Mapping from 2D projection back to 3D point cloud

Question

I have a 3D model consisting of point vertices (XYZ) and eventually triangular faces. Using OpenGL or camera-view-matrix-projection I can project the 3D model to a 2D plane, i.e. a view window or an image with m*n resolution.

The question is how can I determine the correspondence between a pixel from the 2D projection plan and its corresponding vertex (or face) from the original 3D model.

Namely,
What is the closest vertices in 3D model for a given pixel from 2D projection?
It sounds like picking in openGL or raytracing problem. Is there however any easy solution?

With the idea of ray tracing it is actually about finding the first vertex/face intersected with the ray from a view point. Can someone show me some tutorial or examples? I would like to find an algorithm independent from using OpenGL.

Answer 1

Hit testing in OpenGL usually is done without raytracing. Instead, as each primitive is rendered, a plane in the output is used to store the unique ID of the primitive. Hit testing is then as simple as reading the ID plane at the cursor location.