# Distance from point to line segment in Poincaré disk model

I'm trying to build a geometric datastructure in hyperbolic space. For that purpose, I'm using the Poincaré disk model.

The distance between two points can be calculated with the hyperbolic law of cosines, as mentioned here.

As mentioned in the title, I'm interested in the distance between a point and a line segment. Let's say I have points $a$, $b$, and $c$. What is the distance between $c$ and the shortest path between $a$ and $b$?

With appropriate transformations, one could put $a$ in the origin and have the connecting path be a segment of the diameter. Would this make things easier?

• I see that a question about the Distance from a point to a line in the hyperbolic plane has been asked before. But I'm none too happy with the answers currently available there, which is probably mostly because the asker of that question didn't specify a model. – MvG Jun 26 '14 at 16:55

You might want to instead put the single point, $c$, in the origin. Then the line between $a$ and $b$ is a circle but the orthogonal line between $c$ and that is a diameter. So you'd have to find the point on the circle which is closest to the origin, which is a lot simpler. Then you are back to point-point distance.