  We introduce and study a new scheme to construct relativistic observables
from post-processing light cone data. This construction is based on a novel
approach, LC-Metric, which takes general light cone or snapshot output
generated by arbitrary N-body simulations or emulations and solves the
linearized Einstein equations to determine the spacetime metric on the light
cone. We find that this scheme is able to determine the metric to high
precision, and subsequently generate accurate mock cosmological observations
sensitive to effects such as post-Born lensing and nonlinear ISW contributions.
By comparing to conventional methods in quantifying those general relativistic
effects, we show that this scheme is able to accurately construct the lensing
convergence signal. We also find the accuracy of this method in quantifying the
ISW effects in the highly nonlinear regime outperforms conventional methods by
an order of magnitude. This scheme opens a new path for exploring and modeling
higher-order and nonlinear general relativistic contributions to cosmological
observables, including mock observations of gravitational lensing and the
moving lens and Rees-Sciama effects.
