Reprinted with permission from the INSPECT article, "Perfect Book", featured in the September 2010 issue.
By Michael Gibbons
We‘ve all been there. Standing in front of the university photocopier, our Physics 101 textbook laying face-down on the copier glass, pushing down on the lid with all of our strength in a vain attempt to flatten the pages out and make them somewhat legible. Of course, even with this time-honored technique of trying to achieve an undistorted copy of the written word, the result is rarely what is sought after. Even with most digital book scanners, correcting for the distortion produced by the curvature in the area of the spine, or fold, of the book has been almost impossible.
The problem of producing undistorted and evenly illuminated digital representations of the pages of a book, particularly without damaging the book in the process, is one that has faced above all the libraries and museums. Electronic reading of information in paper-bound form by means of optical character recognition (OCR) has also been subject to a high error rate due to the distortion caused by the curvature of the surface. However, distortion-correction methods that use mathematical algorithms often have limitations in the practice.
Figure 1: Zeutschel OS 12000 Bookscanner with Perfect Book, an add-on 3D scanning module
To address these issues, Zeutschel from Tuebingen, Germany, a provider of digital and analog storage systems for the documentation and archive management industry, began work on a new solution. Therefore, the company partnered with OEM industrial image capture system supplier Chromasens from Konstanz, Germany. The result is the Zeutschel OS 12000 Bookscanner with Perfect Book, an add-on 3D scanning module.
The Book Scanner's Setup
The OS 12000 system is comprised of a cradle for supporting a book (opened face-up) and an optical system positioned above the cradle. The placement surface of the book cradle is divided in the center into two height-adjustable page surfaces, enabling a horizontal scan surface. The cradle can be adjusted to accommodate the binding of thicker books up to 10 centimeters thick. A glass plate, typically used in other digital copier systems, does not need to be placed on top of the book. This allows gentle digitization of fragile originals. The OS 12000 optical system consists of a high resolution line scan camera, LED illuminators, a mirror, and a motor controller for the focus and mirror systems. Light from the LED‘s affects the scanning material for a short period of time, which minimizes damage of valuable documents.
Figure 2: The line scan’s projection is moved over the book (green)
while the area scan camera (yellow) is pointing down at the cradle
Creating a 3D Model
In addition to the line-scan camera used for high-resolution texture capturing, the Perfect Book module requires a second area scan camera and uses the principle of light section to capture the 3D surface contour of the page during the scanning process. Accurate calibration of the optical components is a key requirement for the 3D reconstruction. The area scan camera is first calibrated with its lens to generate a camera model, and then with the high resolution line scan camera so that the software knows which pixel in the hi-res image belongs to which 3D point.
The line scan camera used by the OS 12000 is an OEM FireWire color line scan camera developed for Zeutschel by Chromasens, which is equipped with a lens designed for very low longitudinal chromatic aberration. The area scan camera is a board-level Firefly MV IEEE 1394a (FireWire) digital camera from Point Grey. The compact camera, which measures just 40 x 25 mm in size, is equipped with a 4.5 mm C-mount lens from Goyo and is housed in a custom enclosure pointed down at the cradle. The image sensor used by the Firefly MV is a wide- VGA 1/3-inch global shutter monochrome CMOS from Aptina that has near-IR capability in the 850 nm range.
Teamwork of Cameras
The projection of the scanning line and LED illumination line is moved over the original by a system of pivoting mirrors. The movement of the light is synchronized with the line scan camera. Each one-dimensional image from the line scan camera is transferred to the PC, where it is assembled into a high resolution two-dimensional image. During the scan, the Chromasens camera sends a hardware trigger signal to the Firefly MV, which is equipped with a general purpose input/output (GPIO) connector. The Firefly MV supports asynchronous hardware and software triggering. This feature allows the start of exposure, which can range from just 600 μs to 1 ms, to be accurately synchronized between the two cameras.
Creating a Virtual 3D Surface
The individual low-resolution images are streamed from the Firefly MV over the 400 Mbit/s FireWire interface to the host PC, where they are used to construct a 3D representation of the book. The fullyassembled image from the line scan camera is then texture-mapped onto the 3D image to create an exact, virtual 3D surface model of the book. Using the 3D information, all the characters are aligned exactly straight. The software also corrects character spacing and width, and uses interpolation algorithms to remove unwanted elements such as the operator‘ s fingers and inlay sheets. Finally, the dewarped, flattened high-resolution pages are automatically separated and displayed. The system takes just 1.4 seconds to perform a 300 pixel-per-inch (ppi) grayscale scan of an A2-size document. A color scan takes only 4.6 seconds.
Figure 3: With the 3D information, the pages can be dewarped and are displayed separately
"We‘ve recently completed work on a new version of Zeutschel‘s system, the OS 12000 V, which features a v-shaped cradle designed for more valuable books," says Frank Sczech, Team Manager Software Development at Chromasens. "One key difference in the optical system is that it uses two Firefly MV cameras, where each camera is responsible for the 3D generation of either the left or right pages of the book. The source code examples and documentation provided with Point Grey‘s FlyCapture SDK made integrating their cameras quite easy, and when we had questions or issues, we received fast and professional technical support." The cameras are operating without problems in hundreds of scanners in the market. Truly, this combination of camera, optics, software and mechanical design comprises a system, which enables everyone to operate a Zeutschel book scanner with unprecedented ease of use leading to its success.