Re: particle analysis-Feret diameter

On Sunday 27 November 2005 08:57, K. Kachrimanis wrote:
> If this is the maximum Feret and it is selected from all Ferets measured,
> then the only thing missing is to report the minimum and a mean Feret also.

My plugin Particles8_Plus at
http://www.dentistry.bham.ac.uk/landinig/software/software.html

does most of the descriptors mentioned and includes the angle of the maximum 
feret diameter. It also calculates the Breadth, which I think is a better 
parameter than the minimum feret, but it uses a different way to estimate 
perimeter (it uses Freeman's algorithm) so sometimes the results are not 
exactly the same as the built-in analyser.

How do you propose to place the other feret diameters at n angles?
Is this based on the image frame of reference or angles offset from the 
direction of the largest one?
And what should one use, the centroid or the centre of mass of the particle? 
Note that when using the centre of mass, holes in the particle make it 
difficult to standardise the measurements (the same particle filled and with 
holes has different feret legths because the "centre" is in a different 
location.

> a "microshape" descriptor is also available (fractal dimension), 

Fractal dimension is not a microshape descriptor, but a global one which has 
no scale (and therefore is not micro, meso or macro). However I have been 
always suspicious of other commercial programs that "estimate" the f dim of 
particles, for two reasons:
1."particles" I guess are image objects that are most of the time not large 
enough to satisfy the requirement of an order of magnitude in the range of 
scales analysed.
2. fractal dimension is a bit of a statistical measure, it depends of a 
goodness of fit to a particular model, so the "goodness" of the result needs 
to be monitored by some other means (for instance the r-squared of a log-log 
plot linear fit). When particles are all different sizes, these goodness of 
fit are not comparable (it is not the same to look at a 30 pixel particle 
than to a 30000 pixel one, in the latter there will be much larger scales 
that are not present in the former, and so one is not measuring the same 
property unless we already know that the objects are strictly fractal). I do 
not think that it is a good idea to do this blindly.
I think that if f dim is incorporated in the Particle Analyzer, it is going to 
be misused extensively. If there were a vote I would go for "no".

Cheers,

Gabriel