Aligning multiple angle scans manually

One of the problems when scanning irregular, complex objects is that the laser and camera can't see every area, or inside concave areas and holes.  There are simply too many "blind spots". 

One solution to solve this problem, is to change the angle from which you scan the object, and perform multiple scans.  Each scan will see a part of the model which the other scan didn't see.  Once you have most or all the areas covered, you can align the individual scans and get a fairly complete scan.

This tutorial deals with the alignment of these scans, but here is a brief rundown on how I achieved the multiple scans:

The first scan is done by placing the model in the center of the turntable, like we usually do.  For the second scan, the model is brought forward near the front edge of the turntable, This will give a different angle for the front and back, but not much on the sides. Next I tiled the model back at ~45°.  I propped it up using a thin (1mm) wire, simple but effective.  Last I did the same thing but this time I titled it forward.

Each scan was done at 5MP, and 1600 steps (about 50 minutes per scan).  Once you have all the scans, we need to assemble them and align them into a single mesh. 

NOTE:  Cleaning a model is a very important part of this process.  Using a scan with lots of noise will result in a very poorly aligned model, and a poor quality resulting mesh.  So please clean up your scan before attempting to align them.  Check my other posts on how to clean up a model in MeshLab

First we start by opening MeshLab.  Next, click File, Import Mesh, then navigate to where the scanned files are and select them.  You can multiple select by holding the SHIFt key (sequential files), or the CTRL key (non-sequential).

Next, all the meshes load on the main screen overlapping each other.  Notice the angles at which they open.  These are the angles at which they were scanned.  Additionally, the layer screen should automatically open as well.  If it doesn't, just click the Layer view button on the menu bar.

By default MeshLab enables the "Flat" view, but we want to see the actual point cloud, so click "Point" view button, then Click the Align button.  This opens up the Align Tool window. 

The first thing we need to do is change a few things.  Check the "Use Point Rendering", and uncheck Use False Colors.  The false colors option is only useful for meshes without color information.  Next make only the first layer visible, then click the "Glue Here Mesh" (Hmmm... must be Yoda speak for glue mesh here ).  This makes the first layer the reference layer to which the other layers will be aligned.