On Tue, 16 Jul 1996, Ana Pombo wrote:
 
> Is there a procedure (in NIH Image or elsewhere) to automatically obtain the
> gravity centres directly from my images, taking into account the grey level
> information? Could I get the details?
 
   Hello, Ana.
 
   The easiest way we found was just a straightforward mathematical
implementation of the center of mass equation.  While I'm not yet
familiar with NIH Image, I'll assume it supports basic math functions,
which is all you need:
   1) Create an image for each dimention (x and y) that is just a slope
in that dimention.  ie, g1(x,y) = x for the x dimention, g2(x,y) = y for
the y dimention.
   2) Multiply the original image you want the center of by each of the 2
slope images (x and y).  Make sure the multiplication is element by
element, not a matrix mult.
   3) From each result (x and y), sum the values in the product image to
get a single scalar value.
   4) Divide each of these values (x and y) by the sum of the values in
the original image.  The quotients (x and y) are the x and y coordinates
of the center of mass.
   I imagine this process can easily be made into a script or macro that
will perform the task at a keystroke.
 
   BEWARE:  One problem I've encountered is a non-zero black level.  Make
sure that, if your image has a "background" (a black space where the
"mass" object is not), that the values for the black are set to zero.
Scale your image if necessary to provide this.  If you do not, it's much
like finding the center of mass of a pizza with toppings (background and
objects), instead of just the toppings (which are the objects of the
image).
 
   Hope this helps... apologies in advance if this is all stuff you
already knew and I totally misunderstood the question.  :)
 
                                                         -Joe
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