angle_cal.m

彩色图像纹理提取的一种算法

% Taken from www.cs.ucr.edu/~eamonn/205/angle_cal.m

% given 3 points; return 0~2pi angle value of their inner angle 
% the coordinate is based on the image coordinate. 0,0 is upper 
% left corner

% function angle_cal
% Left = [0;2];
% Middle = [1; 3];
% Right = [0.5; 3];
% angle1 = angle1(Left,Middle,Right);

function real_angle = angle_cal(Left, Middle, Right)
%Left = [0, 2];
%Middle = [1, 3];
%Right = [0.5, 3];
if ((Left(1)==Middle(1))&&(Left(1)==Right(1))||((Left(2)==Middle(2))&&(Left(2)==Right(2))))
    real_angle = pi;
else
    LM = sqrt ((Left(1) - Middle(1))^2 + (Left(2) - Middle(2))^2);
    MR = sqrt ((Right(1) - Middle(1))^2 + (Right(2) - Middle(2))^2);
    LR = sqrt ((Left(1) - Right(1))^2 + (Left(2) - Right(2))^2);
    cosval = (LM^2 + MR^2 - LR^2)/(2*LM*MR);
    inner_angle = acos(cosval);% get the angel of such inner angle[0, pi] of the two lines 
    % determin whether such angle will be bigger than pi or not.
    if Left(1) == Middle(1)  % horizonal condition
        if Left(2) > Middle(2)  %  <---
            if Right(1)<Left(1) % above the line by "left and middle"
                real_angle = inner_angle;  % [0~pi]
            else real_angle = 2*pi - inner_angle; % below the line [pi ~ 2pi]
            end;
        elseif Left(2) < Middle(2) % ---->
            if Right(1)<Left(1) % above the line by "left and middle"
                real_angle = 2 * pi - inner_angle;  % [pi ~ 2pi]
            else real_angle = inner_angle; % below the line [0 ~ pi]
            end;
        end;
    else % dealing with the vertical cases
    % testy is used to test the right point is in left or right side of the line
    % testy = (y1-y2)/(x1-x2) * x3 + (x1*y2-x2*y1)/(x1-x2)
        testy = ((Right(1) - Middle(1))*Left(2) + (Left(1) - Right(1)) * Middle(2))/(Left(1) - Middle(1));     
        if Left(1) > Middle(1) % raising trend
            if Right(2) < testy % left to the line [pi ~ 2*pi]
                real_angle = 2 * pi - inner_angle;
            elseif Right(2) > testy 
                real_angle = inner_angle;
                else real_angle = pi;
            end;
        else % down trend
            if Right(2) < testy % left to the line [pi ~ 2*pi]
                real_angle = inner_angle;
            elseif Right(2) > testy 
                real_angle = 2 * pi - inner_angle;
            else real_angle = pi;
            end;
        end;%raising trend
    end;%horizonal condition
    if (real_angle<0 || real_angle>2*pi)
        disp('Angle caution!!');
        pause;
    end;
    
end;