opengl selecting area on model

Question

I need some help in surface area selection on a 3d model rendered in opengl by picking points through mouse. I know how to get a point in world coordinate but cant find a way to select an area. Later I need to remesh that selected area and map an image over it which I know.

Answer 1

Well, OpenGL by itself can't help you there. OpenGL is a drawing API. You draw things, but once the drawing commands have been executed all that's left are pixels in a framebuffer and OpenGL has no recollection about the geometry whatsoever.

You can use OpenGL to implement image based area selection algorithms, for example by drawing each face with a unique index color into an off screen framebuffer. Then by looking at what values can be found therein you know which faces are present in a given area.

Later I need to remesh

This is called topology modification and is completely outside the scope of OpenGL.

that selected area and map an image over it which I know

You can use a image based approach for this again, however you must know in which way you want to make images to faces first. If you want to unwrap the mesh, then OpenGL is of no help. However if you want the user to be able to "directly draw" onto the mesh, this can be done by drawing texture coordinates into another off screen framebuffer and by this reverse mapping screen coordinates to texture coordinates.