On RPi estimate-px-per-mm takes about 6s for 400x400 image

[firepick1]

The current FirePick Raspberry Pi FireSight takes about 6 seconds to execute estimate-px-per-mm. This is a lot of time and one solution would be to support the specification of a region-of-interest to reduce computational load and speed up processing. To specify the ROI, we could add to following parameter to the stage:

"roi":[150,0,100,400]

Such an ROI would be about 4x faster.

PRIORITY: Medium (it takes about 10-20 tries to get a good image that shows correct resolution)

[firepick1]

Oh, I just realized that it's a two-stage pipeline, and I'm not sure where the slowness is. ROI would only help Hough, because Ransac takes points. Not sure if Hough or Ransac is slowest