Enzo simulation is restarted, the file numbers do not necessarily start at zero. |
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NumFields(optional) |
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The number of data fields that are output. Defaults to 1 |
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FieldNames(optional) |
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A space-delimited list of names for the scalar fields. If this is omitted, scalar fields will be named Field_0, Field_1, Field_2, ad nauseum. |
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A typical file output from the UCSD flavor of enzo looks something like this: |
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MovieVersion = 1.3 |
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We don’t document version 1.3 of the movie header
here. In that version, FLOATSize takes the place of both
CoordFloatSize and DataFloatSize. The libc_amr library can
handle this earlier version. However, this movieHeader.dat
file is missing a specification of endianness, which is
needed. In this case, after asking the person who ran the
simulation, we find that the data is stored in little-endian
format. |
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MovieVersion = 1.4 |
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The last two lines are optional. The FileStem line specifies a full path to the data (in case the movieHeader.dat file is stored separately). The FieldNames line gives a more meaningful name to the first scalar field. |
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Matthew Hall <mahall at ncsa dot uiuc dot edu> |