X-ray images generated by fluoroscopes contain complex S-shaped and pincushion distortion

• XMALab calculates the transform required to remove this distortion
• An image of a flat piece of precision-punched metal (McMaster-Carr part number 9255T641) is required
• This precision-punched metal is sometimes called an Undistortion Grid (not to be confused with X-ray antiscatter grids)
• Images of the Undistortion Grid should be recorded before and after video data collection because fluoroscope distortion changes all the time due to changing magnetic fields
• All marker tracking in XMALab is done on distorted images, so the raw XY coordinates are independent of any specific undistortion or calibration
• Different undistortion transforms and calibrations can be applied to the same raw XY coordinates, so it is never necessary to track points (markers) more than once

XMALab opens to the grid images in the Undistortion workspace. Click compute undistortion button.

The undistorted grid images will be displayed. If they look satisfactory, move on to the Calibration workspace. If not satisfactory, there are some tools to modify the result:

• If the whole grid did not fill with detected circles, then it may be necessary to change mouseclick mode to "set center" and try different locations for the starting point for the fill algorithm.
• If the edges look bad, it may be necessary to remove some partial circles from the edges. This can be done manually by changing the mouseclick mode to "toggle outlier" and manually clicking partial circles to remove them from the calculation (detected point turns red) and click recompute undistortion button.
• The remove outlier at border button provides an automatic way to remove a lot of points around the edges. Use this feature carefully because it is almost always better to have more rather than less points for these computations.