by David Legland Geometry library for matlab. Performs geometric computations on points, lines, circles, polygons...
function theta = edgeAngle(edge)
%EDGEANGLE Return angle of edge
% A = edgeAngle(EDGE)
% Returns the angle between horizontal, right-axis and the edge EDGE.
% Angle is given in radians, between 0 and 2*pi, in counter-clockwise
% Notation for edge is [x1 y1 x2 y2] (coordinates of starting and ending
% p1 = [10 20];
% p2 = [30 40];
% rad2deg(edgeAngle([p1 p2]))
% ans =
% See also
% edges2d, angles2d, edgeAngle, lineAngle, edgeLength
% author : David Legland
% INRA - TPV URPOI - BIA IMASTE
% created the 06/04/2003.
line = createLine(edge(:,1:2), edge(:,3:4));
theta = lineAngle(line);