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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 for a single purpose – to cover the farthest ends of the…

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 for a single purpose – to cover the farthest ends of the stage horizontally and vertically with the markers (RED and Blue dots).

Depth camera calibration principles for volumetric video capture
Red and Blue dots in the bounding box represent scanned markers in the end of depth camera calibration

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

Now go back to the Multi-Calibration Principles and make sure you really understood it 🙂
If you did, lets move forward. These are your settings:

Marker auto calibration toolset on EF EVE VolCapp for volumetric capture
EF EVE VolCapp Multi-Marker Calibration tool with functionality description.

For calibrating the multi-markers, you will have collect them a few times. At first we will collect the markers on the ground, later – the markers on a stand. 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.

  • Now put four markers on the floor, in the farthest corners of your Volumetric Capture Stage:
Multi marker automatic calibration first collection for voumetric capture
EF EVE VolCapp Multi-Marker Calibration – Collection number one.
  • Press Collect and wait a few seconds until the progress bars load.
Multi markerautomatic calibration loading bar and index of markers collected.
Multi marker calibration indicators loading bar and markers collected

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, check how many markers were 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.

You have completed the first Collection! Get your marker stand ready, as you will need it in the following steps.

Stand Markers Calibration

The stand markers are necessary for collecting the high points of each Volumetric Capture Stage corner. You will need two stand markers for this.

  • For the best positioning of markers, open the 2D view in the main menu at the top right.
  • Put both markers in the opposite corners. Make sure they fit in every boarder of the capture stage
Markers in the volumetric capture stage for depth camera calibration
Stand markers positioned in the opposite corners of Volumetric Capture Stage.
  • Position them in such angle that all cameras could capture both markers. Take a look at the 2D depth view – if any of the markers has gaps (showing that no data is collected) try to find a different angle. Markers should look as smooth as possible. Image below:
Depth camera view in 2D view window for checking the auto calibration marker position
Correct position of stand markers in the depth camera view
  • In the Multi Marker Calibration section, press the Collect button again and wait until it loads. Make sure that both markers were captured.
    If not, click the Undo button, fix the marker’s angle and collect again:
EF EVE VolCapp showing that both automatic depth camera calibration markers were collected.
Pay attention if both auto calibration markers were collected

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. 

Depth camera auto calibration marker position testing
Test if the calibration marker is seen clearly by all depth cameras. (1) cover with your hand, (2) uncover.
  • Repeat previous actions with the same markers, but in different corners:
Markers in different corners of volumetric capture stage for depth camera calibration
Stand markers positioned in the different corners of Capture Stage than it was before.

After you repeat the steps, you should have 3 green numbers in the count of number of collections done.

depth camera markers calibration showing how many markers were collected
Number of collections done for volumetric capture

To make even more precise calibration, you can collect the markers in different heights and positions up to 10 times.
However, for a first try, 3 times are enough.

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:
Depth camera calibration for volumetric capture. Matching marked points.
Volumetric capture calibration and matching marked points
  • Make these points bigger so you can see them better by clicking the arrow up:
Pointcloud calibration marked points in EF EVE VolCapp
Changing marked points size in the pointcloud
  • Lock the point size by clicking the lock icon, otherwise it will change if you move the scroll wheel on your mouse:
EF EVE VolCapp point cloud calibration
Locking the marked points size for depth camera calibration

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.

Depth camera calibration point cloud for volumetric capture
Marked points before matching the depth camera calibration

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

  • In the Multi Marker Calibration section, click the Match button:
Depth camera calibration in EF EVE VolCapp
Match button for pointcloud calibration
  • Wait a few seconds and you should see the red and blue points coming closer to each other:
Volumetric capture depth sensors calibration matching points
Depth camera calibration Matching points after the Match button is clicked

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:

Depth camera calibration points matched and the view from the top.
Depth camera calibration points matched, view from the top.

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.

Checking volumetric capture quality by turning on red/blue view of pointclouds
Red/blue view of the pointcloud – to check your volumetric capture quality
  • Turn off the Red/blue view
  • Turn off the Marked points

Read next to continue the calibration

Updated on January 30, 2020

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Comments

  1. This is so helpful! And calibration so easy like this. Took me a few attempts though to collect the markers. Somehow the camera didnt get one marker at the first time 😮

    1. Thanks, George, it means a lot!
      Sometimes it might happen if your markers are a bit glossy. If you put the marker in the angle where it reflects light, camera might not capture it.
      However, we already have a new, easier way of calibration with a marker box!
      It’s not yet in the Help center, but you can send us an email and we will explain how it’s done. You won’t have issues like that with the marker box 🙂

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