Scan2CAD raster to vector conversion software

EXPLANATION:

Large format scanner accuracy and calibration

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Stitching errors

The joining together of parts of an image scanned by different cameras or sensor arrays is called "stitching".

Stitching error     Stitching error

Stitching errors

Stitching errors occur when parts of an image scanned by different cameras or sensor arrays are not joined together accurately. The two main reasons for this are camera or sensor misalignment and document transport through the scanner.

Camera or sensor misalignment

Camera or sensor misalignment is normally caused by moving or bumping the scanner or by temperature variations causing expansion and shrinkage of internal components. Although misalignment occurs in both CCD and CIS scanners, CCD scanners are more sensitive to misalignment because they contain more sensitive parts, usually contain a source of temporary heat (e.g. fluorescent tubes and transformers) and because any small errors are magnified by their optical system.

For example, in a CCD scanner with 600 dpi optical resolution, information from each 1/600th of an inch on the document is focussed onto a much smaller sensor. This focussing takes place over a distance that can be up to a metre long and that is "folded" inside the scanner using a system of lights, lenses and mirrors.

In a CIS scanner with 600 dpi optical resolution, information from each 1/600th of an inch on the document is collected at real size (1:1) by a corresponding sensor mounted close to the scan glass (hence CIS - Contact Image Sensor). This provides a more dimensionally rigid optical system, giving much less room for error.

CCD

CIS

In a CCD scanner, each pixel of information is focussed onto a tiny sensor via a long, folded system of lights, mirrors and sensors.

In a CIS scanner there is less room for error - each pixel of information is captured at real size by its own sensor mounted close to the scanner glass.

The sensor arrays in Graphtec and Colortrac CIS scanners are aligned in the factory and are not expected to become misaligned. Both manufacturers provide maintenance software that allows the sensors to be re-stitched in the unlikely event of a misalignment occurring.

Manufacturers of CCD scanners employ various methods for avoiding stitch errors.

Colortrac CCD scanners are supplied with a stitch target - a sheet with printed diagonal lines - to be used in conjunction with an automatic stitching utility. If a camera misalignment occurs, it can be corrected by running the automatic stitching utility.

Contex include a stitch target on their "all in one" calibration sheet and horizontal and vertical camera alignment is checked and corrected every time the Contex "all in one" maintenance software is run. In addition, Contex scanners have special wires just beneath the scan glass. These act as stitch reference points and the scanner uses them to automatically check and correct horizontal camera alignment while the scanner is resting between scans.

Image Access do not supply a stitch target. Instead, they use special stitch marks etched into the scanner glass of their WideTEK scanner. The scanner uses these before every scan to automatically ensure that the cameras are correctly aligned. This means that stitching errors as a result of camera misalignment should not occur, even if expansion and shrinkage of components occurs with temperature changes during scanner use.

Document transport

Document transport can cause stitching errors in a number of ways.

In CIS scanners, staggered sensor arrays capture different parts of the document

Staggered sensor arrays in a CIS scanner (top view).

In CIS scanners, the sensor arrays are set in two staggered rows, front and back. The parts of the document scanned by the front arrays are scanned before the parts of the drawing scanned by the back arrays. If the drawing does not travel smoothly and evenly between the two rows of sensor arrays, for example because the drawing slips, because of wrinkles, because of slight unevenness in the transport rollers or because of poor paper handling by the operator, a stitching error will occur.

A particular type of stitch error caused by staggered CIS sensor arrays is "leading edge stitch" where the leading edge of the document passes through the front arrays slower than the back arrays. This occurs if the scanner is not yet going at full speed at the beginning of the scan.

In CCD scanners, successive cameras are calibrated so that they stitch at a particular point. If the scanner's transport system doesn't press the document tight against the scanner glass at the stitch point, a stitching error will occur as illustrated below.

CCD scanner stitch point

CCD scanner stitch point with folded document

In CCD scanners, documents must be pressed snugly against the scanner glass at the stitch points to avoid stitching errors.

All large format scanner manufacturers aim to minimize stitching errors caused by document transport by designing special transport systems such as Contex's "All Wheel Drive", Colortrac's "Active Paper Transport", Graphtec's rollers and Image Access's individually spring loaded ball bearing pairs. Colortrac have recently been awarded a patent for "Adaptive Stitching" which allows their CCD scanners to automatically mend small stitching errors caused by the document moving away from the scanner glass.

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