Posted by
Michael Schmid on
Jul 15, 2013; 2:26pm
URL: http://imagej.273.s1.nabble.com/Measurement-of-Feret-diameter-tp5003818p5003977.html
Hi,
your sample image shows streaks -- probably scratches due to cutting or polishing. You should try to avoid them by better sample preparation. If this is not possible, as a second-best option, before thresholding, you need to remove them. Do a Fourier transform (FFT), then select the bright wedge with the polygon tool (the selection should be slightly wider than the wedge, but not reach the very center of the image). Doing the selection on one side of the center is enough (ImageJ internally uses mirroring). Then, Process>Math>Set to 0 and do an inverse FFT. This won't remove the streaks completely, but it will help a lot.
Due to noise, you will probably also need some edge-preserving blur plugin such as 'Thresholded blur'.
Finally, after thresholding and converting to an 8-bit mask:
As it seems some particles touch each other, you need Process>Binary>Watershed to fully separate the touching particles.
---
Coming back to your question: the area of an ellipse with major axis 'a' and minor axis 'b' is a*b*pi/4.
[you will also often fine the equation a*b*pi; then a and b are semimajor and semiminor axes; ImageJ gives you the full axes, not the half (semi) axes]
As your particles are solid, there is not much point using the ellipse to measure the area; the 'Area' column of 'Analyze Particles' gives a more accurate result.
Michael
________________________________________________________________
On Jul 12, 2013, at 14:43, Geology Guy wrote:
> Dear Ben,
>
> <
http://imagej.1557.x6.nabble.com/file/n5003955/Example_of_image_I_am_working_on.png>
>
> What I am trying to measure are 2D images of particles that are clustered
> like raspberries (particles within the yellow boundary in the image
> attached). I am doing a project with my Professor and he wants me to use
> Feret's diameter as one of the criteria for measuring the size of the
> individual crystals in the cluster, as well as the size of the cluster
> (framboid) itself. So I will go by the formula for calculating the Feret
> diameter that Michael suggested above. The only other question I have is
> that if you look at the "Fit Ellipse" in image J, it gives only a major and
> minor axis measurement. In order to use Fit Ellipse to measure size, I
> assume I just have to multiply the major and minor axis?
>
> Thanks a lot for responding on time.
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