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Hidden surface removal

You could calculate which triangles are not visible and exclude them from rendering. This is typically performed by enforcing a winding order on the vertices that compose a triangle. Usually vertices are connected in a clockwise order. This allows the graphics engine to calculate a vector that is normal (perpendicular) to the face of the triangle. The triangle will not be displayed if its normal vector is pointing away from the viewer.

This technique operates in object space—as it involves mathematical operations on the objects, faces, and edges of the 3D objects in the scene. It typically has a computational complexity of order n2 where n is the number of faces.

This quickly becomes complicated however as some triangles may be partially visible. For partially visible triangles, an input triangle has to be broken down into several new wholly visible triangles. There are many good online graphics courses that explain various hidden−surface removal algorithms in detail. Use your favorite search engine and search on “hidden surface removal” and you will find lots of useful references.