Why a Solid Object Can Disappear From the Inside
A polygon’s front face is determined by the order in which its vertices are listed. That order gives the renderer a consistent way to tell which side of the surface is meant to face outward. When the camera sees the opposite side, the polygon is treated as a back face.
With backface culling enabled, the renderer skips those polygons before drawing them. This saves work: a closed object usually has many surfaces whose outward-facing sides point away from the camera and cannot be seen from outside anyway. The polygons are not erased from the model; they are simply left out of that view.
What Changes When You Cross the Surface
From inside a closed object, the camera is now looking at the inward-facing sides of its outer shell. If those sides are culled, they vanish, so the scene beyond them appears visible—as if the object had no walls. The effect is especially clear in a game when a camera clips through a building or a character model.
This is a rendering choice, not an unavoidable property of polygons. A material can be set to draw both sides, or the model can use interior surfaces with their own outward-facing orientation. Those options make surfaces visible from within, but drawing both sides costs more work. That tradeoff is why a shell that looks solid from outside can turn transparent the moment the camera slips past it.
