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Calibration

Stereo 3D calibration is the process of calculating the position and orientation of two cameras relative to each other.

Calibration is important if images are captured using stereo 3D and need to be dewarped - without the calibration information, the stereo images will not flatten.

It is recommended that before any work using stereo 3D is done, this calibration process be completed first.

Setting Up Cameras for Calibration

Calibration uses the same setup as stereo 3D capture. Refer to the Decapod Stereo 3D Guide.

Connect Right Camera First (Applies to Tethered Capture Only)

Before doing any calibration of capturing, the right camera must be attached to the Decapod system first, then the left camera. Doing this out of order may cause stereo capture and subsequent dewarping to not perform properly.

To ensure that this is done properly, follow the these steps:

  1. Turn on the right camera.
  2. Remove any memory cards if present.
  3. Connect the right camera to the Decapod system with a USB cable.
  4. Repeat steps 1 to 3 for the left camera.

Note:
If any camera is disconnected during capture, both cameras should be disconnected and reconnected in the manner stated above (left camera first, then right camera). If this is done in reverse order, stereo 3D dewarp may not perform properly.

Taking Calibration Photos

To perform calibration, a 10x7 checkerboard pattern is held in full view of both cameras and pictures are taken. Between taking each pair of photos, the checkerboard is tilted differently or its position changed before another photo is taken.

Download Decapod 10x7 checkerboard
This 10x7 checkerboard is specific to Decapod. Using a different checkerboard with different dimensions may produce undesired results.

A minimum of 23 pairs of photos are required to perform calibration. More calibration images may yield better results.

Once the set of calibration photo pairs are ready (at least 23 pairs), they are processed through the Decapod Stereo 3D Calibration tool. The Stereo 3D Calibration tool generates a file containing the calibration information, which is then saved to a user-specified location.

When the cameras are in place, do not change their positions or orientation relative to each other during the calibration process or during stereo capture. If camera positioning is changed, the images captured up to this point should be deleted and the calibration process restarted.

Calibration capture procedure:

  1. Position checkerboard in front of cameras, and ensure checkerboard is visible from both cameras. If checkerboard is not completely visible, then adjust the cameras higher, or place the subject lower.
  2. Take test captures to ensure proper positioning. These test captures should be deleted and not be included in the final set calibration images.
  3. While holding the checkerboard still, take a picture with both cameras. The Decapod capture tool can be helpful in this situation as it can take pictures from both cameras simultaneously.
  4. Change the tilt / position / orientation of the checkerboard while ensuring it is viewable by both cameras.
  5. Repeat Steps 3 to 4 until at least 23 pairs of photos are taken.
  6. If cameras are bumped or change positions, delete all calibration photos taken so far and start again from step 1.

Example Calibration Capture Process

Calibration Image Filename Convention (Applies to Untethered Capture Only)

Stereo 3D Calibration expects at least 23 image pairs that follow this filename format:

capture-[capture #]-[camera #].[extension]

.

Decapod supports dewarping of TIFF, PNG, and JPEG files.

Example 1: capture-01-1.jpg, capture-01-2.jpg, capture-02-1.jpg, capture-02-2.jpg ...

In this example, images with "XXXXX-01-XX.jpg" are a matching stereo pair - left and right photos of the same checkerboard pattern position. Images ending with "XXXXX-XXX-1.jpg are photos taken with the left camera, and "XXXXX-XXX-2.jpg" are photos taken with right camera.

Example 2: capture-01-0.png, capture-01-1.png,capture-02-0.png, capture-02-1.png ...

Similar to Example 1, but with slightly different file name pattern and different file format. The camera numbers can be any number as long as the numbers identifying the left and right cameras are consistent for all 23 image pairs.
Using Decapod’s Capture Tool to do Calibration

Decapod’s Capture application can make the calibration process easier by:

taking photos simultaneously from both cameras which can be faster than triggering the cameras manually.
the Decapod Capture tool also creates a ZIP file of the images (with proper file names) so it can be used directly in the Stereo 3D Calibration tool without additional work.

Important: Do not Change Relative Camera Positions

Once the calibration photos are taken, do not change the relative positions of cameras. Treat the pair of cameras as a single unit and do not turn or twist any cameras once mounted. Do not bump them accidentally.

If camera positions are changed, then the new positions need to be recorded by re-calibrating the cameras (retaking the 23 photos and re-running the Decapod Stereo 3D Calibration tool). Once the new calibration data is generated, any subsequent photos taken with these new camera positions should only be used with this new calibration data.

Mixing and matching different stereo image sets and different calibration data is strongly discouraged.

How Run Stereo 3D Calibration

  1. Create a ZIP file containing all 23 image pairs (46 images total).
  2. Launch Firefox and load the Stereo 3D Calibration tool by entering the server’s address in the location bar. By default the address is: http://localhost:8080/.
  3. Add the ZIP file containing the 23 image pairs (created in Step 2 above) and select “Calibrate” to begin the calibration process.

Note:

Calibration may take 2 hours to complete with high resolution images.

When calibration is complete, save the file calibration.zip to a location when prompted.

calibration.zip file is needed later for stereo dewarping, so it is recommended that the file be saved in a safe location, preferably the same directory where work is being done.

Note:
It is recommended that all calibration images and calibration data be saved in a location that is associated with the work being done. This way it is clear that certain calibration data belongs with a particular unit of work.

Using the Firefox web browser, load Decapod by entering the Capture web address into the location bar. By default the address is: http://localhost:8081.

If the Decapod server can not be loaded at the default address, or if the server is not running, please see your Decapod administrator for assistance.

When the Decapod Capture interface loads, the attached cameras are checked to ensure they will function properly (you may hear the cameras take pictures at this time). If the cameras are matching and supported by the system, then you will receive a message stating that capture is ready and that stereo 3D capture is available.

Note:
Before capturing in stereo, it is recommended that Stereo 3D Calibration be run. Without the calibration information, dewarping the stereo 3D images will not be possible.
Downloading Images

When all capturing is complete, the stereo images can be saved by selecting the “Download Captures” button. After a short delay, there will be a prompt to download and save the archive containing all the captured images.
Calibration

To dewarp stereo images, the camera positions used for Stereo Capture need to be recorded and its calibration information generated. To do this, it is recommended that Decapod’s Stereo 3D Calibration tool be run before doing stereo capture work.

For more information about stereo 3D calibration, see the Help documentation in the Decapod Stereo 3D Calibration tool, or the Decapod wiki.

Note:

Remove any memory cards from the cameras prior to attaching to the Decapod system. If a memory card is present in the camera and the camera is connected to Decapod, the operating system may mount the memory card and prevent Decapod from using the cameras. You will have to disconnect the USB cable, remove the memory card, and then reconnect the cable.

Page Seperator and Background Colour

Stereo 3D capture in Decapod requires a solid coloured background, and an object to serve as a page seperator.

The background is recommended to be a bright green or bright blue, similar to the characteristics of ChromaKey blue and ChromaKey green. ChomaKey blue has an RGB value (0 0 256), and ChromaKey green has RGB value (0 256 0). When positioning the book on the background, it is recommended that some background is visible by both cameras.

The page seperator is placed in the centre spine (or "gutter") of the book while it is photographed. The page seperator can be a string, a drinking straw, or a pipe cleaner, and must be a solid colour different than the background colour and the colours in the book. Bright red is a good colour if the background is blue or green.
An open book on a green background. Down the middle gutter / spine of the open book is a red string which acts as a page seperator.A solid background and a page seperator is required during stereo 3D capture.

Running Stereo Capture

Filename convention (Untethered capture only)

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