HISTOEQUALIZATION - Histogram equalization of an image.

Contents

Description

Perform histogram equalization, ie. change the values of the input image so that its reordering match the ordered values of another vector/matrix or a given density.

Syntax

X = HISTOEQUALIZATION(x, y);
X = HISTOEQUALIZATION(x, y, 'Property', propertyvalue, ...);
X = HISTOEQUALIZATION(x, 'gauss'/'lin', 'Property', propertyvalue, ...);

Inputs

x : vector/matrix to equalize.

y : reference vector/matrix to operate the equalization of x, or string set to 'lin' or 'gauss' for linear or gaussian equalization resp.

Property [propertyname propertyvalues]

'dir' : string set to operate the equalization along a preferential direction when the input is a multidimensional matrix; it can be either 'col', 'row' or 'vec' to consider the distribution along the columns, the rows or the 3rd dimension resp.; default: dir=[], ie. no preferential direction for equalizing the values, the equalization is performed wrt the distribution of all the values in the matrix x.

'abs' : logical flag set to true to operate only on absolute values; default: abs=false.

'lam' : scalar in [0,1] to interpolate between the two histograms; default: lam=1.

Outputs

X : equalized version of x.

See also

Ressembles: HIST. Requires: HISTOEQUALIZATION_BASE.

Function implementation

function X = histoequalization(x, varargin)

parsing parameters

error(nargchk(1, 16, nargin, 'struct'));
error(nargoutchk(1, 1, nargout, 'struct'));

if ~isnumeric(x)
    error('histoequalization:inputparameter','vector or matrix required in input');
end

p = createParser('HISTOEQUALIZATION');
p.addRequired('y', @(x)isnumeric(x) || ...
    (ischar(x) && any(strcmpi(x,{'lin','gauss'}))));  % has to be entered!!!
p.addParamValue('dir',[], @(x)isempty(x) || ...
    (ischar(x) && any(strcmpi(x,{'row','col','vec'}))));
p.addParamValue('lam', 1, @(x)isscalar(x) && x<=1 && x>=0);
p.addParamValue('abs', false, @(x)islogical(x));

% parse and validate all input arguments
p.parse(varargin{:});
p = getvarParser(p);

checking variables

if ~isempty(p.dir) && ischar(p.dir) && isnumeric(p.y)
    if strcmpi(p.dir,'col') && ~size(x,2)>1 || ~(size(x,2)==size(p.y,2))
        error('histoequalization:inputerror', ...
            'x and y must have same number of columns');


    elseif strcmpi(p.dir,'row') && ~size(x,1)>1 || ~(size(x,1)==size(p.y,1))
        error('histoequalization:inputerror', ...
            'x and y must have same number of rows');

    elseif strcmpi(p.dir,'vec') && ~size(x,3)>1 || ~(size(x,3)==size(p.y,3))
        error('histoequalization:inputerror', ...
            'x and y must have same number of dim 3');
    end
end

main computation

X = histoequalization_base(x, p.y, p.dir, p.abs, p.lam);

end % end of histoequalization