Converting Three.js Matrices to Blender

The 90-degree offset you're experiencing is likely due to the different coordinate systems used by Three.js and Blender. Let me help you with the conversion process.

Understanding the Coordinate Systems

• Three.js: Uses a right-handed Y-up coordinate system (X right, Y up, Z out of screen)
• Blender: Uses a right-handed Z-up coordinate system (X right, Y into screen, Z up)

Fixing Your Matrix Conversion

Your approach is generally correct, but the axis conversion parameters might need adjustment. Here's a modified version of your code:

PYTHONCopied
import bpy
import mathutils
import bpy_extras

# Transformation matrix from Three.js (row-major order)
matrix_values = [
    1, 0, 0, 0,
    0, 1, 0, 0,
    0, 0, 1, 0,
    0, 0, 0, 1
]

# Convert to Blender's column-major order
matrix = mathutils.Matrix((
    (matrix_values[0], matrix_values[4], matrix_values[8], matrix_values[12]),
    (matrix_values[1], matrix_values[5], matrix_values[9], matrix_values[13]),
    (matrix_values[2], matrix_values[6], matrix_values[10], matrix_values[14]),
    (matrix_values[3], matrix_values[7], matrix_values[11], matrix_values[15])
))

# Convert Three.js coordinate system to Blender coordinate system
axis_conversion_matrix = bpy_extras.io_utils.axis_conversion(
    from_forward='Z',  # Three.js forward is +Z
    from_up='Y',       # Three.js up is +Y
    to_forward='-Y',   # Blender forward is -Y
    to_up='Z'          # Blender up is +Z
).to_4x4()

# Apply coordinate system conversion
blender_matrix = axis_conversion_matrix @ matrix

# Apply transformation to the active object
obj = bpy.context.active_object
if obj:
    obj.matrix_world = blender_matrix
else:
    print("No active object found in Blender.")

Alternative Approach

You might also try this simpler approach which directly applies the necessary rotations:

PYTHONCopied
import bpy
import mathutils

# Transformation matrix from Three.js (row-major order)
matrix_values = [
    1, 0, 0, 0,
    0, 1, 0, 0,
    0, 0, 1, 0,
    0, 0, 0, 1
]

# Convert to Blender's column-major order
matrix = mathutils.Matrix((
    (matrix_values[0], matrix_values[4], matrix_values[8], matrix_values[12]),
    (matrix_values[1], matrix_values[5], matrix_values[9], matrix_values[13]),
    (matrix_values[2], matrix_values[6], matrix_values[10], matrix_values[14]),
    (matrix_values[3], matrix_values[7], matrix_values[11], matrix_values[15])
))

# Create a rotation matrix for coordinate system conversion
# Rotate -90 degrees around X axis to convert Y-up to Z-up
conversion_matrix = mathutils.Matrix.Rotation(math.radians(-90), 4, 'X')

# Apply coordinate system conversion
blender_matrix = conversion_matrix @ matrix

# Apply transformation to the active object
obj = bpy.context.active_object
if obj:
    obj.matrix_world = blender_matrix
else:
    print("No active object found in Blender.")

Make sure to update your Three.js matrix extraction correctly and test with different orientations to ensure the conversion works as expected.