Emulating constant size of point sprites using OpenGL ES

I am trying to emulate point sprites in Android using OpenGL, specifically the characteristic of the size staying the same if the view is zoomed.

I do开发者_运维百科 not want to use point sprites themselves as they pop out of the frustum when the "point" reaches the edge, regardless of size. I do not want to go down the route of Orthographic projection either.

Say I have a billboard square with a size of 1, when the use zooms in, I would need to decrease the size of the square so it looks the same size, if the user zooms out, I increase it. I have the projection and model matrices to hand if these are required as well as the FOV. My head just goes blank every time I sit down and think about it! Any ideas on the necessary algorithm?

Ok, By changing field of view to zoom into the environment I divide the quad size by (max_fov / current_fov). It works for me.