| Re: Area of a Blob [message #33200 is a reply to message #33196] |
Thu, 12 December 2002 09:07   |
![Stein Vidar Hagfors H[2] is currently offline](theme/default/images/xoffline.png.pagespeed.ic.XRkd1fkXye.png "Stein Vidar Hagfors H[2] is currently offline")
Stein Vidar Hagfors H[2]
Messages: 28
Registered: October 2002
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Junior Member |
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David Fanning <david@dfanning.com> writes:
[...]
> What is the area of a blob on an image?
>
> The answer, of course, is that it depends on who
> is asking.
[...]
> Area by .....
> Simple Count: 7390.00
> Russ Method: 7236.50
> PolyfillV Method: 7313.00
> IDLgrROI computeGeometry: 7236.50
> IDLgrROI Mask Method: 7391.00
Now, without knowing the details of the different methods, I'd say
that it's not at all surprising to get different answers using
different methods, because the question isn't well defined as stated,
without a qualification of "what do you *mean* by the area of a blob
(and how do you specify a blob, anyway)".
If you cound a blob as those pixels that are picked by a series of ROI
indices, then just count the pixels. If you mean the geometrical area
inside a polygon in units of square pixels, then you get a different
answer (pixels may be bisected by the border polygon). If you mean
"the number of pixels fully enclosed by the polygon" you get a third
answer, if you mean "the number of pixels partially *or* fully
enclosed by the polygon" you get a fourth, and so on (ROI methods
may be returning one of the above).
> What do you make of this? Does anyone have any insight?
> Does it matter how you computer area as long as you are
> consistent? Or is one method more accurate than others?
> What is the *real* answer?
>
> Appreciate your thoughts. :-)
Not sure if the above helps you, but as long as you appreciate it, I'm
happy ;-)
--
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Stein Vidar Hagfors Haugan
ESA SOHO SOC/European Space Agency Science Operations Coordinator for SOHO
NASA Goddard Space Flight Center, Tel.: 1-301-286-9028
Mail Code 682.3, Bld. 26, Room G-1, Cell: 1-240-354-6066
Greenbelt, Maryland 20771, USA. Fax: 1-301-286-0264
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