however, I still dont get the function of “recalculating” normal…
For example, when I make one side of a ginger man, and duplicate that side and flip one of them 180 degrees, ie, a side which is a reflection of another, and then, I set smooth all of them… However, you can see a funny line in the middle… by pressing “recalculating normal” function, the line disapears.
It is possible to determine which side of the mesh is outside/inside if it is manifold. Recalculate normals exploits this fact and turns all normals so that they point outside.
If you have a non-manifold mesh, meaning that it’s impossible to determine which side is outside and which side is inside, recalculating normals doesn’t work normally.
ctrl-shift-alt-m shows you the part of the mesh that are non-manifold. It is a very useful tool for finding mistakes in mesh.
You can see the normals drawn if you hit “Draw Normals” in Editing\Mesh Tools 1 while in Edit Mode.
The line in the middle is caused by ‘bad’ normals, or normals that are flipped the opposite way to the surrounding ones. Before you recalculated the normals, you would have noticed that when you select the object and press ‘F’, the normals in the center would have been pointing inwards. The recalculate normals option just flips all the normals outside.
I hope you understand what I am trying to explain…