Incorrect prefix with recyprocal units when it needs to convert

[din14970]

I noticed that the scalebar incorrectly handles the prefix for reciprocal units, I noticed this when I was trying to measure distances in a Fourrier transform of an image. My original image has a pixel size of 0.007 nm/pixel, but everything is stored in meters, so 7e-12 m. The scalebar has no problem interpreting this value and puts a scalebar in nm on the image.

To calculate the scale of my Fourrier transform, I take the meters value and convert. My image is 4096x4096 pixels so the scale in the FFT is 1/(7e-12 * 4096)=3.4e7 m^-1/pixel. This is equivalent to 0.034 nm^-1/pixel. When I pass the scale in nm^-1 to the scalebar, it uses the correct nm^-1 units. However, when I pass the scale in m^-1, the scalebar uses Gm^-1.

[din14970]

Might be easily fixed if you associate the reciprical factor with the prefix, line 124 in dimensions.py

[din14970]

Edited it on my own computer and seems to work

[din14970]

the tests in test_dimension were also wrong:
0.02 1/m = 0.0002 1/cm or 20 1/km
0.002 1/m = 2 1/km
Updated them in a second commit

[ppinard]

Thank you for finding and fixing the issue. I know release version 0.6.2 with the fix.