Flipped Normals
Faces pointing inside out
Also known as: Inverted normals · Reversed normals · Face orientation · Black faces · Inside-out mesh
Quick answer
Flipped normals are faces whose normals point into the model instead of out of it, which makes them render dark, shade backwards or vanish because of backface culling. The fix is to recalculate or flip the normals so they point outward. In Blender, turn on the Face Orientation overlay (blue is outside, red is inside), select the red faces and use Mesh > Normals > Recalculate Outside.
How to fix it
Normals flip most often after mirroring with negative scale, boolean operations and bad imports. Check them before exporting: fix flipped faces in your 3D tool, apply negative scale (Ctrl+A in Blender, Freeze Transformations in Maya) and re-export. If only a few faces are red, flip those; if the whole mesh is inside out, recalculate all of it. A normal map with the wrong green channel looks similar but only affects lighting detail, not whether a face is visible.
Tools that use Flipped Normals
See also
Backface Culling
Why faces vanish when seen from behind
Two Sided Material
Show a surface from both sides
Normal Map
Per-pixel surface detail
DirectX vs OpenGL Normal Map
Which way the green channel points
Smoothing Groups
Per-face shading split
Apply Scale in Blender
Fix stretched bevels and uneven modifier results
Solidify Modifier
Adds thickness to a surface
Maya Faces Black or Inside Out
Dark or invisible faces from reversed normals