scanned target calibration

clear
close all
rgb = imread('Scan_Pic0003.jpg');
%imshow(rgb);

%imshow(rgb);
% [I3 RECT3] = imcrop;
% save pix2 RECT3

load pix2
I3 = imcrop(rgb,RECT3);
imshow(I3);
im = im2double(rgb2gray(I3));
zm = 0.5;

Warning: Image is too big to fit on screen; displaying at 50%

find_edges;

Warning: Image is too big to fit on screen; displaying at 50% 
Warning: Image is too big to fit on screen; displaying at 50% 
   n    a    b  len
   1   79  368  298
   2  384  654  286
   3  669  955  289
   4  658  666    9
   5  661  668    5
   6   67   70   10
   7    1    4    4
   8    5    5    1
   9   16   17    2
  10   56   60    6
  11   41   42   15
  12   29   33    3
  13    7   26   19
  14   45   54   10

15 allocated segments
957 total edgels

ans =

     3

Warning: Image is too big to fit on screen; displaying at 50%

dpi = 599.406; % value from ruler1 (no filtering)
dpmm = dpi/25.4;

xpos = ones(1,3);
for n = 1:3
    subplot(3,1,n);
    [x y] = getseg(seg(n),u,v,next);
    npts = length(y);
    xmean = mean(x);
    xpos(n) = xmean;
    x = x - xmean;
    c = polyfit(y,x,1);
    fprintf('n %d slope %g intercept %g\n',n,c);
    fprintf('mean x %g rms %g\n',xmean,std(x));
    ydx = [min(y) max(y)];
    plot(y,x,ydx,polyval(c,ydx));
end

n 1 slope 0.000606112 intercept -0.086293
mean x 234.185 rms 0.422997
n 2 slope -0.000249657 intercept 0.0355994
mean x 1095.93 rms 0.220096
n 3 slope 0.00101544 intercept -0.143787
mean x 1955.58 rms 0.347632

cal = diff(xpos)/dpmm;
fprintf('white square width %g mm, black square width %g mm\n',cal);

white square width 36.5166 mm, black square width 36.4281 mm