Registration moves one object so that it fits another object as closely as possible. Use it to fine-align two overlapping scans of one part, or a scan and its CAD model, before you merge or compare them. The tool only refines: the objects must already be roughly in place.
You find the tool in Transform › Registration.
How Registration Works
Registration uses the Iterative Closest Point (ICP) algorithm. It takes sample points on both objects, pairs each sample with the closest point on the other object, drops pairs that look wrong, and moves the floating object to make the pairs closer. Then it repeats these steps with the new position until a stop condition is met.
Because every step looks only for the closest points, ICP finds the best fit near the start position. If the objects start far apart or turned against each other, it can stop in a wrong position. That is why you align the objects roughly first with Align Manually, Move Object or Transform Object.
The Method setting decides what "closer" means:
- Point-to-Point reduces the distance between the two points of each pair. It is the safest method and works on any data, but it needs more iterations.
- Point-to-Plane reduces the distance from a point to the surface (the tangent plane) at its pair. It converges much faster, but it needs normals: meshes always have them, a point cloud has them only after Make Normals or if the scanner saved them. This is the default method.
- Combined uses Point-to-Point for the first two iterations and Point-to-Plane after that.
Before You Start
- Select exactly two objects, each a mesh or a point cloud. Otherwise the tool is disabled and its tooltip says Two valid objects must be selected, each either mesh or point cloud.
- The object that is higher in the Scene tree becomes the reference: it stays in place. An object with the Reference Model tag becomes the reference instead, wherever it is in the tree. The other one is the floating object: it moves. You can exchange the roles in the panel.
- The objects must overlap and be roughly aligned.
Register Two Objects Step by Step
- In the Scene tree, click one object, then Ctrl+click (Cmd+click on macOS) the other one.
- Go to Transform › Registration. The panel shows the names in Reference Obj. and Floating Obj.; while the tool is open the reference object is painted red. If the roles are wrong, click Swap.
- Look at the Information section and at the black pair lines in the scene. They show which points ICP has paired and how far apart they are.
- Click Run. The floating object moves, the Information values update, and the line under the buttons tells how many iterations ran and why ICP stopped.
- If the RMS values still went down noticeably, click Run again: it starts from the current position.
- To go back, click Reset Objects: both objects return to the positions they had when you opened the tool. To keep the result, just close the panel. Ctrl+Z (Cmd+Z) also undoes a run.
Auto Initial places the floating object automatically before you click Run. It matches the main axes of the two objects (their covariance), so it works for clearly elongated or asymmetric objects that start far apart. For symmetric objects it can choose a wrongly turned position; align such objects with Align Manually instead.
Example: Two Scans of an Owl
Two scans of an owl figurine are loaded as point clouds. The second scan was moved and turned out of place to show the effect. On opening, the panel sets Max Distance to 201 mm (the object size) and Sampling Spacing to 3.01 mm (1.5% of it). Before Run, 3 289 point pairs are active and the RMS point-to-point distance is 2.33 mm.
One Run took 29 iterations and stopped with No improvement iterations limit reached. The scans now lie on each other: 5 854 pairs are active, and the RMS point-to-point distance fell to 0.35 mm.
The same scans without normals show why Method matters: with Point-to-Plane, Run did not move the floating scan at all, and RMS point-to-plane distance showed 0. After switching to Point-to-Point, one Run aligned the scans with an RMS point-to-point distance of 0.44 mm.
How to Read the Result
The Information section shows four values:
| Value | Meaning |
|---|---|
| Samples | How many sample points ICP uses. |
| Active point pairs | How many pairs passed the filters and take part in the fit. |
| RMS point-to-point distance | Root mean square of the distances between the points of the active pairs. |
| RMS point-to-plane distance | Root mean square of the distances from the points to the surface at their pairs. |
The RMS values are in scene units. Lower means a closer fit. A good result is when the RMS drops after Run and ends near the point spacing or noise level of your scans, while Active point pairs stays high. A low RMS with only a few active pairs is not a good sign: it may mean that only a small area matches.
The line under the buttons starts with Performed N iterations. and gives the reason ICP stopped:
| Message | Meaning |
|---|---|
| No improvement iterations limit reached. | The fit stopped improving: the usual end of a good run. |
| Limit of iterations reached. | ICP used all Max Iterations. Click Run again or raise the limit. |
| Required mean square deviation reached. | The RMS reached Deviation Reached. |
| No solution found. | ICP could not compute a step, for example because too few pairs are left. |
Before the first Run, the line says Not started yet.
To see where the objects still differ, open Surface Deviation after the registration.
Typical Errors
- Run is greyed out. ICP needs at least three active point pairs. The tooltip says Objects are too far apart. Perform a rough initial alignment using Align Manually, Transform, or Move object. Align the objects roughly first, raise Max Distance, or select Register Whole Objects.
- Run does not move the floating object, and RMS point-to-plane distance shows 0. Both objects are point clouds without normals, and Method is Point-to-Plane, which needs normals. Set Method to Point-to-Point, or run Make Normals on the clouds first.
- The result is turned or shifted along a flat or round surface. Flat, cylindrical and symmetric parts can slide without changing the RMS much. Pick the start position with Align Manually using distinctive points, or limit Transformation, for example to Translation Only.
- The scans overlap only partly, and the fit is pulled sideways. Pairs from areas that exist in only one scan pull the object. Lower Max Distance or Far Distance Factor, or select Mutual Closest Points in Each Pair.
- The objects have different sizes, for example a scan in other units than the CAD model. Set Transformation to Rigid + Scale, or fix the units first with Scale.
For three or more scans, register them all at once with Global Registration. To merge registered point cloud scans into one mesh, use Scans to Mesh.
Parameter Reference
The same ICP settings appear in Align Manually, Global Registration and Scans to Mesh. "Object size" below is the diagonal of the bounding box of the larger object. The defaults are set when you open the tool for the first time.
Pairs Forming Settings
| Parameter | What it does | Default and tip |
|---|---|---|
| Register Whole Objects | Uses all point pairs without filtering; the filters below are hidden. | Off. Turn on when the objects are far apart and Run is disabled. |
| Normals Max Angle | A pair is used only if the normals at its two points differ by no more than this angle. | 45.6°. |
| Max Distance | A pair is used only if its points are closer than this distance. | The object size. Lower it when the scans overlap only partly. |
| Far Distance Factor | A pair is used only if its distance is below the current RMS distance times this factor. | 1.5 (range 1–10). Lower values drop more outlying pairs. |
| Mutual Closest Points in Each Pair | A pair is formed only if each point is the closest one to the other. | Off. Helps with partial overlap. |
| Sampling Spacing | Size of the sampling cell: one sample point per cell. | 1.5% of the object size. Smaller is more precise and slower. |
| Sample Target | Which object provides the sample points: Both Objects, Floating Only or Reference Only. | Both Objects. |
| Method | The value ICP minimizes: Combined, Point-to-Point or Point-to-Plane (see How Registration Works). | Point-to-Plane. Use Point-to-Point for point clouds without normals. |
| Transformation | Which movements are allowed (see the table below). | Rigid Body. |
| Transformation | Allowed movement |
|---|---|
| Rigid + Scale | Move, rotate and scale uniformly. |
| Rigid Body | Move and rotate. |
| Orthogonal Axis Rotation | Move and rotate, but not around the axis you enter in Fixed rotation axis. |
| Fixed Axis Rotation | Move, and rotate only around the axis you enter in Allowed rotation axis. |
| Translation Only | Move without rotation. |
Stop Conditions
| Parameter | What it does | Default |
|---|---|---|
| Max Iterations | Maximum number of ICP iterations in one run. | 100 |
| No Improvement Iterations | ICP stops after this many iterations in a row without improvement. | 20 |
| Deviation Reached | ICP stops as soon as the RMS distance falls to this value. | 0 (off) |
Visual Settings
In the Visual settings section, Show pair lines, Show points and Show normals display the active pairs, their points and the normals at them (all on by default). Lines always front draws the pair lines over the objects, so you see them even behind a surface.
