I managed to work out:
1. photograph gear on contrasting background
2. crop, convert to heightfield
3. take a slice
4. use trace to measure the width of the gear
5. make an educated guess about number of teeth
6. use trace to take at least twice that number of samples around the perimeter
7. use a change in the direction of the measurement, compared to the radius
determined in (4) to increment the counter

preliminary results so far:

RA0-1179 (69 teeth)
counted 69 teeth

RU6-0190 (193 teeth)
http://www.sunworth-tec.com/productshow.asp?id=1270
counted 192 teeth

I may just need to take more samples.  But that's as far as I got before I hit
the sack.

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