Another Rookie Question (thresholding)

Posted by Rick Simmons on Jun 03, 2010; 5:27pm
URL: http://imagej.273.s1.nabble.com/Another-Rookie-Question-thresholding-tp3688072.html

Hello,

I am taking pictures of what are called nodules (phosphate bearing ore
that has been calcined).  The pictures are taken above a moving conveyor
(see example image below).  We are interested in getting a relative
measurement of the particle size distribution of this material.  My
strategy involves using either the Red slice or the Brightness slice of
the color image and then thresholding the slice to isolate individual
nodules.  The thresholded image is smoothed (by subtracting background),
then converted to a binary image.  One the binary image is prepared, we
perform some math on the pixels that fall "under" vertical lines that
are sequentially drawn across the image (20 to 30 individual lines) from
left to right.  The math records the number of black pixels in each
vertical line as well as the number of transitions from black to white
for each line.   These numbers result in an average "black
pixels/nodule" found in each vertical line.  This is all automated in a
macro.  

 

Problem:  On some images, thresholding isolates the background (mostly
the shadows between the nodules) and on others it isolates the
foreground (the nodules) - which is the desired result (see examples
below).  I think this is partly due to the lighting of the original
image (which is not entirely controllable).  Any suggestions on how to
manipulate the image to consistently achieve the desired binary image
(with black nodules) would be greatly appreciated.

 

Thanks in advance

Rick Simmons

 

 

Here is a "good" thresholding result:

 

 

Here is a "bad" thresholding result (the same original image is used for
simplicity):

 

 

 

 


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