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How do I improve the camera calibration?

Improving the camera mounting calibration in RiSOLVE using tie-points

In order to check the quality of the mounting adjustment you need some objects with known coordinates you can clearly identify in at least one of the images acquired with the camera. In the example shown below we scanned an area with some retro-reflectors, and we took some images with the flash switched on. Furthermore, we automatically extracted the reflectors in the image and we extracted the reflectors from a corresponding scan to fill up the TPL (SOCS) with the coordinates of the reflectors in the SOCS system. To get the best accuracy we recommend to "finescan" the reflectors.

The image below shows a part of the image where we activated the display of the TPL Image (yellow) and TPL SOCS (red).

You can see the deviation in the pixel coordinates which is caused by a not perfectly calibrated mounting matrix.

Preparation of a calibration field

Set out a series of 50mm retro-reflective discs. This will be your calibration field. Make sure to observe the following points:

  • use at least six targets in your calibration field
  • the calibration field should cover the field of view of a single image
  • aim for a good spread of targets in both vertical and horizontal directions
  • aim for a good spread of distance from the scanner - between 5 and 30 metres
  • avoid areas with lots of reflective objects (car headlights, number plates, shiny metal, etc)

Capture the calibration data

Position the scanner in front of the calibration field. Rotate the scanner so that one of the images will cover the calibration field.

Switch on the flash on the camera, and set the camera to use the flash. Note - correct exposure of the image is not so important, as you want the targets to stand out brightly in order for the software to automatically extract them later. If you don't have a flash, you will need to manually identify the targets later, so ensure the exposure is set to facilitate this.

Perform a normal Panorama scan (e.g. Pano_40) and acquire images. Ensure the flash fired, it you have one.

Once the results are displayed, tap the context menu icon and from the drop-down, and tap on "Search reflectors"

   

A number of reflectors will be found - tap on "x reflectors found" to visualise them.

The results will be shown overlaid onto the scan, marked with red crosses. You can select the correct targets by tapping on them - they will turn yellow - or by selecting them in the list below.

Once you have selected all of the correct targets, tap the green button at the bottom to fine-scan them.

Now you have captured all the calibration data, the next steps are performed in RiSCAN PRO / RiSOLVE, so copy the project onto a SD card or USB stick and transfer it to your processing PC.

Import the project and extract image tie-points

Create a new project in the normal way, by importing the .riproject / .PROJ folder. During import, the fine scans of the targets will be imported, and the coordinates will be added to the tie point list, called the TPL (SOCS).

Extract the image tie-points

The image tie-points now need to be extracted. The method for doing this depends on whether you had a flash available or not.

If you had a flash...

If you had a flash, right-click on the image that contains the calibration field, and choose "Find reflectors". Set the options as follows:

The option "Use TPL SOCS" is important here, as this will enable the software to link the image tie-points with the scanner tie-points automatically.

If you didn't have a flash...

Open the image that contains the view of the calibration field. Enable the display of the scanner tie-points by expanding the "Show reflectors" button and checking "Show TPL SOCS". Do the same again, but check the "Show TPL Image" this time.

Now, you will need to manually mark the centre of each target by left-clicking it, which places a small crosshair marker.

When you are happy that it is in the correct place, right-click the marker and choose "Add point to TPL"

Now you need to link the image tie-point with the scan tie-point. To do this, hold down SHIFT and drag a box around both tie-points - they will both turn red:

Then right-click near the box the choose "Link tie-points together"

Repeat this process for all the targets in the photo.

Important note: In order to improve the mounting calibration you have to use at least 3 corresponding points.

Improve the calibration

Once you have finished establishing the correspondences, start the optimization by displaying the "Re-Adjustment of camera mounting" page. The are two different places where this is stored, depending on which scanner you are using:

  • CALIBRATIONS > MOUNTING
  • CALIBRATIONS > SYSTEM DESCRIPTIONS

Right-click the relevant entry, and choose "Adjust Camera Mounting with Tie Points..."

Switch to the "Re-adjustment of camera mounting" tab.

RiSOLVE searches all scan positions for links you have established before and displays the links in the upper part.

For optimization the following options are available:

SETTINGS

  • Calculation mode:
    • Modify rotation only: Only the three rotation parameters out of the six parameters defining the mounting calibration are optimized. As the position of the camera is very well defined by the mounting support this default option should be used.
    • Modify rotation and translation: All six parameters are optimized. This option should only be used with a sufficient number of well distributed signals (reflectors). Use this option with care!
    • Modify translation only: Do not use this option.
  • Figure of merit:
    • Least squares fitting: Optimization is based on minimizing the sum of the squares of the residuals. This is the standard setting. Note that the result may be changed significantly even by a single outlier arising from establishing a wrong link.
    • Robust fitting: Optimization is based on minimizing the absolute value of the residuals. Note that this option reduces the sensitivity to outliers.
Start optimization by clicking the button "Start re-adjustment".

Note that this process may take some time if the camera-calibration has to be prepared.

The result for the optimization of the matrix is shown at the bottom. The LOG area displays the improvement in the residuals.

Check the quality of the mounting calibration by displaying the tiepoints again within the image (compare image below). Update the display by switching the display of the tiepoints off and on again.

Upload the new calibration to the scanner

If you are satisfied that the new calibration is correct, you will need to upload it to the scanner so that it can be used for future projects. This process for this varies depending on the scanner and firmware, so please contact support for advice.