A common misunderstanding about tangent space normal maps is that
this representation is somehow asset independent. However, normals
sampled/captured from a high resolution surface and then transformed
into tangent space is more like an encoding. Thus to reverse the
original captured field of normals the transformation used to decode
would have to be the exact inverse of that which was used to encode.

This presents a problem since there is no implementation standard for
tangent space generation. Every normal map baker uses a different
implementation and, additionally, there is no standard for how the
interpolated frame is to be used to transform the normal into tangent
space.

The math error which occurs from this mismatch between the normal map
baker and the pixel shader used for rendering results in shading
seams. These are unwanted hard edges which become visible when the
model is lit/shaded. They are a source of wasted time and frustration
to an artist/designer.

To make matters worse it is not enough to use the same tangent space
generation code. Most implementations have order-dependencies which
can result in different tangent spaces depending on which order faces
are given in or the order of vertices to a face. Others generate
different results if the index list changes (multiple/single/single
with duplicate vertices). And others again produce different results
if degenerate primitives are removed.

Order-dependencies also result in mirrored meshes not always getting
correct mirrored tangent spaces. See for instance an example of a
commercial product used to produce tangent spaces, on a mirrored
model, in figure 17a on page 45 in the master's thesis Simulation of
Wrinkled Surfaces Revisited by Morten S. Mikkelsen.

There are additional examples of problems with different commercial
products shown on pages 44, 52-56. These problems do not just occur
with mirroring. The problem is general and is mostly visible at
tangent space splits in locations where the vertex normal is shared
among surrounding faces. The unwanted hard edge is the result of the
decoded normal deviating from the original captured normal.

The tangent space generation code of Morten S. Mikkelsen overcomes
these problems. The implementation is designed, specifically, to make
the generation of tangent space as resilient as possible to a 3D model
being moved from one application to another. That is generate the same
tangent spaces even if there is a change in index list(s), ordering of
faces/vertices of a face, and/or the removal of degenerate primitives.
Both triangles and quads are supported.

The implementation was made by Morten S. Mikkelsen during the
development of his master's thesis and is free for anyone to use. It
is contained in the two standalone files mikktspace.h and mikktspace.c. This makes it
easy for anyone to integrate the implementation into their own
application and thus reproduce the same tangent spaces. This also
makes the code a perfect candidate for an implementation standard. We
hope the standard will be adopted by as many developers as possible.

The standard is used in Blender 2.57 and is also used by default in
xNormal since version 3.17.5 in the form of a built-in tangent space
plugin (binary and code).

In regards to the interpolated tangent space the baker in both
Blender and the xnormal plugin will keep the vertex normal and the
vertex tangent normalized in the vertex shader. However, in the pixel
shader the "unnormalized" and interpolated vertex normal and tangent
are used to decode the tangent space normal. The bitangent is
constructed here to avoid the use of an additional interpolater and
again is NOT normalized.

`// vNt is the tangent space normal`

vB = sign * cross(vN, vT);

vNout = normalize( vNt.x * vT + vNt.y * vB + vNt.z * vN );

The key to get flawless results is that the baker is designed to do
the EXACT inverse of this very transformation allowing the pixel
shader to remain fast and simple.

Note that mikktspace returns the tangent spaces in an unindexed form.
Do NOT try to average these over an existing index list. It will NOT
work correctly. If you need the tangent spaces to be indexed you can
use a welder to create a new index list. If you do not have such an
implementation you can get a free one here:

weldmesh.h
and weldmesh.c

This welder works with an arbitrary number of floats per vertex.

Even though mikktspace supports quads these will eventually be split
into triangles before baking and pixel shading. Furthermore, though
mikktspace guarantees a consistent choice of tangent frame, at the
quad vertices, the interpolated tangent frame is heavily affected by
which diagonal split is chosen later on. This is not a big issue when
the tangent frame transitions slowly across the face. For a
well-behaved low polygonal mesh this is true for most regions.

When there is a great change in the tangent frame a problem occurs
when the tools pipeline (prior to pixel shading) and the baker do not
choose the same diagonal split. One possible solution is to choose
your split based on an order-independent strategy. For instance, split
all quads by the shortest diagonal using the vertex positions. If
these have the same length then split by the shortest diagonal using
the texture coordinates. This will lead to an order-independent choice
and works with mirrored meshes. The triangulator in xNormal has
supported this since 3.17.5 and triangulation is performed after the
mikktspace plugin is done. Reproducing the same splits in your own
tools pipeline is trivial to do.

Another more trivial solution which any artist can perform on his/her
own is to simply triangulate the model before baking and export which
will also ensure consistent triangulation. However, this way the quad
information is lost which may or may not be of any importance
depending on the game engine and how the asset is to be used.

We stress that in most cases this is NOT a visible issue. But in the
few cases where it is a problem this section explains why and how to
deal with it.