I have created the map of a huge terrain (10,000 x 12,000) vertices, which results in about 240 million triangles. Loading all these is very heavy and slows down the scene significantly. To make it more efficient, I am thinking of using a fraction of the vertices when the camera is away (say one for every 10 vertices) and as the camera zooms in on the map, I would increase the number of vertices.
The problem is, even when the camera is zoomed in and a small fraction of the map is viewed at full resolution, the scene is still loading very slowly because of all the drawings that are happening outside the viewport.
My question is, how do I calculate which vertices are in the viewport so that I don't draw them. Or, alternatively, is there a better way of handling increasing the resolution as I zoom in (note that I am not using textures)