Alignment with Tags

Reconstruct provides tags that can be printed and posted at a project site to aid in the alignment of processed point clouds captured by 360 videos. Each tag has a unique identifier, labels, and an April Tag or unique set of shapes for computer visual recognition of a spatial point of reference. These tags can be placed on several locations at the job site and are viewable in Reconstruct. For each captured tag, a Control Point is created and added to the Control Point list in the Alignment Tool to quickly align point clouds.

Please follow the steps in the workflow below to understand how they help in alignment. 

Workflow:

To quickly align subsequent captures after an initial capture, follow these steps.

1. Before the initial capture of a particular capture area, download the Tag Collection File attached in the following of this article.
2. Print at least 6 different tags on 11 x 17 or A3 paper size prior to the capture (download file below). A minimum of 3 non-collinear tags (control points) are required for alignment and adding another 3 provides redundancy in case of detection failure and to improve the accuracy of alignment.
3. Plan a capture path and determine the best locations to place tags. Consider how future capture paths and tag locations may be affected by construction. 
4. Post the tags at multiple locations in the area that are visible in the planned capture path, following these instructions:
1. Label the tags
2. Only post tags in a safe space following all safety requirements
3. Post tags on flat surfaces (no round columns or curves)
4. Post tag at a height between 3-7 feet and as close to the height of the camera 
5. Do not move the targets after placement
6. Use unique targets only once per project
7. Equally, distribute the tags in the capture area. Do not concentrate them in one corner. 
8. Post the tags so that not any set of three of them are along one line in the space
9. If a target must be removed, discard it and do not reuse it in another location
5. Capture the area and the 6 tags while slowly moving in front of each tag at a distance between 3-5 feet.
6. Once the capture is complete, download the files to your computer, stitch them if necessary, and upload them to Reconstruct.
7. After completion of the point cloud process, open the Data Manager, and click on the Point Clouds tab. Verify the number of tags detected in each point cloud in the Tags column. If the Tags column is not visible, check “Tags” under Show Columns. 
8. Go to the Viewer, open up the Alignment Tool. 
9. For each captured tag, a Control Point with a naming convention of TAG_CP_# is automatically created and added to the list. If you jump into an image near a tag control point, the control point will be placed at the top, left corner of the tag in the 3D space.
10. Align the initial point cloud to BIM using the Alignment Workflow.
11. At this step, all subsequent captures that capture at least 3 of the already-in-place tags captured in the initial point cloud following the guidelines above can be used for quick alignment.
12. Create a new capture, upload, and process the point cloud with or without aligning to an existing point cloud.
13. After the point cloud is processed, open the Alignment Tool in the Viewer, select the recent capture as the "To-Align" point cloud, and the initial point cloud as the "Reference". Both of these two point clouds should have at least three noncollinear mutual tag control points. 
14. Check the control point list to make sure no old control points are already in use. Select and add the tag control points, click “Align” and verify that the alignment is correct.