## Tuesday, April 3, 2018

### Geometry Lesson: find point closest to set of lines

In [1] a simple optimization problem is posted:

Find a single point $$q$$ in 3d space that is closest to a number of given lines. Each line $$L_j$$ is defined by two points $$p_{j,a}$$ and $$p_{j,b}$$.

This looks like a problem for which specialized algorithms have been developed. Let's just ignore that and see if we can use standard mathematical programming models to tackle this. My geometry is a bit rusty, but the necessary formulas can be found in [2]. To recap, the squared distance $$d^2$$ between a point $$q$$ and a single line through $$p_a$$ and $$p_b$$ is given by:$d^2=\frac{|p_a-q|^2 |p_b-p_a|^2 - \left[(p_a-q)\cdot (p_b-p_a) \right]^2}{|p_b-p_a|^2}$ where $$\cdot$$ indicates the dot product.

Actually just finding the shortest distance between a given line and a given point can be interpreted as an optimization problem:

 Shortest distance between a line and a point

We have multiple lines and the position of the point is not given. An optimization model to find the best place for our point $$q$$ can be formulated as:
$\min_q\>\sum_j \frac{|p_{j,a}-q|^2 |p_{j,b}-p_{j,a}|^2 - \left[(p_{j,a}-q)\cdot (p_{j,b}-p_{j,a}) \right]^2}{|p_{j,b}-p_{j,a}|^2}$ After simplifying some constants we have: $\min_q\>\sum_j \frac{|\alpha_j-q|^2 |\beta_j|^2 - \left[(\alpha_j-q)\cdot \beta_j \right]^2}{|\beta_j|^2}$ This thing is convex, so it is somewhat easy to solve as a QP (Quadratic Programming) problem.

 2D example: Minimize sum of squared distances

Actually I cheated a bit: I minimized the sum of the squared distances. When really minimizing the sum of the distances we need to minimize: $\min_q\>\sum_j \sqrt{\frac{|\alpha_j-q|^2 |\beta_j|^2 - \left[(\alpha_j-q)\cdot \beta_j \right]^2}{|\beta_j|^2}}$ This can be solved as an NLP (Nonlinear Programming) model. Of course the objective with the quadratic distances will put more weight on the larger distances.

How do results compare?  For a 3D problem with 10 random lines I see:

----     54 PARAMETER results

x           y           z         obj

squared dist       0.015      -0.069      -0.282       4.087
dist               0.050      -0.142      -0.413       5.634


Here the $$x,y,z$$ values are the coordinates of $$q$$. The column marked obj indicates the objective function value. In our 2D example we can see the differences more clearly:

 Minimization of sum of distances

The differences are a little bit more pronounced than I would have predicted.

#### Minimax Model

A different objective can be formed by minimizing the maximum distance:\begin{align} \min\> & z\\ &z \ge d_i\end{align} We get the same solution if we minimize the maximum of the squared distance: \begin{align} \min\> & z\\ &z \ge d^2_i\end{align} This means we have a convex quadratic model. This model puts all the emphasis on the largest distance and thus will shift our location to the right:

 Minimize maximum of distances

A 3d visualization would be interesting.

#### SOCP Model

In the comments it is suggested to use a Second Order Cone Programming (SOCP) model. This would allow us to solve the minimum sum of distance problem using solvers like Cplex, Gurobi, Mosek and Xpress. There is no need to resort to a general purpose NLP solver. In [3] I have some results.

#### References

1. Non-linear Optimization about Point to Lines, https://stackoverflow.com/questions/49594754/non-linear-optimization-about-point-to-lines
2. Eric Weisstein, Point-Line Distance -- 3 Dimensional, http://mathworld.wolfram.com/Point-LineDistance3-Dimensional.html
3. SOCP reformulations of a min distance problem, http://yetanothermathprogrammingconsultant.blogspot.com/2018/04/socp-reformulations-of-min-distance.html