Extracting Raw Data From an Image in MATLAB

% digitize.m
function [x,y] = digitize(imgname)

    [img,map] = imread(imgname);
  image(img);
  colormap(map);

    Left = input('Enter the left X coordinate: ');
  Right = input('Enter the right X coordinate: ');
  Bottom = input('Enter the bottom Y coordinate: ');
  Top = input('Enter the top Y coordinate: ');

    disp('Click on the lower-left corner');
  [localLeft,localBottom] = ginput(1);
  disp('Click on the upper-right corner');
  [localRight,localTop] = ginput(1);

    disp('Begin digitizing points. Click right mouse button when finished.');
  b = 0; bold = 0;
  while(b~=3)
     [xp,yp,b] = ginput(1);
     if (b == 1)
        if (bold ~= 0)
           line([xold; xp], [yold; yp]);
           x = [x; xp];
           y = [y; yp];
        else
           x = xp;
           y = yp;
        end
        xold = xp; yold = yp; bold = b;
     end
  end

    x = (x - localLeft) / (localRight-localLeft) * (Right-Left) + Left;
  y = (y - localBottom) / (localTop - localBottom) * (Top-Bottom) + Bottom;

I want to know what is the logic behind the formula in getting x and y in the last two lines of the code.

Answer 1

[Image Analyst]

They're getting an x and y in pixels, then getting rid of the offset (the pixel row and column where the axes are located) then scaling to the "real world" coordinates, i.e. the units of the graph rather than pixels.

[Priyanka]

How would we know exactly if the final coordinates are right?

[Image Analyst]

Do you trust your user to click on the right point or not? Who's to say what's "right"?