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.
Too much white |
Too much black |
Optimal |
Too much white |
Too much white |
Too much black |
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 ...
... simply move the slider bar or bars at the side of the screen until the threshold is correct.
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.
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 Contex WIDEImage, Graphtec Scanning Master 21.)
While this approach does allow you to set the threshold visually it is slower than the Colortrac ScanWorks 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.
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Q&A: ASK AN EXPERT
Q: I'm told that 48-bit colour capture is best. Is this true?
A: No, it's untrue that 48-bit colour capture is best. 36-bit colour capture is as good and if you are buying a scanner for scanning technical drawings it's irrelevant anyway!
While some large format scanners capture colour in 24-bit and output it as 24-bit, most scanners capture colour in 48-bit or 36-bit and output "the best" 24 bits as 24-bit.
To give you some idea of the numbers of colours we're talking about:
* 48-bit - 281475 billion colours
* 36-bit - 68.7 billion colours
* 24-bit - 16.7 million colours
* 8-bit - 256 colours
36-bit colour capture captures the entire range of colours that can be represented on the best quality scannable media (film transparency).
As 36-bit colour is capable of capturing all the colours that can be represented the additional colours captured using 48-bit colour add nothing to the range of colours that can be captured or used - they simply slow down the scanning process by piling in more data and creating bigger files to work with. And that takes time.
So, in conclusion, the statement which we think is most correct is "36-bit colour capture is best". It gives you all the benefits of 48-bit colour capture without the time and hassle involved in processing extra, useless information.
Why has this misconception occurred? Because bigger sounds better!
We're happy to answer questions! Click here to email us.
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