Occasionally, you may find that models that you import from other programs will appear to be inside out. This is because polygons have two sides: a front and a back. The direction that the front side faces is called the normal, which is a mathematical term. It doesn't mean what the word 'normal' normally means in this context. Sometimes ZBrush will get confused about which side is which. Since the back side of a polygon is invisible by default, the model can seem to be inside out. This problem is easily corrected.
Let's open up the DemoSoldier and expand the Display Properties sub-palette. Right now, there's no problem with the normals. But if we click the Flip button, the active subtool looks like it's backwards. You see now we're seeing the inside of the model. This is because ZBrush has just flipped which side of the polygons are front and which are back. Clicking Flip again will set the Normals in the correct direction.
Let's hit Flip once again in order to see what Double does. Double makes both sides of the model visible instead of just the front-facing side. One situation where you would want to flip normals is if you're sculpting a cave. I am just going to make a Sphere3D primitive to work with and let's click Make PolyMesh so that we can sculpt on it. I am going to hit F to zoom in. All right, so let's switch to the Move brush. I am going to hit B+M+B, and I am just going to start making this sphere look a little bit more organic, kind of like a cave would look naturally.
Now if you want to start sculpting details like stalactites and stalagmites inside the cave you'll want to see it from the inside. Reversing the normals allows us to see and sculpt from within the model. It really comes down to your preference whether you flip your normals or double them; however, it might make a difference if you export the model to a different program, since some softwares interpret normal direction differently.
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