UofK SCANNER CALIBRATION

Updated 11-26-2009

The calibration program is calibrate.exe

STEP 1: MAKE A CALIBRATION GRID PATTERN

The roof grid is one of the easiest grids to make because it is based on a 2-D pattern. The roof grid also lends itself to being constructed to be folded into a portable form. For surround scanning, it can be configured to surround a box so that all scanners are aligned to the same coordinate system. You will want the following downloads:

1. Printable Patterns: Use any one that is convenient and verify that the print matches the desired dimensions. PNG, TIF, PDF

2. Excel Sheet: Adjust the X_off, Y_off, X_plane, Y_plane and theta values for your grid. Excel will calculate the world coordinates {x_w, y_w, z_w} that you need to manually enter into calibrate.exe which will be stored in calgridG.byt.

The generic format of a roof grid: The layout and measurement is performed on the flat 2-D print out and then Excel performs the affine transformations to get the world coordinates. Try to follow the indexing pattern because our auto-calibration algorithms are based on that. You must have a minimum of 6 total features but the accuracy will improve as you include more features.

We try to follow the right hand rule for the world coordinates. It is not required and will result in different orientations of your captured surface data. The above shows the calculations used to convert your 2-D measurements of your printout, to the 3-D coordinates of the calibration grid.

STEP 2: MAKE A CALIBRATION GRID FROM THE GRID PATTERN

Folded Calibration grid made from foam board. The backside of the grid has hinges made from paper. One main hinge and one for the support. I used Elmers glue to assemble the foam board and a spray on rubber cement (you can get at most craft stores or Office Depot) to mount the grid patterns. After the grid patterns are mounted and aligned, one of them will need to be trimmed because of the way the hinged corner edge comes together. Look closely at the photo below. The key is to carefully position the grid patterns so their adjacent edge is exact. It is also important to get a right angle when the grid is fully deployed.

Roof calibration grid. Note that the spine or roof edge would align with the primary epipolar plane defined by the camera and projector. The close up shows the non-symmetry in the corner (sorry for the blurring from my cell phone camera).

STEP 3: CREATE THE CALIBRATION G FILE FOR THE FIRST TIME

Now that you have a physical calibration grid, you know the indexing of each feature along with its world coordinates, you are ready to create the calibration G file.

There are two ways of creating a new “machine” data structures.

3a (option A) The easiest way is to copy a UofKScannerX folder in scanner_settings and change the machine number in the folder name from X to Y. For example, if you have a folder named “UofKScanner7”, just select it, copy it and paste it. The new folder name will have “copy” at the end. Then rename the folder to “UofKScanner8”. Using calibrate.exe, you can select that machine number, “8” and then in File-Files, edit the number of points in the grid.

Once you edit the number of points, go back to the main dialog screen and go to “Setup - World Coord” dialog.

Select “Open” and then use the point spinner to step through each point and manually enter the coordinates and also select “Enable” for each point.

3b (option B) The more difficult method is to run calibrate.exe directly, select a new Machine Number that is not already used such that

Figure: Opening dialog in calibrate.exe allows you to select a machine number and you can index to new number so that you can create the control data.

After entering the primary dialog screen in calibrate.exe, you can create the 0G.byt via File-Files dialog, select the “Create Blank 0G.byt” button such that you see:

Figure: File-Files dialog box. You can create a blank G.byt file and then select the proper number of features that you want. You must have at least 6 features. Avoid changing the Number of Points later because you will lose your data. ALWAYS BACK UP YOUR scanner_settings folder.

Figure: Select “Open” and then use the point spinner to step through each point and manually enter the coordinates and also select “Enable” for each point.

Once you have the G file, you can run the UofKScanner application in calibration mode to complete the process.