enhance.m

function [ enhanced_polar_frame ] = enhance( polar_frame, range_start, range_stop )
%ENHANCE YEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAH
%   CSI-style enhancements for sonar imagery
%
%   Assumes 
%   - image is 512x96, where the first row is the nearest range bin
%   (range_start) and the last (512th) is the farthest;
%   - image range is [0 1], which corresponds to [0 90] dB
%   
%
%   Pedro Vaz Teixeira, June 2014
%   pvt@mit.edu

enhanced_polar_frame = polar_frame; % all subsequent operations on this variable

%% transmission loss (geometric+absroption)
%{
delta_r = (range_stop- range_start) / 512;

alpha = 0.945;  % absorption coefficient [dB(re 1m)/m]

r = range_start:delta_r:(range_stop-delta_r);

H = 2*alpha.*r + 20*log(r.^2);

% subtract transmission loss per each range bin.
for i=1:512;
    enhanced_polar_frame(i,:) = max(((90*polar_frame(i,:) - H(i)*ones(1,96))/90), zeros(1,96));
end
%}


%% beam pattern 'taper'
%

% experimental coefficients (from SOUNDMETRICS)

k1 = -0.0089;
k2 = 0.8515;
k3 = -20.0994;

i = 0:95; % beam index
k0 = 0.6; % my own, additional, correction factor
a = (k0)*(k1 * i.^2 + k2 * i + k3*ones(1,96));

s = 1/70; % this is the MuPerDB (image intensity over dB, eg. 255/(70dB) )
a = -s*a;% + 0.5*ones(1,96);

offset = repmat(a', [1 512]);

%enhanced_polar_frame = enhanced_polar_frame + offset;
%}

%% cross-talk reduction
%

% psf creation (isotropic, simplified)
beam = zeros(1,96);
beam(1,[1 9 17 25 33 41 49 57 65 73 81 89]) =[  24 24 24 27 32 40 70 40 32 27 24 24];
PSF = (1/sum(sum(beam)))*beam';

% apply wiener filter deconvolution
estimated_nsr = (0.0018); % replace with experimentally determined value (variance!)
enhanced_polar_frame = deconvwnr(enhanced_polar_frame, PSF, estimated_nsr);

% normalization
%
enhanced_polar_frame = (1/max(enhanced_polar_frame(:)))*enhanced_polar_frame; % normalize to the [0,1] range
enhanced_polar_frame = max(polar_frame(:))*enhanced_polar_frame; % match to the same max intensity as the original image
%}
%enhanced_polar_frame(enhanced_polar_frame<0)=0; % this does nothing.