Numerical relativity coordinate system displayed In a picture or video of a numerical relativity simulation, such as a neutron star merger into a black hole, how do they set up their coordinate system? Lets take the point in a video corresponding to x=10km, y=20km, z=30km, t=1ms. Spacetime itself is distorted, in a very complex way, so how do you make sense of these numbers?
Website to find some nice videos:
http://numrel.aei.mpg.de/images
Just to clarify: There are simulations in which space-time is fixed to the a well defined metric (e.g. Kerr black hole accretion disk MHD simulation with no disk self-gravity). But for true numerical relativity, in which the shape of space-time itself has to be simulated, there is no "clean" metric.
 A: While I do not work on GRHD/GRMHD simulations, I am fairly certain that the numbers you would see in a simulation (displayed as an image or a video, or even the initial conditions file) would be the laboratory reference frame.
A: There is a huge variance in how these coordinates are set up, and very often the coordinate systems are chosen for computational convenience (having more data points in place where the metric varies a lot, and fewer far from, say, your colliding black holes), in addition to more physical choices.  Once you have run the simulation and have found a solution, however, you can apply math and create any coordinate system you wish for visualizations.  
A: In the program David Madore has used to simulate Kerr black holes, the description mentions

Note: this program does not handle correctly the region where two horizons cross (where neither ingoing nor outgoing Kerr coordinates are adequate). This will result in some “snow” around such regions. This is a numerical artefact. 

Thus, the program is using ingoing and outgoing Kerr coordinates internally for the computations, but that might not be the best solution.  See also some notes on what all the different coordinate systems mean.
For visualization, a free falling camera is used.
