linegradient.m

function [ lines ] = linegradient( I, lines )

    I = double(I);

    for i=1:length(lines)
        ldx=0;ldy=0;
        p1=lines(i).point1;p2=lines(i).point2;

         k1=lines(i).k;
         b1=lines(i).b;
         if k1>-1 && k1<1
            for ii=min(p1(1),p2(1)):max(p1(1),p2(1))
                jj = round(k1 * ii+b1);
                [dx,dy]=pointgrad(I, ii, jj);
                ldx=ldx+dx;
                ldy=ldy+dy;
            end
        else
            for jj=min(p1(2),p2(2)):max(p1(2),p2(2))
                if k1~=Inf
                    ii =  round((jj-b1)/k1);
                else
                    ii = p1(1);
                end
                [dx,dy]=pointgrad(I, ii, jj);
                ldx=ldx+dx;
                ldy=ldy+dy;
            end      
        end
        
         lines(i).gradient=[ldx,ldy]./max(abs(ldx),abs(ldy))*15;%,atan(ldy/ldx)
    end

end
function [dx,dy] = pointgrad(I, i, j)

i=floor(i);
j=floor(j);
    s=size(I);
    if j<2||j>=s(1)||i<2||i>=s(2)
        dx=0;
        dy=0;
    else
        dx = [I(j,i+1) - I(j,i-1)]/2;
        dy = [I(j+1,i) - I(j-1,i)]/2;
    end
end