title: find and plot locations of minimum pixel intensity in each column
body: <p>i have a gray scale image. i would like to scan column by column to find the location of the darkest pixel in each column, obtain the (x,y) locations and plot a mark on it. the output of the prog should be an image with marks on each column's darkest pixel. </p>
  
  <p>my attempt resulted in no output at all. please help me.</p>
  
  <pre><code>[row col] =size(i11);
  for j=1:col
      for i=1:row
          darkestPixelValue = min(i11(i,j));    
          [i,j]=find(i11==darkestPixelValue);
          plot (i,j);
      end
  end
  </code></pre>