Step 4. Volumetric Capture Multi-Marker Calibration in VolCapp

Multi-Marker Calibration Principles

In volumetric capture, Multi-Marker Calibration is a powerful tool which precisely calibrates multiple pointclouds within a single coordinate system. Before we start, let’s understand the principles! You will be positioning markers across the Volumetric Capture Stage to cover the farthest ends of the stage horizontally and vertically with the markers (RED and Blue dots).

RED and Blue dots must be inside the yellow invisible boarder – this means your markers must also stand inside the capture stage

Multi-Marker Calibration

These are your settings:

For calibrating the multi-markers, you will have to collect them a few times. Please make sure you are using Aruco markers received in your email. At first we will collect the one marker on the ground in the middle of your stage(slave and master calibration), later – the markers on a stand. Please make sure you first do the master and slave calibration then start following information on this page.

The markers on the stand will collect data for mid height (60cm) and upper height (160cm) of your stage while the one on the floor takes your ground position (master and slave). Therefore you have a number of collections done indicated below the Collect button. You will need to use this Collect button a few times for different markers.

Remember: The numbers of detected markers will become green as well as the first number in the number of collections done section.

Every time you collect data, check if the markers where collected. If you’re using two computers’ network, check on both screens. If any of markers are missing, undo the action, and try to change the missing marker position so the cameras can see it better. Then collect again.

Stand Markers Calibration

The stand markers are necessary for collecting the mid (60cm) and top height(160cm) of each Volumetric Capture Stage mid centre and corner point. You will need one stand markers for this and one marker printed twice. Basically two copies of the same marker. You need to attache these two markers on a piece of board or strong paper to form double sided board. Place this double sided marker on a stand. Please, make sure that none of the printed markers are bent.

• Position the stand with the marker at mid point between the corner and the centre diagonal of your Capture stage.
• For the best positioning of markers, open the 2D view in the main menu at the top right.
• Lower the stand with double sided markers to 60cm height.

• Position the marker in such angle that 2 cameras could see only one side of the double sided marker and are standing right in the middle diagonal the corner and centre of the stage. Take a look at the 2D depth view – if any of the marker sides has gaps (showing that no data is collected) try to find a different angle. Markers should look as smooth as possible. Image below:

• In the Multi Marker Calibration section, press the Collect button again and wait until it loads.
  If not, click the Undo button, fix the marker’s angle and collect again:

Both cameras have to fully capture the marker. To test if it does, you can simply cover the marker with your hand, then uncover it. This way it’s easier to notice the angle. If the marker is not clearly seen in both cameras, try to turn it more to one side or another. 

Repeat previous actions with the same marker, but for different corners as well as the middle of the stage. At the end you will get 5 collections.1Centre and 4 mid diagonals of the stage.

After you repeat the steps, you should have 5 green numbers in the count of number of collections done.

Now you must repeat the same steps with your stand adjusted to capture the top height of the stage. The stand has to be adjusted to 160cm height.

Next steps: Matching the calibrated points

For volumetric capture, firstly you need to calibrate the cameras and then match the calibrated points in space. As you’ve collected all the markers, you probably see them splitting at the top.

It’s now time to match the pointclouds from both cameras.

• At the menu on the right, in the Calibration tab, scroll down to see the Inspector section. Click the checkbox marked points:

• Make these points bigger so you can see them better by clicking the arrow up:

• Lock the point size by clicking the lock icon, otherwise it will change if you move the scroll wheel on your mouse:

Your pointcloud should now have blue and red dots. Blue dots show the calibrated points from one camera, red dots from another. The closer red dots are to the blue, the better.

Your goal is to match these dots one with another, as close as possible.

• In the Multi Marker Calibration section, click the Match button:

• Wait a few seconds and you should see the red and blue points coming closer to each other:

Sometimes you might need to click the Match button for a few times until you get the best result.

A good practice is to rotate the pointcloud and check the matching points from top and/or other sides:

To check how much of your pointcloud each camera covers, you can click the checkbox “Red/blue view” in the Inspector section. Make the matching points smallest size and zoom in your pointcloud. The places which are seen by both cameras, should have a very good overlay of red and blue dots.

• Turn off the Red/blue view
• Turn off the Marked points

Read next to continue the calibration