COMPASSEDGE - Compass edge detection.

Contents

Description

Nothing else than a wrapper for the Generalized Compass Operator for images [RT99a,RT99b,RT01], which contains various information related to edges and corners in an image. See source code [COMPASS] by Ruzon & Tomasi.

Algorithm

The compass operator uses a circular window centered at a junction where 4 pixel squares meet. The needle is a diameter at a given orientation. The color distributions of the two semicircles are computed, and the distance between them is measured using the Earth Mover's Distance (EMD). The maximum EMD over all orientations gives the edge strength and orientation at that point.

Syntax

   [S, O, A, U] = COMPASSEDGE(I);
   [S, O, A, U] = COMPASSEDGE(I, sigma, 'Property', propertyvalue, ...);

Inputs

I : input image, possibly multichannel, with size (X,Y,C); when the variable 'gray' (see below) is set to true, then the function GREYCOMPASS_MEX is applied over the different channels of the input image (even in the case C=3 channels); when 'gray' is set to false and C=3 channels, the function COMPASS_MEX is used: if I is of class uint8, the values are treated as RGB values, if I is of class double, the values are assumed to be in the CIE-Lab color space.

sigma : (optional) scalar defining the standard deviation of the Gaussian used to weight pixels in the Compass Operator (operator's radius is 3*R); default: sigma=1.

Property [propertyname propertyvalues]

'gray' : (optional) logical value set to true when the channels of the image I are treated independently using GREYCOMPASS_MEX, even in the case C=3; default: gray=false, ie. if C=3, the function COMPASS_MEX is applied, GREYCOMPASS_MEX is still applied for all other C values.

'angles' : (optional) angles subtended by the Generalized Compass Operator; if angles is a vector, the Compass Operator will detect corners and edges at different angles, and the output arguments will have multiple rows; default: angles=180 (edges only).

'nwedges' : (optional) number of wedges (orientations) in one-quarter of the circle; nwedges defines the set of possible orientations and angles, so the values in the angles vector must match; default: nwedges=6 (90/6 = 15 degree increments).

Outputs

S : strength of the Compass Operator, lying in the interval [0,1].

O : orientation of the edge or corner, in degrees; for edge detection, this number lies in [0,180) and is measured with respect to the positive X-axis; for corners, the interval is [0,360) and refers to the orientation of the "right" side of the corner.

A : the Abnormality of the Compass Operator at that point; this is the minimum value over all orientations.

U : the Uncertainty of the Compass Operator; this is the number of degrees over which, at each orientation, the compass operator gives similar responses.

References

[RT99a] M. Ruzon and C. Tomasi: "Color edge detection with the Compass Operator", Proc. IEEE CVPR, vol. 2, pp. 160-166, 1999. http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=784624

[RT99b] M. Ruzon and C. Tomasi: "Corner detection in textured color images", Proc. ICCV, vol. 2, pp. 1039-1045, 1999. http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=790384

[RT01] M. Ruzon and C. Tomasi: "Edge, junction, and corner detection using color distributions", IEEE Transactions on Pattern Analysis and Machine Intelligence, 23(11):1281-1295, 2001. http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=969118

[COMPASS] code available at http://ai.stanford.edu/~ruzon/compass/.

Acknowledgment

This function calls the mex functions implemented in the original [COMPASS] code by Ruzon & Tomasi.

See also

Ressembles: EDGECORNER, CANNYEDGE, CANNYEDGEPROD, ROTHWELLEDGE, CONGRUENCYEDGE, SDGDEDGE, ANISOEDGE, ELDERZUCKEREDGE, KOETHEDGE, PETROUEDGE. Requires: COMPASSEDGE_BASE.

Function implementation

function [S, O, A, U] = compassedge(I, varargin)

parsing parameters

error(nargchk(1, 16, nargin, 'struct'));
error(nargoutchk(1, 4, nargout, 'struct'));

if ~isnumeric(I)
    error('cannyedge:inputparameter','a matrix is required in input');
end

p = createParser('COMPASSEDGE');
p.addOptional('sigma', 1, @(x)isscalar(x) && x>=0.05);
p.addParamValue('angles', 180, @(x) (isscalar(x) || isvector(x)) && ...
    all(x>=0 & x<=360));
p.addParamValue('nwedges', 6, @(x)isscalar(x) & x>=1);
p.addParamValue('gray', false, @(x)islogical(x));

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

calculation

[S, O, A, U] = compassedge_base(I, p.sigma, p.angles, p.nwedges, p.gray);

display

if p.disp
    figure, imagesc(S), axis image off, title('compass strenght');
    if size(S,3)==1;  colormap gray;  end
end

end % end of compassedge