Help needed for world orientation and matrices

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1

Hi.

I am working on a script that parses a ‘Camera’ file into Blender. The file I am parsing uses a 4 x 4 matrix to transform the world into camera space as per the camera transform in a RIB file.

eg.

filmback 24.892 18.669
frame 1
#focal length 35
fov 29.97002051
transform
0.7071067812 -0.3312945782 0.6246950476 0
-2.775557562e-17 0.8834522086 0.4685212857 0
-0.7071067812 -0.3312945782 0.6246950476 0
3560.565558 -2127.414591 3464.230949 1
frame 2
#focal length 35
fov 29.97002051
transform
0.7071067812 -0.3312945782 0.6246950476 0
-2.775557562e-17 0.8834522086 0.4685212857 0
-0.7071067812 -0.3312945782 0.6246950476 0
3560.565558 -2127.414591 3464.230949 1

=====================================

THE PROBLEM

The problem is that this camera is based on a Maya camera and world which is of course Y-Up as opposed to Blenders Z-Up.

I have managed to create a camera in Blender and bake the animation, I just need to find a way to rotate the camera so it respects Blender’s world orientation.

Help please!

R.

ps. I understand that this is something that has probably been addressed before, but I am having a hard time to find the correct solution. I found this snippet where someone had a similar problem and multiplied their matrix to re-align everything. I tried similar things, but my camera went haywire. I feel I am so close, yet so far.

===================================================

All you have to do is multiply your vector or matrix from the Z-up system with the transformation matrix which converts Z-up into Y-up:

yUp = someZupVectorOrMatrix * matrix3 [1,0,0] [0,0,1] [0,-1,0] [0,0,0]

This flips the axes by taking the +Z as +Y and the +Y as -Z and will convert rotation and scale parts correctly, too.

To convert from Y-up back to Z-up, just use the inverse() of the conversion matrix above which is
zUp = someYupVectorOrMatrix * matrix3 [1,0,0] [0,0,-1] [0,1,0] [0,0,0]

==================================================

Script examples will be appreciated :wink:

2

If the data is taken from maya then it’s possible that the matrices are formed differently to Blender. Blender’s matrices are column-major rather than row-major link. This means that in addition to flipping an axis to convert from right-handed to left-handed coordinate systems (or is it the other way around? :spin:) you might need to take the transpose of the matrices that you are importing.

The transpose of a matrix is a matrix that is similar, but with the rows and columns swapped around. Kinda like mirroring the matrix along the diagonal from top left to bottom right.

3

Assuming you are still having trouble, I think maybe performing the following might convert from Mayas right-handed xyz coordinate system to Blender’s left-handed xzy coordinate system:

blender_camera_matrix = transform1_matrix * maya_camera_matrix * transform2_matrix

where:

maya_camera_matrix is the 4x4 matrix: (using your example)
[0.7071067812,      -0.3312945782, 0.6246950476, 0]
[-2.775557562e-17,   0.8834522086, 0.4685212857, 0] 
[-0.7071067812,     -0.3312945782, 0.6246950476, 0]
[3560.565558,       -2127.414591,   3464.230949, 1]

transform1_matrix is the 4x4 matrix:
[1, 0,  0, 0]
[0, 0,  1, 0]
[0, -1, 0, 0]
[0, 0,  0, 1]

transform2_matrix is the 4x4 matrix:
[1, 0, 0, 0]
[0, 0, 1, 0]
[0, 1, 0, 0]
[0, 0, 0, 1]

Don’t have Maya to test… so maybe it works maybe it doesn’t. I’d write a little code snippet for you to run, but honestly I don’t know how to construct matrices in python :stuck_out_tongue:

The file I am parsing uses a 4 x 4 matrix to transform the world into camera space as per the camera transform in a RIB file.

I don’t know what a RIB file is, but make sure you know what your matrix represents.

Because… a camera’s world matrix transform’s a camera from local space to world space. While a matrix that transform’s the world into camera space is a totally different thing. That would be what is commonly referred to as a view matrix (the inverse of a camera’s world matrix). So make sure you know which one your dealing with or things will be off. :eyebrowlift:

4

Thx guys for the quick response. Will run a couple of tests tomorrow and post results.