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Global Registration fine-aligns several objects to each other at the same time. Use it when you have three or more overlapping scans of one object: aligning them in pairs one after another adds up the small errors, while Global Registration fits all scans together. For two objects, Registration gives you more control.
You find the tool in Transform › Global Registration.
Before You Start
- Select at least two meshes or at least two point clouds. Otherwise the tool is disabled.
- The scans must overlap and be roughly aligned. Global Registration uses the same ICP algorithm as Registration: it only refines positions that are already close. Place the scans roughly first with Align Manually or Move Object.
- The first object in the Scene tree stays in place; all other objects move to fit it. An object with the Reference Model tag stays in place instead, wherever it is in the tree.
In this example, the scan objects are already positioned close to each other, but the fit between them still needs to be improved.
Register Several Scans Step by Step
- In the Scene tree, select the scans: click the first one, then Ctrl+click (Cmd+click on macOS) the others.
- Go to Transform › Global Registration. While the tool is open, each object is shown in its own color.
- Expand Information and check Active point pairs and the RMS values. If Apply is disabled, see No Active Pairs below.
- Click Apply. The tool moves the objects, updates the Information values and shows under them how many iterations ran and why the calculation stopped, for example No improvement iterations limit reached.
- If the RMS values still dropped noticeably, click Apply again: it continues from the new positions.
- To return the objects to the positions they had when you opened the tool, click Reset Transforms. To keep the result, close the panel.
How to read Samples, Active point pairs and the RMS values is explained in Registration: How to Read the Result. In short, a good fit has a low RMS and many active pairs.
To turn the registered point cloud scans into one mesh, use Scans to Mesh: it runs Global Registration as its first step and then cleans and meshes the merged cloud.
No Active Pairs
If no point pairs pass the filters, Apply is disabled, and its tooltip says No active pairs found, please consider changing parameters. Usually the scans are too far apart for the current Max Distance:
- Expand Pairs Forming Settings.
- Increase Max Distance (by default it is 10% of the bounding box diagonal of the largest object) or Normals Max Angle.
- Check that Active point pairs in Information is above zero, then click Apply.
If the scans are far apart or turned against each other, a larger Max Distance is not enough: align them roughly first with Align Manually.
Settings
The Pairs Forming Settings and Stop Conditions work as in Registration; see the parameter reference. "Object size" below is the bounding box diagonal of the largest object. The differences:
| Setting | What it does | Default |
|---|---|---|
| Max Distance | A pair is used only if its points are closer than this distance. | 10% of the object size (Registration uses 100%). |
| Sampling Spacing | One sample point per cell of this size. | 1.5% of the object size. |
| Far Distance Factor | Drops pairs farther apart than the RMS distance times this factor. | 1.5 |
| Transformation | Always Rigid Body: objects are moved and rotated, never scaled. | Rigid Body |
| Max Group Size | The objects are registered in groups of up to this many objects. | 64 (range 2–512) |
| Cascade Mode | How the groups are formed: Sequential divides the objects by their order in the scene, which suits objects comparable in size to the whole scene; AABB Tree groups objects by their bounding boxes, which suits many objects that are much smaller than the scene. | Sequential |
| Max Iterations / No Improvement Iterations | Stop conditions. | 100 / 20 |
The pair settings and the Information values recalculate as soon as you change a setting, so you can watch Active point pairs while you adjust Max Distance.
Freeform Adjust
Freeform Adjust is an optional step after Apply. It does not only move the objects: it slightly deforms them to improve the fit, the same way Freeform Align does for two objects. Use it for scans that are bent a little, for example by scanner drift, when a rigid fit leaves a systematic gap.
Adjustment Grid sets the number of nodes of the deformation grid along X, Y and Z (2–9 each, default 5). Fewer nodes give a smoother, more global correction; more nodes allow more local changes. Click Adjust to run it.
Reset Transforms does not undo the deformation, because it restores only the object positions. To undo Adjust, press Ctrl+Z (Cmd+Z on macOS).
Extra Statistics with Experimental Features
With Show Experimental Features turned on in the settings, Global Registration shows two extra windows. With the default settings they do not appear.
- A graph of the RMS value across the iterations appears after Apply. Export to CSV under it saves the graph data to a file.
- A statistics window in the corner of the viewport shows the same values as Information, either for all objects or for one object. In Object Id, select −1 for the summary of all objects, or the number of one object; its name appears below the field.
In the statistics window, select Deviation Map to color the objects by their deviation from the other scans. A color scale appears next to the objects; High Deviation sets its upper value (by default 0.5% of the object size).
When −1 is selected in Object Id, the deviation map is displayed for all objects.
When you select an individual object ID, the deviation map is displayed for that object, while the remaining objects are shown in gray.
