From BlenderWiki

Face Tools

Here we will see mainly some of the options in the Specials/Face Specials and Mesh » Faces menus. There are many more advanced options for faces, that we will see later on.

Set Smooth or Shade Smooth

Set Solid or Shade Flat

These are options regarding mesh smoothing.

Bevel

This is a “subdivision” tool, described in details here.

Face Mode Set

Face Mode Clear

UV Rotate

UV Mirror

These are UV-texturing tools.

Color Rotate

Color Mirror

These are vertex color tools.

Make Edge/Face

It will create an edge or some faces, depending on your selection. We have already discussed about this tool in the editing basics page.

Fill and Beautify Fill

These are more advanced face creation tools.

The Fill option will create triangular faces from any group of selected edges or vertices, as long as they form one or more complete perimeters. These triangles are not always created “in the best way” it is possible (there might results in long and thin triangles) – but I don’t know the algorithm used by this tool…

An unclosed perimeter that won’t work with Fill.

A good closed perimeter and its Filled result.

Beautify Fill works only on selected existing faces. It is a sort of “complementary” tool of the Fill command, as it tries to rearrange the triangles in the selected groups of faces, to obtain more “balanced” ones (i.e. less long thin triangles).

Convert Quads to Triangles

As its name intimates, this tool converts each selected quadrangle into two triangles. Remember that quads are just a set of two triangles… Note that it does not work on FGons.

Join Triangles

Another quite self-explaining name! This tool converts the selected triangles into quadrangles.

However, this operation is less trivial than its counter part – it even has its own buttons group in the Mesh Tools panel of the Editing context. But let’s see some general rules first:

• Of course, to create a quad, this tool needs at least two adjacent triangles.
• If you have an odd number of selected adjacent triangles, not all of them will be converted to quads!
• However, even if you have an even number of selected triangles, it is also possible not to obtain only quads. In fact, this tool tries to create “squarishest” quads as possible from the given triangles, which implies it might let some lonesome triangles on the way…

All the menu entries and hotkey use the settings defined in the Mesh Tools panel, so it’s time to see them:

Join Triangles

The “action” button, doing exactly the same thing as hotkey or menu entries.

Threshold

This numeric filed (values between 0.0 and 1.0) controls how much non-squarish quads this tool can create. With a threshold of 0.0, it will only join adjacent triangles that form a perfect rectangle (i.e. right-angled triangles sharing their hypotenuses).

The four buttons of the bottom row are toggle options to further limit the triangles reunion possibilities:

Delimit UVs

When enabled, it will prevent reunion of triangles that are not also adjacent in the active UV map. Note that this seems to be the only option working…

Delimit Vcol

When enabled, it will prevent reunion of triangles that have no matching vertex color. I’m not sure how this option works – or even if it really works…

Delimit Sharp

When enabled, it will prevent reunion of triangles that share a “sharp” edge. I’m not sure either if this option works, and what is the “sharp” criteria – neither the Sharp flag nor the angle between triangles seem to have an influence here…

Delimit Mat

When enabled, it will prevent reunion of triangles that do not use the same material index. This option does not seem to work neither…

Flip Triangle Edges

This command will cycle between all possible combinations of triangles inside a selected island of faces. Each time you apply it, a new organization is proposed (but always with the same number of triangles), until you have seen all, then it returns to the initial state, etc.

In an island of only two triangles, there are only two possibilities, and this tool does exactly the same thing as edge rotating…

Of course, for this tool to work, there must be islands of at least two adjacent triangles in the selection – quads and FGons are completely ignored here!

Normals

As normals are mainly a face “sub-product”, we describe there few options here also.

Flip

Well, it will just reverse the normals direction of all selected faces. Note that this allows you to precisely control the direction (not the orientation, which is always perpendicular to the face) of your normals, as only selected ones are flipped.

Recalculate Outside and Recalculate Inside

These commands will recalculate the normals of selected faces so that they point outside (respectively inside) the volume that the face belongs to. This volume do not need to be closed. In fact, this means that the face of interest must be adjacent with at least one non-coplanar other face. For example, with a Grid primitive, neither Recalculate Outside nor Recalculate Inside will never modify its normals…