given 2 lines with equations:

\vec{r_0}+s \vec{n_0} =0

\vec{r_1}+ z \vec{n_1} = 0

the shortest distance between them can be found by 3 methods.

***** method 1 ******

\hat{n} = \frac{\vec{n_0}\times \vec{n_1}}{|\hat{n}|}

d = |(\vec{r_1}-\vec{r_0})\cdot \hat{n}|

***** method 1 ******

the shortest distance line make right angle with 2 given lines. Thus, bu simple geometry, we have

\vec{r_0}+s \vec{n_0} + d (\vec{n_0} \times \vec{n_1}) = \vec{r_1}+ z \vec{n_1}

where d is the distance between 2 lines. there are 2 unknowns : s, d, z.  the 3 components of the vectors give 3 equations. Thus, by solving the matrix equation:

N\cdot (s, z, d) = \vec{r_1}-\vec{r_0}

Notice that the matrix N is formed by 3 independence vectors when the 2 given lines is not parallel, therefore, it has an inverse and the there is always an unique solution.

***** method 2 ******

another method is minimized the distance between 2 lines. the distance between 2 lines is:

d= |\vec{r_0} + s \vec{n_0} - \vec{r_1} - z \vec{n_1}|

by minimize the distance square for variate the parameter s, z, i.e.

\frac{\partial d^2}{\partial s} = (\vec{r_0}-\vec{r_1})\cdot \vec{n_0} + s - z \vec{n_0}\cdot \vec{n_1} = 0

\frac{\partial d^2}{\partial z} = (\vec{r_0}-\vec{r_1}) \cdot \vec{n_1} +s \vec{n_0}\cdot \vec{n_1} -z =0

by solving the matrix equation:

\left(  \begin{array}{cc}  -1 & \vec{n_0}\cdot \vec{n_1} \\  \vec{n_0}\cdot \vec{n_1} & 1  \end{array}  \right) \cdot    \left(  \begin{array}{c}  s \\  n  \end{array}  \right)=    \left(  \begin{array}{c}  \left(\vec{r_0}-\vec{r_1}\right)\cdot \vec{n_0} \\  \left(\vec{r_0}-\vec{r_1}\right)\cdot \vec{n_1}  \end{array}  \right)

solving and get s , z. then get d.

*********** remark *****************

it is interesting that, the method 2 separate the problem into 2 parts, while method 1 solve it at once. And reduced a 3 dimensional problem into 2 dimensional.

Advertisements