I have a triangulated isometric grid, like this:
[![alt text](https://i.stack.imgur.com/uCx7k.gif)](https://i.stack.imgur.com/uCx7k.gif)  

(source: [mathforum.org](https://web.archive.org/web/20160328125659/http://mathforum.org/workshops/sum98/participants/sanders/isomgrid.gif))

In my code, triangles are grouped by columns.

As I hover the mouse, I want to calculate what triangle the mouse coordinates are in. Is there a simple algorithm to do that?