[Tutor] What Centroid Algorithm Is This?

[Wayne Watson]

I'm looking at a GUI program that handles meteor images. They leave a trail 
across the sky. Each video frame (30 fps) provides data on the track, and a 
centroid is found around each track point. 'm not familiar with the 
algorithm that does this, but part of it is in the three def stmts below. 
Maybe someone can hazard a guess? It looks like someone had fun doing this.


     def MakeSelectsAndWeights( self ):
         global SELECT1, SELECT2
         global WEIGHT1, WEIGHT2

         a1 = indices((640,))[0]

         a2 = a1 + a1 * (701.0-640.0)/702.0
         a3 = floor(a2)
         SELECT1 = a3.astype( intc )

         SELECT2 = SELECT1 + 1

         WEIGHT2 = a2  - a3
         WEIGHT1 = 1.0 - WEIGHT2

One more
     def Centroid( self, ref, curr, mask ):
         slopes = indices((488,722),int32)
         nada   = zeros((488,722),uint8)

         result = where(curr > ref, curr-ref, nada)
         result = where(result > mask, result-mask, nada)

         xresult = result * slopes[1]
         yresult = result * slopes[0]
         total   = sum(ravel(asarray(result,int32)))
         count   = sum(ravel(result > 0))
         if total == 0:
             return 360,240,0,0

         xpos = sum(ravel(xresult))/total
         ypos = sum(ravel(yresult))/total

         return xpos,ypos,total,count

Another
     def Central( self, ref, curr, mask ):
         slopes = indices((488,722),int32)
         nada   = zeros((488,722),uint8)

         result = where(curr > ref, curr-ref, nada)
         result = where(result > mask, result-mask, nada)
         total   = sum(ravel(asarray(result,int32)))
         count   = sum(ravel(result > 0))
         if total == 0:
             print "Count: %d Total: %d" % (count,total)
             return 361,244,0,0

         blur  = result[0:-2, 0:-2]
         blur += result[0:-2, 1:-1]
         blur += result[0:-2, 2:  ]
         blur += result[1:-1, 0:-2]
         blur += result[1:-1, 1:-1]
         blur += result[1:-1, 2:  ]
         blur += result[2:  , 0:-2]
         blur += result[2:  , 1:-1]
         blur += result[2:  , 2:  ]

         index = argmax(ravel(blur))
         ypos, xpos  = divmod(index, 720)
         xpos += 1
         ypos += 1

         return xpos,ypos,total,count

-- 
            Wayne Watson (Watson Adventures, Prop., Nevada City, CA)

              (121.01 Deg. W, 39.26 Deg. N) GMT-8 hr std. time)

          Copper and its alloys have been found effective in hospital
          sinks, hand rails, beds, ... in significantly reducing
          bacteria. Estimates are 1/20 people admitted to a hospital
          become infected, and 1/20 die from the infection.
                        -- NPR Science Friday, 01/16/2009

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