2.8. Generating a Point Cloud

To capture detailed surface shape 3DSOM Pro can calculate a point cloud — a dense set of 3D points in space. This data is generated using pairs of photos that were taken close together (or optionally pairs taken with the 3DSOM Pro stripe pattern projected onto the object surface). The Point Cloud Wizard is used for this task.

Using Projected stripes

If you are using projected stripes, for each rotation angle of the object, a digital projector should be used to illuminate the object with a pattern of horizontal stripes. You can use two or more cameras to capture the appearance from several elevation angles (see Using projected stripes). Alternatively, you can use a single camera placed at two elevation angles as long as the calibration target(s) is visible in all the shots. If you are moving a single camera, you can either use a computer controlled turntable to take a complete sequence at each elevation or move the camera for every object rotation angle.

Point cloud without stripes

The latest version 3DSOM Pro can now use pairs of photos taken from slightly different viewpoints to build a dense point cloud. For more detail on how to take these shots see Easy workflow 2 - Camera on tripod moving around object and Workflow 3 - Single camera with tripod.

Starting Point

Although not essential, it is usually better to have an initial surface generated — the initial unoptimised mesh generated from masks defines the region of space where 3D points can lie.

At any stage you can recalculate a point cloud using the existing geometry to constrain where the points can be found.

Step 1

Open the Point Cloud Wizard by clicking the ID_CALC_POINTCLOUD.jpgGenerate Point Cloud button.

Step 2

Adjust the "density" slider to vary the number of points that will be calculated. A high density cloud will require more processing power but result in more 3D points which can result in better accuracy. A multi-core fast processor is recommended for calculating a high density cloud.

Figure 2.8.1. The effects of choosing different point density settings.

Move the slider to the left to generate a quick sparse set of 3D points ("draft" quality).

The "Remove outliers when done" option should be selected to interactively eliminate erroneous outliers after generating the 3D points. If you already started from an existing approximate point cloud you should not need to select this option.

The "Restrict image pairs" parameters are used to determine which pairs of images are used if you are using projected stripe patterns. If you are using the naming convention in File Naming Convention you will use the "...using trailing digits" (default) option - which selects pairs of images with the same numerical suffix. If you don't have projected stripes then the default unchecked options should be used.

The "...using timestamp" option can be used if the images taken from both cameras were taken simultaneously (or within a 0.5 seconds).

Step 3

Click the Next > button to start the process. The points will be displayed in the 3D view as they are calculated. You can rotate the scene using the viewpoint controls as normal.

TIP:

Use the ID_SHOWPOINTS.jpgShow points only button to see the point cloud more clearly without the current mesh. Pairs of images are processed in turn and the resulting points displayed with a lighting effect to show the point orientation (normal direction).

Click the Next > button button to stop processing and use the current point cloud or wait for the wizard to complete processing and then hit the Next > button.

Step 4

If the "Remove outliers when done" option was selected the "Filtering Point Cloud" page is displayed.

You will need to wait a short while as the points are pre-processed. You can then interactively eliminate outlier points and see the results immediately.

Try to eliminate points that appear to be incorrect (i.e. do not lie on the surface of the object) as these points could result in an inaccurate 3D model when you optimise the surface.

Move the slider to the left to eliminate more points.

Select the "remove points outside masks" option to remove points that lie outside any mask when projected into each view. If your masks are accurate this can help eliminate incorrect points.

When you are happy select the Finish button.

Step 5

At any point you can remove outliers in the point cloud by selecting the ID_DFILTERCLOUD.jpgFilter point cloud outliers button.

To update the surface geometry using the generated point cloud you will now need to run the Surface Wizard. See Optimising the surface geometry.

Two pass point cloud generation
If you use the ID_MAKE_ALL.jpgMake all button at higher quality settings you may notice a point cloud is generated twice. Once a point cloud is generated, re-running the wizard uses the previous "cleaned-up" initial cloud to help capture a more accurate point cloud the second time. This technique allows more accurate detailed points to be obtained.