matrix_basis isn't affected correctly

Question

I'd like to set the matrix_basis of an object. Unfortunately, the matrix isn't get updated correctly.

This is my test scene, consisting of an plane and a cylinder object:

My objective is to get alpha, the plane's x angle, and apply it in some way to cylinder.

Here is the code:

import bpy
from math import *
from mathutils import *

plane = bpy.data.objects['plane']
cylinder = bpy.data.objects['cylinder']
 
alpha = plane.rotation_euler[0] # x rotation
print('alpha=%.2f' % degrees(alpha))

'''
# normal rotation matrix around x axis
# works fine
m = Matrix(( \
    [1, 0, 0, 0], \
    [0, cos(alpha), -sin(alpha), 0], \
    [0, sin(alpha), cos(alpha), 0], \
    [0, 0, 0, 1] \
    ))
'''

m = Matrix(( \
    [1, 0, 0, 0], \
    [0, cos(alpha), 0, 0], \
    [0, sin(alpha), 1, 0], \
    [0, 0, 0, 1] \
    ))

cylinder.matrix_basis = m
bpy.context.scene.update() # is this call needed?

# prints different matrix than m
print(cylinder.matrix_basis)

# for debugging
print(cylinder.matrix_world)

This is the console output of it:

alpha=-17.10
<Matrix 4x4 (1.0000,  0.0000, 0.0000, 0.0000)
            (0.0000,  0.9890, 0.1478, 0.0000)
            (0.0000, -0.1478, 0.9890, 0.0000)
            (0.0000,  0.0000, 0.0000, 1.0000)>
<Matrix 4x4 (1.0000,  0.0000, 0.0000, 0.0000)
            (0.0000,  0.9890, 0.1478, 0.0000)
            (0.0000, -0.1478, 0.9890, 0.0000)
            (0.0000,  0.0000, 0.0000, 1.0000)>

The first printed matrix is the matrix_basis. As you can see, it differs from m. For example, the element of the second row, third column have to be 0 but is 0.1478.

I haven't apply any constraints to cylinder. So whats wrong by setting the basis_matrix in this way?

Edit:

In addtion to @mont29 's answer, I've found the following document: issues in orientation-matrix in Blender <2.50

Current implementation of orientation-matrix system in Blender is not reliable: there are limitations / some nasty problems with mirrored/non-uniform-scaled geometry in Blender

Perhaps, it is not a bug, but a system design side-effect: insufficient/incomplete representation method of object orientation data with "all-in-one loc/rot/scale-matrix".

Yes, m isn't orthogonal (m.is_orthogonal == False), so Blender can't handle it.

Answer 1

matrix_basis only supports valid transform matrices - not a specialist, but by the look of it your m looks more like a shearing one?

Note that internally, there is no matrix_basis at all, Blender only stores translation/rotation/scaling separately, and converts them to/from RNA's matrix_basis. So if a matrix cannot be represented by a translation, rotation and scaling, then you cannot expect it to work.