You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: retina.m
    path of file: ./repos/gkaModelEye/model/eye/species/+human
    
    the code of the file until where you have to start completion: function retina = retina(eye)
% Returns the retina sub-field of an eye model structure
%
% Syntax:
%  retina = human.retina( eye )
%
% Description:
%   The retinal surface (i.e., the vitreous chamber / posterior segment) is
%   modeled as an ellipsoidal quadric surface, with adjustments in size for
%   the spherical ametropia of the eye. Atchison 2006 provides the semi-
%   radii of a triaxial ellipsoid model of the retinal surface, with these
%   values varying by spherical ametropia:
%
%       Atchison, David A. "Optical models for human myopic eyes." Vision
%       research 46.14 (2006): 2236-2250.
%
%   The Atchison model is arranged around the visual axis, with the
%   vitreous chamber tilted and shifted. In the current model, the retinal
%   ellipsoid is aligned with and centered on the optical axis.
%
% Inputs:
%   eye                   - Structure.
%
% Outputs:
%   retina                - Structure.
%
% Examples:

    
function retina = retina(eye)
% Returns the retina sub-field of an eye model structure
%
% Syntax:
%  retina = human.retina( eye )
%
% Description:
%   The retinal surface (i.e., the vitreous chamber / posterior segment) is
%   modeled as an ellipsoidal quadric surface, with adjustments in size for
%   the spherical ametropia of the eye. Atchison 2006 provides the semi-
%   radii of a triaxial ellipsoid model of the retinal surface, with these
%   values varying by spherical ametropia:
%
%       Atchison, David A. "Optical models for human myopic eyes." Vision
%       research 46.14 (2006): 2236-2250.
%
%   The Atchison model is arranged around the visual axis, with the
%   vitreous chamber tilted and shifted. In the current model, the retinal
%   ellipsoid is aligned with and centered on the optical axis.
%
% Inputs:
%   eye                   - Structure.
%
% Outputs:
%   retina                - Structure.
%
% Examples:


% Semi-radii of the ellipsoid as a function of ametropia, values taken from
% Atchison 2006.
retinaRadiiEmmetrope = [10.148 11.455 11.365];
retinaRadiiAmetropiaSlope = [-0.163 -0.043 -0.090];
retinaRadii = ...
    retinaRadiiEmmetrope + retinaRadiiAmetropiaSlope.* eye.meta.sphericalAmetropia;

% The current model holds the depth of the anterior chamber constant across
% ametropia, consistent with Atchison.
%
% To position the retina, we need to know the axial length of the eye. If
% this value is not provided, it is derived from the Atchison equation.
if isempty(eye.meta.axialLength)
    axialLength = 23.58 - 0.299*eye.meta.sphericalAmetropia;
else
    axialLength = eye.meta.axialLength;
end