Auto Align

A blender add-on that automatically re-aligns wrong axis objects.

Usage

There are three options available in the 3D Viewport > Sidebar > Item > Auto Align.

• Rotate : Rotate the selected object to match the world axis.
• Rotate & Bake : Same as Rotate, but rotation is applied.
• Keep Position & Bake : Keep the object fixed and only change the local axis.

Also there is a symmetry option. This option can be used to align symmetrical objects, even though there are not many orthogonal faces to the axis. It's a bit slower, but effective when aligning organic objects, such as people.

Note

• The object should be symmetric or have enough orthogonal faces.
• When using symmetry option, the model doesn't need to be completely symmetrical because the algorithm considers the outlier.
• The symmetry plane detection algorithm is far from perfect. If the object is complex (>10⁵ vertices?), symmetry detection will fail with a high probability. Also, if a single object has multiple planes of symmetry, the alignment can be done in an unintended manner.

Algorithm

Orientation

The proper orientation of the object is calculated as follows:

1. RANSAC (Random sample consensus)

   • Randomly select two orthogonal faces to create a candidate orientation.
   • Find the faces that corresponding to the axis within a specific threshold (<5 deg).
   • Criterion is to maximize the sum of the areas of the face.

2. Correction

   • Fine-tune the candidate orientation.
   • Find the weighted median of the points on each axis and rotate the orientation.
   • Repeat until convergence.

3. XYZ adjustment

   • To avoid continuous flips and maintain consistency while repeating the same operation.
   • Determine XYZ axis to have the least difference from the initial mesh orientation.

For each object, only up to 10⁴ faces are considered in the process. This is due to performance reason. If there are more than 10⁴ faces, larger faces would be considered first.

Symmetry Plane Detection

When the symmetry option is on, the symmetry plane must be derived before RANSAC.

1. Vertex pair matching
   • Randomly extract vertex pairs that satisfy symmetry conditions.
   • Vertex normals must be consistent with their relative positions.
2. Plane voting
   • 4 parameters of fitting plane is calculated from each pair.
   • From 4D voting space, choose plane parameters that receive the most votes.
3. Correction
   • From the planes within a certain distance, calculate the median of each parameter.