Now consider the exact same list of triangles with the only change being to flip the normals at each vertex. If you try this on your own, it is probably best done by modifying your vertex list in the rendering code, for reasons that will be explained shortly.
With backface culling turned on, you will still see the cylinder in each image, except the lighting will be flipped in the second one. And if you turn off backface culling, then the same images will be rendered, and it will be done so with the same number of triangles, because there are no backfacing triangles in the given example. Don't think that the GPU is rendering the backside and then going over it a second time when you have backface culling turned off for the front side, unless you actually did specify two triangles in that big list of triangles with vertices in the exact same locations. Assuming you understand that what I described is not what typically happens, let me now explain the point that I'm making. When you flip the normals in a 3D modeling package, it's doing more than just changing the vertex information; it's also changing that triangle list. This is because the front and back face of a triangle are determined through the novel concept of winding order, which is quite simply that the front of a triangle is the side of the face where vertices are viewed counterclockwise. Suppose you have a mesh with no normal data, just positions in 3D space for vertices. You can still use backface culling just fine as long as your triangle list is specified in the proper order, a point that I think is often missed by designers and artists when trying to understand backface culling. If you want the proof in the pudding, here's the OpenGL call to specify culling: glFrontFace(GLenum mode); Where mode is either GLCW or GLCCW, standing for clockwise and counterclockwise, notice that this has nothing to do with vertex normals! And here's a diagram to illustrate which side is the front for a triangle by default (you could assume that triangles are front clockwise as your standard if you really want to, but the default is counterclockwise). If the points defining your triangle are listed in an order that would cause the diagram on the left to be true after transformed into screen-space, then that triangle is front facing:
So what are we culling again? We're culling triangles facing away from the camera, which are going to be obstructed by the forward facing polygons on a closed mesh. Those triangles that would be drawing the inside of that cylinder are just going to be covered up by the front-facing triangles that you would actually see. With the winding order in hand, the triangle's face normal can be determined and checked against the camera. I made this little diagram (inspired by the great explanation for backface culling in Real-Time Rendering). The triangles on the backside of the full cylinder are facing away from the camera, so they are culled (indicated by the dotted part).
So on a closed mesh like a full cylinder, you can avoid doing the work of rendering all of those triangles that you already know are going to be obscured. This is why disabling backface culling is typically not the correct answer when you have triangles culled that you didn't intend to, and it's also bad because that usually means that the backfacing triangles are being shaded incorrectly with backwards normals. If you do intentionally cut that cylinder in half, the easy fix is to add front facing triangles along the inside of the mesh. I believe there is functionality these days (DX10 maybe?) to figure out which side of the triangle is being rasterized. Theoretically you could have the shader flip the normals based off of that information to still have correct lighting, but if you just read a post on the basics of backface culling, I bet that's not what you're looking to do. Because 3D modeling programs are automatically adjusting the winding order for you based off of what direction your normals are, it's easy to mistakenly think of backface culled triangles as triangles with incorrect normal information, when really it's winding order that determines it. This is why sometimes you can really end up in weird situations while using a 3D modeling package. I know that as a Freshman in college, there were many models at game jams that had those stray triangles that artists just couldn't get to show up, and maybe if they did, the lighting got all weird. Perhaps thinking about what's actually happening can help alleviate those pains. [This piece was reprinted from #AltDevBlogADay, a shared blog initiative started by @mike_acton devoted to giving game developers of all disciplines a place to motivate each other to write regularly about their personal game development passions.]