  A general procedure for obtaining frame-independent, three-dimensional
light-front coordinate-space wave functions is introduced. The third spatial
coordinate, $\tilde z$ , is the frame independent coordinate conjugate to the
light-front momentum coordinate $x={k^+\over P^+}$ which appears in the
momentum-space light-front wave functions underlying generalized parton
distributions, structure functions, distribution amplitudes, form factors, and
other hadronic observables. These causal light-front coordinate-space wave
functions are used to derive a general expression for the quark distribution
function of hadrons as an integral over the frame-independent longitudinal
distance (the Ioffe time) between virtual-photon absorption and emission
appearing in the forward virtual photon-hadron Compton scattering amplitude.
Specific examples using models derived from light-front holographic QCD show
that the spatial extent of the proton eigenfunction in the longitudinal
direction can have very large extent in $\tilde z$.
