I'm trying to undo some transformations coming from an external tool*. I'm getting different results depending on subtle differences in the input and wondering how to convert to a rotation about the Z-axis only.
The transformations are expressed as a
matrix = translation * rotation * -translation. I want to decompose the resulting matrix into a single translation and rotation around Z -- I know this is possible given the source material (2D plane).
My problem is coming from GLM
decompose. Given a matrix that looks like this:
[ -0.5 | 0.866025 | 0 | 0 ] [ -0.866025 | -0.5 | 0 | 0 ] [ 0 | 0 | 1 | 0 ] [ 0 | 0 | 0 | 1 ]
If I call
decompose, then take the eulerAngles of the Rotation I end up with either:
- ( 0, -0, 2.09439 ) from quat( 0.5, 0, 0.866025, 0 )
- ( 3.14159, 1.0472, 3.14159 ) from quat( 0.5, 0, 0, 0.866025 )
The difference depends on how the matrix was generated, whether the rotation was 120degrees or -240degrees. The display must be clipping the floating point, introducing a subtle change.
I'm assuming both these rotations are actually the same.
How do I force/convert the result to be a rotation about the Z axis only.
*The external tool is Inkscape which uses the CSS/SVG function
rotate(r, cx, cy) instead of a rotation and transform. That function results in the matrix: translate(cx,cy,0) * rotate(r, (0,0,1)) * translate(-cx,cy,0)