How to measure image coincidence in an optical rangefinder

Question

I have a couple of USB webcams (fixed focal length) setup as a simple stereoscopic rangefinder, spaced N mm apart with each rotated by M degrees towards the centerline, and

Answer 1

the problem is that you can not assume pixel perfect align of cameras

so let assume x-axis is the parallax shifted axis and y- axis is aligned. You need to identify the x-axis image distortion/shift to detect parallax align even if you are aligned as much as possible. The result of abs difference is not guaranteed to be in min/max so instead of substracting individual pixels substract average color of nearby area of that pixel with radius/size bigger then the align error in y-axis. Let call this radius or size r this way the resulting difference should be minimal when aligned.

Approximation search

You can even speed up the process by r

1. select big r
2. scan whole x-range with step for example 0.25*r
3. choose the lowest difference x-position (x0)
4. change r to half
5. go to bullet 2 (but this time whole x range is just between <x0-2.0*r,x0+2.0r>
6. stops if r is smaller then few pixels

This way you can search in O(log2(n)) instead of O(n)

computer vision approach

this should be even faster:

1. detect points of interest (in booth images)
   • specific change in gradient,etc ...
2. cross match points of interest between images
3. compute average x-distance between cross-matched points
4. change parallax align by found distance of points
5. goto bullet 1 until x-distance is small enough

This way you can avoid checking whole x-range because the align distance is obtained directly ... You just need to convert it to angle or what ever you use to align parallax

[notes]

You do not need to do this on whole image area just select few horizontal lines along the images and scan their nearby area.

There are also another ways to detect align for example for short distances the skew is significant marker of align so compare the height of object on its left and right side between cameras ... If near the same you are aligned if bigger/smaller you are not aligned and know which way to turn ...