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Scanning Checklist

 

You can eliminate many vectorization problems by scanning your drawings using the checklist below.

 

1. Color, grayscale or monochrome

 

See Also: Color depth

 

Most scanners give you the option of scanning in color, grayscale or monochrome. These options have different names depending on the make of scanner you have.

 

Color

Your scanner’s color option will normally create a raster image that contains 16.7 million colors.

You should only use this option if you are scanning a color drawing with a view to converting it to a color DXF file. Do not use your scanner’s color option if you are scanning a black and white drawing – it is easy to do this by accident as most scanners default to color.

If you are scanning a color drawing with a view to converting it to a color DXF file, experiment with your scanner’s settings until the colors on the raster image are as high contrast, vibrant and saturated as possible.

Warning: Color images can be very large. An E or A0 size drawing scanned in color at a resolution of 300 dpi will take up about 385Mb of memory.

 

Grayscale

Your scanner’s grayscale option (often called black and white photo) will normally create an image that contains 256 shades of gray.

Grayscale images are not normally suitable for raster to vector conversion. You should only select this option if you are going to use Scan2CAD’s threshold functions to create a black and white image after scanning. See About thresholding, Simple Threshold and Adaptive Threshold.

Warning: Grayscale images can be very large. An E or A0 size drawing scanned in grayscale at a resolution of 300 dpi will take up about 128Mb of memory.

 

Monochrome

Your scanner’s monochrome option (often called line art, black and white drawing, 1 bit) will create an image that contains two colors – black and white. This is the option you should normally choose when scanning a drawing for raster to vector conversion.

However be aware that 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 raster image and which to set to white (thresholding).

 

If your drawing is clean and sharp this is not normally a problem. However if your drawing has faint lines or a dirty or tinted background you will have to experiment with your scanner’s settings until you get a raster image where, as far as possible, the parts of the raster image that are supposed to black are black and the parts that are supposed to be white are white.

 

Too much white Too much black Optimal

 

If your scanner or scanning software sets too much of the drawing to white in the raster image, it may contain breaks and holes. Faint parts may be lost.

If your scanner or scanning software sets too much of the drawing to black in the raster image, text characters may “bleed” so that white spaces within them or between them become filled. Speckles and dirt may appear in the background.

 

Rather than experiment with your scanner settings until you get optimal levels of black and white you may find it easier to scan your drawing using your scanner’s grayscale option. You can then use Scan2CAD’s threshold functions to create a black and white image after scanning. This will allow you to experiment with different levels of black and white without having to rescan the drawing. See About thresholding, Simple Threshold and Adaptive Threshold.

 

2. Resolution

It is not true that “the higher the scanner resolution, the better the vectorization results”. In fact, scanning at too high a resolution can decrease vectorization quality.

However, bear in mind that:

If you scan at too high a resolution, you can always lower the resolution after scanning.
If you scan at too low a resolution, you can never increase the resolution after scanning and recover the details you have lost.

So, our advice is to err on the side of higher rather than lower resolution.

 

Line drawings

For most line drawings a resolution of 200 to 400 dpi (dots per inch) is optimal.

Ideally, aim for scanned lines that are about 5 pixels thick, and make sure that the resolution you choose is enough to separate close together parallel lines and consecutive text characters.

 

Logos

Because logos tend to be very small you will normally need to scan them at a higher resolution than line drawings. If the edges of the logo are “stepped” after scanning, the resolution is too low. The edges of the logo must be smooth and well-defined.

 

Resolution too low – stepped image

Optimal resolution – smooth, well-defined image

 

3. Saving raster images

We recommend you save your scanned image as TIFF. TIFF files make economic use of disk space and do not reduce the quality of the scan.

However, DO NOT save your image as a tiled TIFF file which Scan2CAD does not support.

DO NOT save your image as JPEG. Saving your image as JPEG will normally reduce its quality. The blurring of the text and entities in the image below and the gray speckles surrounding them are typical problems caused by saving a drawing as JPEG.

 

 

4. VERY IMPORTANT: CHECK YOUR SCAN!

After scanning, check your scan. For example:

Make sure that the extents of the drawing have been captured.
Make sure that the entities on the drawing are solid.
Make sure that any close together lines are separated by white space.
Make sure that the text is legible and that consecutive characters are separated by white space.

 

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