possible error in docs?

[Aryeh Weiss]

I was looking for how ImageJ calculates roundness, and I found the
following:
/
Shape descriptors/ Calculates and displays the following shape descriptors:

  *
    /Circularity/ /4/π/ × [/Area/][/Perimeter/]^2 / with a value of 1.0
    indicating a perfect circle. As the value approaches 0.0, it
    indicates an increasingly elongated shape. Values may not be valid
    for very small particles. Uses the heading*Circ*.
  *
    /Aspect ratio/ The aspect ratio of the particle’s fitted ellipse,
    i.e., [/Major/ /Axis/][/Minor/ /Axis/]. If */Fit Ellipse/* is
    selected the*Major* and *Minor* axis are displayed. Uses the
    heading*AR*.
  *
    /Roundness/ 4 × [/Area/]/π/ × [/Major/ /axis/]^2 or the inverse of
    /Aspect Ratio/. Uses the heading *Round*.

https://imagej.nih.gov/ij/docs/guide/146-30.html

I think that the 4  in the Roundness  definition is not correct, and it
should be Area/[pi*(Major Axis)^2],
and not 4 x Area /[pi*(Major Axis)^2]

Am I correct?

--aryeh

--
Aryeh Weiss
Faculty of Engineering
Bar Ilan University
Ramat Gan 52900 Israel

Ph:  972-3-5317638
FAX: [phone]


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ImageJ mailing list: http://imagej.nih.gov/ij/list.html

[Avital Steinberg]

Hi Aryeh,
The area of an ellipse is: PI*(major/2)*(minor)/2

So the roundness would be: Area/(PI*major^2) =( 4*(major/2)(minor/2) )/
PI*major^2

And the 4 cancels out  - what you get for the roundness is minor/major.
This is 1 for a perfect circle.

Does this help?
Avital

On Mon, Sep 26, 2016 at 3:07 PM, Aryeh Weiss <[hidden email]> wrote:

> I was looking for how ImageJ calculates roundness, and I found the
> following:
> /
> Shape descriptors/ Calculates and displays the following shape descriptors:
>
>  *
>    /Circularity/ /4/π/ × [/Area/][/Perimeter/]^2 / with a value of 1.0
>    indicating a perfect circle. As the value approaches 0.0, it
>    indicates an increasingly elongated shape. Values may not be valid
>    for very small particles. Uses the heading*Circ*.
>  *
>    /Aspect ratio/ The aspect ratio of the particle’s fitted ellipse,
>    i.e., [/Major/ /Axis/][/Minor/ /Axis/]. If */Fit Ellipse/* is
>    selected the*Major* and *Minor* axis are displayed. Uses the
>    heading*AR*.
>  *
>    /Roundness/ 4 × [/Area/]/π/ × [/Major/ /axis/]^2 or the inverse of
>    /Aspect Ratio/. Uses the heading *Round*.
>
> https://imagej.nih.gov/ij/docs/guide/146-30.html
>
> I think that the 4  in the Roundness  definition is not correct, and it
> should be Area/[pi*(Major Axis)^2],
> and not 4 x Area /[pi*(Major Axis)^2]
>
> Am I correct?
>
> --aryeh
>
> --
> Aryeh Weiss
> Faculty of Engineering
> Bar Ilan University
> Ramat Gan 52900 Israel
>
> Ph:  972-3-5317638
> FAX: [phone]
>
>
> --
> ImageJ mailing list: http://imagej.nih.gov/ij/list.html
>

--
ImageJ mailing list: http://imagej.nih.gov/ij/list.html

[Aryeh Weiss]

Hi Avital

Thank you for your reply.

You are correct -- I forgot to divide the axes by 2.

Best regards
--aryeh

On 26/09/2016 3:37 PM, Avital Steinberg wrote:

> Hi Aryeh,
> The area of an ellipse is: PI*(major/2)*(minor)/2
>
> So the roundness would be: Area/(PI*major^2) =( 4*(major/2)(minor/2)
> )/ PI*major^2
>
> And the 4 cancels out  - what you get for the roundness is
> minor/major. This is 1 for a perfect circle.
>
> Does this help?
> Avital
>
> On Mon, Sep 26, 2016 at 3:07 PM, Aryeh Weiss <[hidden email]
> <mailto:[hidden email]>> wrote:
>
>     I was looking for how ImageJ calculates roundness, and I found the
>     following:
>     /
>     Shape descriptors/ Calculates and displays the following shape
>     descriptors:
>
>      *
>        /Circularity/ /4/π/ × [/Area/][/Perimeter/]^2 / with a value of 1.0
>        indicating a perfect circle. As the value approaches 0.0, it
>        indicates an increasingly elongated shape. Values may not be valid
>        for very small particles. Uses the heading*Circ*.
>      *
>        /Aspect ratio/ The aspect ratio of the particle’s fitted ellipse,
>        i.e., [/Major/ /Axis/][/Minor/ /Axis/]. If */Fit Ellipse/* is
>        selected the*Major* and *Minor* axis are displayed. Uses the
>        heading*AR*.
>      *
>        /Roundness/ 4 × [/Area/]/π/ × [/Major/ /axis/]^2 or the inverse of
>        /Aspect Ratio/. Uses the heading *Round*.
>
>     https://imagej.nih.gov/ij/docs/guide/146-30.html
>     <https://imagej.nih.gov/ij/docs/guide/146-30.html>
>
>     I think that the 4  in the Roundness  definition is not correct,
>     and it should be Area/[pi*(Major Axis)^2],
>     and not 4 x Area /[pi*(Major Axis)^2]
>
>     Am I correct?
>
>     --aryeh
>
>     --
>     Aryeh Weiss
>     Faculty of Engineering
>     Bar Ilan University
>     Ramat Gan 52900 Israel
>
>     Ph: 972-3-5317638 <tel:[phone]>
>     FAX: [phone] <tel:[phone]>
>
>
>     --
>     ImageJ mailing list: http://imagej.nih.gov/ij/list.html
>     <http://imagej.nih.gov/ij/list.html>
>
>

--
Aryeh Weiss
Faculty of Engineering
Bar Ilan University
Ramat Gan 52900 Israel

Ph:  972-3-5317638
FAX: [phone]


--
ImageJ mailing list: http://imagej.nih.gov/ij/list.html