Posted by
Gabriel Landini on
Feb 26, 2010; 10:49am
URL: http://imagej.273.s1.nabble.com/Questions-about-equations-of-Shape-Descriptors-tp3688338p3688350.html
On Thursday 25 Feb 2010 10:17:02 you wrote:
> 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.
In Particles8 I compute the aspect ratio as the ratio of Maximum Feret to the
Breadth of the particle. In IJ, that seems to be done on the fitted ellipse.
They are measuring different things.
> As far as I know, AR is calculated by the other way around: minor/major.
As far as I am aware that is not written in stone, so it is a matter of
definition.
> 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
As you know, since you will be computing all your ARs with the same procedure
it should not make a difference in the result. If you want them the way round
do 1.0/AR. That is why one has to be aware to the definition of the
parameters. Sometimes these do not match across different packages (or
plugins). The advantage here is that you can check the source to see exactly
how things are done. Try that in any "commercial" package.
> 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) !
Perhaps I confused you in my reply. The formula is correct
4π*area/(perimeter)^2
I just pointed out that "sqrt" (no "t") is the square root in the macro
language. But I now realise that this was not what you were asking.
> 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.
This is confusing but only because the way IJ computes AR.
As mentioned before, AR in IJ is for the fitted ellipse, not the particle.
You need to write your own AR or use Particles8 to compute the AR of the
particle instead.
Cheers
G.