Scan2CAD raster to vector conversion software

QUALITY SCANNING:

Creating high quality scans for raster to vector conversion

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Threshold

When you scan a drawing in monochrome your scanner or scanning software has to make a decision about which parts of the drawing to set to black in the scan and which to set to white. This is called thresholding.

There are two types of thresholding, normally called simple and adaptive. In simple thresholding, a single threshold value is applied to the whole scan. All the parts of a drawing that are whiter than the threshold value are set to white and all the parts of the drawing that are blacker than the threshold value are set to black. In adaptive thresholding, the scan is divided up into local areas and a different threshold value is calculated for each area.

If your drawing is clean and sharp thresholding is not normally a problem and you will probably get a good scan using your scanner's default threshold settings. However, if your drawing has faint lines or a dirty or tinted background you will have to experiment with your scanner's threshold settings until you get a scan where, as far as possible, the parts of the drawing that are supposed to be black are black and the parts that are supposed to be white are white.

If you set a too much of the drawing to white, the scan may contain breaks and holes and faint parts may be lost. If you set too much of the drawing to black, text characters may "bleed" so that white spaces within them or between them become filled and speckles and dirt may appear in the background.

Unsuitable for raster to vector conversion - too much white

Too much white

Unsuitable for raster to vector conversion - too much black

Too much black

Suitable for raster to vector conversion - optimal threshold

Optimal

Unsuitable for raster to vector conversion - parts of image missing

Too much white

Unsuitable for raster to vector conversion - holes in lines

Too much white

Unsuitable for raster to vector conversion - entities have merged

Too much black

Suitable for raster to vector conversion - separated concentric entities

Optimal

Using large format scanner software to threshold

While some scanners have software that makes setting an appropriate threshold easy, getting the best threshold on other scanners requires endless rescans. Also, some scanners' software only allows simple thresholding, which is less effective than adaptive thresholding on many poorer quality drawings.

Colortrac's ScanWorks software is the best we've seen to date for quickly and easily setting a simple or adaptive threshold.

Using ScanWorks, you scan the drawing and it appears on the screen. If the threshold is incorrect, as in the example below ...

Unsuitable for raster to vector conversion - entities not separated

... simply move the slider bar or bars at the side of the screen until the threshold is correct.

Suitable for raster to vector conversion - entities separated

Once you've set the threshold, it's easy to zoom in and out and pan around the scan to ensure that the threshold you have chosen is appropriate for the whole scan. There is no need to scan the drawing again.

Contex's new Nextimage software works very similarly, with quick and easy selection of threshold settings using a slider bar. The downside of Nextimage is that the settings are made on a preview scan and the drawing then has to be re-scanned. It is possible to re-scan the drawing using different settings from those seen in the preview, so what you see in the preview is not necessarily what you get in the final scan.

Other scanning software offers one of two approaches to selecting threshold settings:

  • Choose a portion of the drawing. Keep rescanning that portion of the drawing with different threshold settings until the setting is correct. When the setting is correct, rescan the whole drawing. (For example Graphtec Scanning Master 21.)

    While this approach does allow you to set the threshold visually it is slower than the ScanWorks / Nextimage method, makes it hard to zoom around the drawing to see if the threshold you have chosen is appropriate to the whole drawing and holds the drawing in the scanner while you set the threshold. (It is sometimes helpful to look at the drawing while setting the threshold but because the part of the drawing you are thresholding is held in the scanner you cannot do so.)

  • Guess what the correct threshold might be, then scan the whole drawing. If the threshold is wrong, guess what adjustment to make. Then scan the whole drawing again. Repeat until the threshold is correct. (For example Graphtec Scanning Arts.)

    This is too time consuming if you have a lot of drawings where the default threshold settings are inappropriate.

If your scanner's software makes it hard to select an appropriate threshold, you might want to consider third party scanning software. For example, Scan2CAD software includes a simple and adaptive threshold facility where the threshold can be set on the fly using a slider bar. If your scanner is TWAIN compliant you can scan directly into Scan2CAD. If your scanner is not TWAIN compliant you can load an image in any common raster format such as TIFF.

For more information on thresholding, see also:
Explaining the threshold – a vital setting for successful raster to vector conversion

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