Posted by
Ruszkai Ákos on
Feb 25, 2010; 11:28pm
URL: http://imagej.273.s1.nabble.com/Questions-about-equations-of-Shape-Descriptors-tp3688338p3688340.html
On 25 February 2010 23:34, Gabriel Landini <
[hidden email]> wrote:
> On Thursday 25 February 2010 21:10:46 you wrote:
> >
http://rsbweb.nih.gov/ij/docs/menus/analyze.html says:
> >
> > *Round* (roundness): 4*area/(π*sqr(major axis)), or the inverse of the
> > aspect ratio.
> >
> > So... does area in this equation mean "area of best fit ellipse" ?
>
> No, it is area of the blob, which in IJ is the number of pixels.
> Note that other plugins (such as the particles8 and 4 plugins) compute area
> differently than IJ, and so the result values might not be exactly the
> same.
>
> Ok, thank you, that sounds reasonable.
However...
> Major axis, here means "maximum Feret diameter" of the blob.
>
Then it's either very misleading, or even worse, not true at all. For Aspect
ratio, the site says:
*AR* (aspect ratio): major axis/minor axis. Enable "Fit Ellipse" in
*Analyze>Set
Measurements* to have the major and minor axis displayed.
As far as I know, AR is calculated by the other way around: minor/major. Or
am I false here again? I'm aware of the thing that it simply shows how
elongated the area I measure, and it only has real relevance if I have
predefined axis's which I compare to, but somehow eg. 3:4 is just doesn't
feel the same like 4:3.
> > Also, does sqr mean pow(major axis, 2) aka (major axis)^2 ?
> > In that case, how'd you write square root?
>
> pow (n, 2) is n^2.
> sqrt(n) is the square root of n, see the macro functions.
>
Umm... are we exactly sure that circularity is 4π*area/sqr(perimeter) ?
Because
http://www.ncbi.nlm.nih.gov/pubmed/17933272 tells me circularity is:
4π*area/(perimeter)^2 which is != 4π*area/sqr(perimeter) !
Another misleading thing for me: if AR is calculated from the fit ellipse
axis's (either way why would the descriptin say **Enable "Fit
Ellipse" in *Analyze>Set
Measurements* to have the major and minor axis displayed), why does it say
"major axis" again in *
Round* (roundness): 4*area/(π*sqr(major axis)), or the inverse of the aspect
ratio.
?
To be exactly sure, I've double checked with graphical experiment, and Feret
diamater is != Major axis of best fit ellipse.
I hope it's not only my brain that can't add things up here, sorry if I'm
annoying or too much incomprehent.
Regards,
Ákos