Troubleshooting

The errors people actually hit in their first hour, with the message Mera prints and what to do about it. If something here is wrong or missing, please open an issue — that is the fastest way to get it fixed for the next person.

"File or folder does not exist"

[Mera]:  File or folder does not exist: /no/such/path/output_00300/info_00300.txt !

Mera builds the filename from the output number and the path, so this message shows you exactly what it looked for. Two things to check:

Is the output number right? getinfo(300, path) looks for output_00300 inside path. Use checkoutputs to list what is actually there:

checkoutputs("/path/to/simulation")

Is the path the parent folder? It should contain the output_XXXXX directories, not be one of them.

Are you pointing at non-RAMSES data? Code detection falls back to RAMSES when it recognises nothing, so a PLUTO/Athena++/FLASH/GADGET folder that isn't detected produces this same RAMSES-shaped message about a missing info_XXXXX.txt — which is confusing, because your data has no such file and never should. Name the code explicitly to find out:

info = getinfo(path, code=:pluto)     # :pluto, :chombo, :athena, :flash, :gadget, :ramses

If that works, detection failed rather than the file being missing — please report it with your directory listing, since detection is meant to handle it. See Multi-code support for what each reader looks for.

The tutorials point at a folder I don't have

Every tutorial builds its paths from one variable, so you do not have to edit the cells:

ENV["MERA_EXAMPLES"] = "/path/to/your/simulations"   # before `using Mera`

If you have no simulation output at all, several pages run on synthetic data with nothing downloaded — see Clump Finding, Statistics and Uniform Grid. synthetic_clumps() builds real Mera objects in memory, so everything downstream works exactly as on a real snapshot.

For a public snapshot to follow along with, the RAMSES samples linked from Cosmological Runs and Magnetic Fields are freely downloadable.

`UndefVarError: Figure not defined` (or `heatmap`, `Axis`, `plot`)

Mera draws nothing itself. The tutorials plot with CairoMakie, and a few older projection pages use PyPlot; neither is a Mera dependency:

using Pkg
Pkg.add("CairoMakie")   # what most tutorial figures use

Mera's Makie support is a package extension — it activates by itself once a backend is loaded, with nothing else to install.

`FieldError: type ScalesType003 has no field ...`

FieldError: type ScalesType003 has no field `kpcc`, available fields: `Mpc`, `kpc`, `pc`, ...

A unit name that doesn't exist. The message lists the valid ones. See getunit for how units are resolved, and viewfields(info.scale) for the full list of a given simulation.

`KeyError: key :something not found`

A quantity name getvar doesn't know. Call getvar with no arguments to print everything available for your data type:

getvar()          # lists every quantity, grouped by data type

`TypeError: in keyword argument weighting, expected Vector, got Symbol`

Hydro, gravity and RT projections take the weighting as an array; only particle projections take a bare symbol:

projection(gas,  :T, weighting=[:volume])   # hydro / gravity / RT
projection(part, :sd, weighting=:mass)      # particles

Julia runs on one thread

Threads.nthreads()    # 1 unless you asked for more

Threads are set when Julia starts, not from inside a session:

julia -t 8
# or
export JULIA_NUM_THREADS=8

See Multi-Threading. Note that more threads is not automatically faster — a single light projection is serial-fraction dominated and stays flat; the measured numbers are in Projection benchmarks.

Out of memory on a large output

gethydro(info) loads the whole box. Read only what you need instead — the selection happens during the read, so the memory is never allocated:

gas = gethydro(info, lmax=10)                               # cap the refinement level
gas = gethydro(info, xrange=[-10,10], yrange=[-10,10],
               zrange=[-2,2], center=[:bc], range_unit=:kpc) # or a spatial window

usedmemory reports what an object costs, and Load by Selection covers the options in full.

My `.jld2` file won't open in Python

It isn't meant to. savedata stores the Julia object and compresses with LZ4, so h5py can open the container but cannot reconstruct the table. To hand data to Python, use export_vtk, write the columns out yourself, or call Mera from Python via JuliaCall.

The first call is slow

Julia compiles as it runs, so expect several seconds on the first call in a session and near-instant afterwards. This is normal and not a sign anything is wrong — see Julia for Simulation Analysis.

Something else

viewfields on any Mera object shows what it actually contains.
provenance reports the Mera version, output and simulation code behind a result, which is the first thing to include in a bug report.
Reader support differs by simulation code — see how mature is each reader before assuming a gap is a bug.