I have to tell you this one.  Its the best free software I have seen
in years!
http://www.ddsdx.uthscsa.edu/dig/itdesc.html
I was looking to buy some image analysis software and was able to
download some very advanced software from this site for free.
One tip, use the software to convert colour pics to grey scale before
analysis.
I tried it out on muscle biopsies and it can identify the fiber types
and do stats on size, length, width etc.
It has a huge range of filters and maths functions to edit the image.
The statistical output can be exported and opened in Excel.

Best Wishes
Steve Machin UK

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