Record: the plate stores the intensity of the sum of an object wave E_o (spherical, from the object point) and a plane reference wave E_r at angle theta:
I(y) = |E_o + E_r|^2 = |E_o|^2 + |E_r|^2 + 2 Re(E_o E_r*)
visibility V = 2 sqrt(r) / (1 + r), r = Io/Ir
local period = lambda / |sin(phi_o(y)) - sin(theta)|
Replay: the plate (transmittance I(y)) is lit by the reference only; summing Huygens wavelets E(x,y) = sum_j I_j E_r(y_j) exp(ikr_j)/sqrt(r_j) reproduces a term proportional to E_o, so the diffracted light diverges as if from the original object point (virtual image), plus a conjugate real image and the undiffracted beam.