In this video, we have covered the Back Face Detection Method from Computer Graphics in detail. 

The back face detection method is also called as plane equation method, it is used to check visible surfaces whenever an object is projected from 3D space to a 2D plane. 

With the help of this method, it is possible to detect, which parts of an object will be visible. The idea here is to find out if a surface is facing away from the user or not. If its face is away from the user then discard the current frame and move on to the next one.

Every surface has a normal vector. If the normal vector is projecting in the direction of the center of the projection, then it is the front face and it can be seen by the viewer. If the normal vector projects away from the direction of the center of the project then it is back face and it cannot be seen by the viewer.

Let the normal to a surface S be N = Ni + Ny] + Nzk and the viewing vector be, V = V1 + Vyj + Vzk
Then, the surface S will be back face if, V.N<0  or, |V|.|N.|cos(angle) < 0  or, cos(angle) < 0 or, 180° > O > 90°

Thus the viewing angle would be greater than 90 degrees and less than 180 degrees for the front face. For the front face, Angle<90 degrees

This method is used for convex polyhedra but is not recommended for concave polyhedra. It is applied on solid objects modeled as a polygon mesh.

Back Face Detection: https://www.geeksforgeeks.org/back-face-detection-method/