Manual Pre-registration among grid point clouds

Manual Pre-registration (among grid point clouds)

The pre-registration technique allows you to manually compute a rough alignment between two grid point clouds. The alignment can be later refined automatically, using ICP registration.

Accessing the preregistration dialog

You can access this dialog from the button Cloud-to-Cloud Registration in Reconstructor®'s top toolbar.

Defining the reference and the moving grid

From the list of grids in the dialog, choose the reference model and press Set reference grid, then select the grid to move and press Set moving grid. The two grids appear as range images on the right and left panel of the dialog respectively.

Finding corresponding points

The preregistration procedure works by finding three couples of corresponding points among the reference and moving grids. If needed, you can register one moving grid to more reference grids: to do so, change the reference grid to another from the loaded ones, the image-grid point pairs are stored for each grid, and are accumulated.

Keyboard/mouse event	Effect
LMB double click	Selects a point. Selected points are listed in the tables moving grid and reference grid at bottom of the dialog
Mouse wheel rotates	Zooms the range image in/out centering on the hovered point
Key 'z'	Toggles the Zoom views shown at the bottom of the screenshot
Ctrl + LMB + move mouse	Moves the zoomed region inside the global view
Space bar	Rotates the range image of 90°
Shift + LMB + move mouse	Translates the range image
RMB on a selected point	Deletes the point
LMB on a point + move mouse	Moves the point around
Alt + central mouse button	Fit image to window
Alt + RMB	Reset image to original size

When three or more couples of points are selected, the table error per point shows the alignment error per point couple. The mean registration error is instead shown at bottom left of the dialog. In general a pre-registration error of 2 cm is acceptable. However, this depends on the scale of your project and of the final output you want to get. If a point couple introduces too much error, you may try to move slightly around the points after zooming on them, to improve that couple's error.

Try to change the color type of the grid on top of the window to improve the contrast of features. Press the right arrow to show a menu of available commands for the current color type for better adjustment.

The list of points is shown in the table, where every cell is manually editable to force the values, like for geo-referentiation.

Finishing the preregistration

When at least 3 point correspondences are found the Mean error value appears in the bottom of the dialog. When the acceptable value is reached, press Compute. A dialog shows you the mean registration error and gives you the options to immediately apply the registration to the moving grid, to copy the registration transform to clipboard to apply it later, or to cancel if you don't accept the error. You may want to copy the transform to the clipboard if you want to move more grids according to the same transform. This is useful for example if you have a group of say 10 grids already registered among them, to be registered against other 10 already registered among them.

To apply “later” a registration transform stored in the clipboard, select in the project window the grids you want to apply the transform to, right-click on them and select Registration->matrix transform from the context menu. Then select Apply registration transform in the Matrix transform dialog.

When the pre-registration is finished, the grids are unloaded from the project to free memory. Please reload them to see them rendered.