[Bf-committers] matrix multiplication in Blender SVN of this morning (W32 Vista mingw compiled)

[Benoit Bolsee]

Hi, I repeat once more: mathutils matrices are COLUMN-MAJOR. This means
that the top elements in the definition list are columns, not rows,
despite the fact that they are printed horizontaly. So the following
code:

m1 = Matrix([[ 1,  0,  2], [-1,  3,  1]])
m2 = Matrix([[3, 1],[2, 1],[1, 0]])
print(m1*m2)

Translates into this in ordinary notation:

(1 -1) X (3  2  1) = (2  1  1)
(0  3)   (1  1  0)   (3  3  0)
(2  1)               (7  5  2)

Which is exactly what blender returns.

Mathutils matrices are column-major because that's how Blender stores
the matrices internal, and Blender uses that convention because openGL
uses it. 


> -----Original Message-----
> From: bf-committers-bounces at blender.org 
> [mailto:bf-committers-bounces at blender.org] On Behalf Of 
> bf-committers-request at blender.org
> Sent: mercredi 27 juillet 2011 12:00
> To: bf-committers at blender.org
> Subject: Bf-committers Digest, Vol 84, Issue 26
On Tue, 26 Jul 2011 16:49:54 +0200, "Peter K.H. Gragert"
<pkhgragert at gmail.com> wrote:
> 
> Hallo,
> First try ...
> 
> Put this text in the console and run it
> ============code start=============
> import bpy
> from mathutils import Matrix
> print("\n----START---")
> m1 = Matrix([[ 1,  0,  2],
>         [-1,  3,  1]])
> print(m1, "\nelement R(2,3)")
> 
> m2 = Matrix([[3, 1],
>         [2, 1],
>         [1, 0]])
> print(m2, "\nelement R(3,2)")
> 
> print("m1 * m2 =", m1*m2, " should be element R(2,3) * R(3,2) 
> => R(2,2) but is R(3,3) so math it looks like m2 * m1 so it 
> is STRANGE!")
> 
> print("m2*m1 = ", m2*m1, " should be element R(3,2) * R( 2,3) 
> => R(3,3) but is R(2,2) so math it looks like m1 * 
> m2,(consistantly) STRANGE")
> 
> # checking what *= means
> m1_Copy = m1.copy()
> m1_Copy *= m2
> m1_BlStar_m2 = m1_Copy
> print("m1 BLender *= m2",  m1_BlStar_m2 ,"should be element 
> dependant of what *= means , right or left multiplication") 
> print("result shows it is interpretated as math: m1 * m2, 
> \n\n ==> so  math RIGHT  multiplication!")
> print( "check:",   m1 * m2 == m1_BlStar_m2 )
> print("\n+++++++++++++ now again?!!??? ")
> m1_BlStarm_m2_Copy = m1_BlStar_m2.copy()
> m1_BlStarm_m2_Copy *= m2
> m1_BlStar_m2_BlStar_m2 = m1_BlStarm_m2_Copy
> print(" m1 BlStar_m2_BlStar_m2 =\n", m1_BlStar_m2_BlStar_m2, 
> "\nshould be ERROR R(3,3) * R(3,2) => R(3,2) but is R(3,3) ERROR!")
> 
> ============code end=============
> 
> 
> ==>   matrix multiplication is NOT like math matrix multiblication
> 
> m1 * m2 in Blender correspondent with m2 * m1 in math 
> (STRANGE not nice .... but ... let it be so)
> 
> But THEN  *=
> m2  *=  m1 behaves very strange and to my opinion wrong!
> 
> Greetings
>    Peter K.H. Gragert
>