bottom 1D: sidewall based thinning

Problem

1. broken bottom
2. get the real 1D ones
3. simple features: median radius, lowest points, not satisfactory

Analysis

1. bottom = "lowest sidewall tip band"
2. from outer band to inner band, the extreme radial positions are sidewalls for sure?
3. If there is just one point, then no sidewall to analyze.
4. The acquired band is usually 2D, tangentially thick layered. So extracted 1D should be a vector instead of one single point.
5. Then the real 1D point, should be picked from the vector according to p-to-line dist.
6. #4 does the radial picking, #5 does tangential picking
7. Need to check the z-contour of bottom: under various resolution, they show certain trail of stylus tip across band, though extremely noisy.
   
8. cannot assume all bottom points are distributed across the groove center.
9. so we have to make sure which of them belong to which side of the walls.
10. ASSUMPTION: the 1D point is among the detected points.
    
    

Implementation

1. find center by in/out edge.
2. divide 2d band into in/out wall groups.
3. linear fitting on both sides,
   
4. find the cross as the center.
5. if one sidewall is missing, assume its delta_r/z with prev is proportional with the edges'.