Re: Measurement of Feret diameter

Posted by BenTupper on
URL: http://imagej.273.s1.nabble.com/Measurement-of-Feret-diameter-tp5003818p5003935.html

Hi,

On Jul 11, 2013, at 9:29 AM, Michael Schmid wrote:

> Hi everyone,
>
> there was a recent thread on the accuracy of the Feret diameter in ImageJ.
>
> 'Analyze Perticles' does not use the code of shapeRoi but the code in Roi.java.  There, the maximum Feret (and the Feret angle, which refers to the maximum width) is calculated accurately, by a double loop over the corner pixels.
>
> Varying the angle (in 0.5-degree steps) is used only for the minimum Feret.  This is quite accurate unless the particles are elongated extremely much.
>
> [Shape Rois are created only as a result of merging or intersecting rois; this does not happen when using 'Analyze Particles']
>

Thanks for that clarification.

Cheers,
Ben

> Anyhow, if you have objects that can be approximated as circles (2D section through spherical particles or 2D projection of spheres), the area or 'Fit Ellipse' give you much better indications of size than the Feret diameters.
>
> If you want to use the Feret nevertheless, assuming ellipsoidal particles, one would typically use sqrt(minFeret * maxFeret), not the arithmetic mean.
>
> Michael
> ________________________________________________________________
> On Jul 11, 2013, at 14:38, Ben Tupper wrote:
>
>> Hi,
>>
>> On Jul 8, 2013, at 1:12 PM, Geology Guy wrote:
>>
>>> Good Morning Everyone,
>>> The Feret's diameter that is measured is a measurement of the "maximum
>>> diameter" I believe and the MinFeret is also provided. I read a paper saying
>>> that just one Feret measurement of a non spherical particle would be
>>> meaningless as the Feret measurement is based on a statistical average after
>>> rotating the particle through all different possible angles. Does that mean
>>> that I have to add the Feret and MinFeret and divide by 2 to get the average
>>> value or what would be the best way to go about this?
>>>
>>
>> The computation of ferets already involves rotation - you can see the details in the source code for ShapeRoi.  The rotation is divided into 2 degree increments - which is different than "all different possible angles".  Getting the latter would require a good deal of patience.
>>
>> http://rsb.info.nih.gov/ij/developer/source/ij/gui/ShapeRoi.java.html
>>
>> Are you looking for a representative size measurement?
>>
>> Cheers,
>> Ben
>>
>>
>>
>>> Thanks for your reply
>>>
>>>
>>>
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>>>
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