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Merge pull request #1698 from ombre5733/patch-2 

Clarify the matrix layout in the documentation
pull/1699/head^2
Kim Kulling 2018-01-12 09:40:08 +01:00 committed by GitHub
parent b128cb9c4c 33ddbf9aa0
commit 7c16a589e7
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1 changed files with 16 additions and 6 deletions
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doc/dox.h
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@ -561,17 +561,27 @@ The output UV coordinate system has its origin in the lower-left corner:
@endcode @endcode
Use the #aiProcess_FlipUVs flag to get UV coordinates with the upper-left corner als origin. Use the #aiProcess_FlipUVs flag to get UV coordinates with the upper-left corner als origin.
All matrices in the library are row-major. That means that the matrices are stored row by row in memory, A typical 4x4 matrix including a translational part looks like this:
which is similar to the OpenGL matrix layout. A typical 4x4 matrix including a translational part looks like this:
@code @code
X1 Y1 Z1 T1 X1 Y1 Z1 T1
X2 Y2 Z2 T2 X2 Y2 Z2 T2
X3 Y3 Z3 T3 X3 Y3 Z3 T3
0 0 0 1 0 0 0 1
@endcode @endcode
with (X1, X2, X3) being the X base vector, (Y1, Y2, Y3) being the Y base vector, (Z1, Z2, Z3) with <tt>(X1, X2, X3)</tt> being the local X base vector, <tt>(Y1, Y2, Y3)</tt> being the local
being the Z base vector and (T1, T2, T3) being the translation part. If you want to use these matrices Y base vector, <tt>(Z1, Z2, Z3)</tt> being the local Z base vector and <tt>(T1, T2, T3)</tt> being the
in DirectX functions, you have to transpose them. offset of the local origin (the translational part).
All matrices in the library use row-major storage order. That means that the matrix elements are
stored row-by-row, i.e. they end up like this in memory:
<tt>[X1, Y1, Z1, T1, X2, Y2, Z2, T2, X3, Y3, Z3, T3, 0, 0, 0, 1]</tt>.
Note that this is neither the OpenGL format nor the DirectX format, because both of them specify the
matrix layout such that the translational part occupies three consecutive addresses in memory (so those
matrices end with <tt>[..., T1, T2, T3, 1]</tt>), whereas the translation in an Assimp matrix is found at
the offsets 3, 7 and 11 (spread across the matrix). You can transpose an Assimp matrix to end up with
the format that OpenGL and DirectX mandate. To be very precise: The transposition has nothing
to do with a left-handed or right-handed coordinate system but 'converts' between row-major and
column-major storage format.
<hr> <hr>