1. A method for numerically modeling evolution of an image propagating through a medium, the method comprising: representing an image using image data comprising a plurality of spatially-indexed amplitude values, the image data comprising a center located relative to the plurality of spatially-indexed amplitude values; providing a propagation medium model comprising quadratic phase properties which are defined relative to a propagation centerline of the propagation medium model; aligning the propagation centerline of the propagation medium model relative to the center of the image data; approximating the propagation medium model with a numerical operator for applying an index-shifted numerical fractional Fourier transform operation on the image data, the numerical operator having original-domain indices and transform-domain indices, wherein the original-domain indices comprise a zero original-domain origin that is centered within the original-domain indices, and the transform-domain indices comprise a zero transform-domain origin that is centered within the transform-domain indices; and aligning the zero original-domain origin relative to the center of the image data to produce transformed image data comprising a zero frequency-domain origin that is centered within the transform-domain indices.