[Insight-users] SignedDanielssonDistanceMap: squared distances inexact?

[lynx.abraxas at freenet.de]

Dear Dan Mueller,


Thank You for Your quick reply with the examples.
What   I   still   wonder   is   why   I   don't   get   2   when  I  ask  the
SignedDanielssonDistanceMap for squared  distance  output.  Squared  distances
should  be  always  integers  (if  I'm  not mistaken) and should go in the tif
unmodified (eg. 2).
Anyway, I changed my intermediat pixel format to float and  the  output  of  a
single white pixel is as I'd expect.
However  if I run my little watershed test program there is one blob (dark red
blobl in blobs_ws03.tif) that is surrounded by four split blobs but itself  is
not  split  although  it  has some crease. I even made the effort to count the
pixels by hand. The  minima-line  is  8  pixels  off  the  rim  in  horizontal
direction.  Now  accounting  for the additional distance to the left up to the
end of the minima-line I'd say the gray values should decrease  further  since
the  distanc  to the left has to be added to the 8 pixels up or down.But still
blobs_rm01.tif has just one minimum. Are my expectations wrong here?
I ran my test prog like this for the pics in the tbz  (h-min  height=  0,  all
fully connectd):
watershed05b blobs.tif blobs_sddm_01.vtk blobs_hm01.tif blobs_rm01.tif blobs_cc01.tif blobs_ws01.tif blobs_ws02.tif blobs_ws03.tif 0 1 1


Thanks for hints or help
Lynx


On 26/07/09 15:33:53, Dan Mueller wrote:
> Hi Lynx,
> 
> The SignedDanielssonDistanceMap will perform the distance computation
> using real (ie. float/double) precision. If the output is integer (ie.
> signed short in your case) it will cast the result. The corner pixels
> (which you expect to be 2) are actual sqrt(2) = 1.414, which when cast
> to signed short come out as 1.
> 
> Please find attached an input image for which the floating point
> signed distance (InsideIsPositive=False) has been computed -- the
> result is as expected.
> 
> HTH
> 
> Cheers, Dan
> 
> 2009/7/26  <lynx.abraxas at freenet.de>:
> > Hello!
> >
> >
> > Using  the  SignedDanielssonDistanceMap and squared distances (signed short as
> > output) I wonder why a distance of 2 has the same grey value as  the  distance
> > 1.
> > eg. input image (x white, space black):
> > |   |
> > | x |
> > |   |
> >
> > The relative distance map I get looks like:
> > |111|
> > |1x1|
> > |111|
> >
> > I'd expect:
> > |212|
> > |1x1|
> > |212|
> >
> > Is  my  expactation wrong or is the SignedDanielssonDistanceMapImageFilter not
> > that accurate? Would flout as output and not squared distances exchage the  2s
> > by sqrt(2)?
> >
> > Thanks for any hint or help
> > Lynx





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