[crazygringo]

Before OCR'ing it converts to black-and-white using a brightness threshold. Keeping an evenly lit background is particularly important, because otherwise a shadow area can easily fall under the threshold of all-black.

A license or ID will almost certainly have medium-contrast elements in the background that will show up as dark. But if you were able to manipulate the contrast/brightness appropriately in advance, you could probably get it to work.