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Raster Quality Checklist

 

The most common reason for poor vectorization results is a poor quality raster image. Nowhere is the saying “Garbage In, Garbage Out” truer than in raster to vector conversion.

 

Make sure that your raster image is suitable for vectorization by going through the Raster Quality Checklist below.

Do this even if you think your raster image is perfect. Your image may look fine when viewed full screen but once you zoom into it and look properly you may find that it is not as good as you think.

 

See also the Real World Guide to vectorization, accessible from Scan2CAD’s Help menu. The Real World Guide tells you how to handle less than perfect scans and how to tweak vectorization settings. It will help you to get the best possible vectorization from any vectorizable image.

Click here to open it.

 

1. Is your image negative (white lines on a black background)?

 

Negative image Positive image

 

Scan2CAD converts positive images (black or colored lines on white paper). If your image is negative you need to click to convert it to positive.

 

2. Does your image have a color depth greater than two?

Look at the Title Bar at the top of the Scan2CAD screen. There is a number after the name of your raster image.

If this number is 16, 256 or 16.7M it means that you may need to whiten your image’s background, even if it looks white.

See Whiten the raster image background for details of what you need to do.

 

If you have already performed a simple or adaptive threshold on the image you do not need to whiten its background even if the number is 256.

 

3. Is your image skew?

 

Skew image After deskew

 

If your image is slightly skew, deskew it using Raster Effects Menu > Rotate > Auto Deskew () or Raster Effects Menu > Rotate > By Line ().

Deskewing a very skew image can cause significant deterioration in image quality. If your image is very skew, the best thing to do is to rescan your drawing taking care to get the drawing straight on the scanner.

 

4. Is your image dirty?

If your image is very dirty, you may not be able to clean it well enough to produce a meaningful vectorization or cleaning it might take too long to be worthwhile.

For example, there is no point trying to clean an image that looks like the one below because there are solid black dirty areas obscuring the drawing.

 

 

However if the speckles and dirty areas do not interfere with the drawing itself you will be able to clean the image quickly and easily.

 

Speckled image with dirty area

After despeckling and area erase

 

Use Raster Effects Menu > Clean > Remove Speckles () to despeckle an image.

You can erase dirty areas using Scan2CAD’s raster erase tools.

There are often dirty areas around the edges of raster images. You can delete these using Raster Effects Menu > Crop.

 

5. Does your image have “hairs”

Place your cursor over your image. Press M to Magnify. Press M again and again until your image is highly magnified. Or, zoom in by scrolling your mouse wheel forward. (To zoom out again, click or press the Home key.)

 

Image with “hairs” After Raster Effects Menu > Smooth

 

If the lines on your image look like the left hand diagram above, use Raster Effects Menu > Smooth () to remove the “hairs”.

 

6. Does your image contain holes?

Place your cursor over your image. Press M to Magnify. Press M again and again until your image is highly magnified. Or, zoom in by scrolling your mouse wheel forward. (To zoom out again, click or press the Home key.)

 

Image with holes

After hole removal and flood fill

 

If the lines on your image contain small holes you can remove them using Raster Effects Menu > Clean > Remove Holes ().

To remove large holes use Flood.

 

If your image has holes it may have been scanned at too high a resolution. You may want to try reducing its resolution. You can do this using File Menu > Raster > Statistics.

 

7. Is your image dithered?

Place your cursor over your image. Press M to Magnify. Press M again and again until your image is highly magnified. Or, zoom in by scrolling your mouse wheel forward. (To zoom out again, click or press the Home key.)

 

Dithered image

 

If the lines on your image look like the line in the diagram above, the best thing to do is to rescan your drawing. Experiment with your scanner’s settings until you get a scan that has solid, continuous lines and is not dithered.

 

If rescanning is not an option, try Raster Effects Menu > Thicken > Pixels ().

You may need to run Thicken Pixels several times to improve the quality of a dithered image to the point where it can be vectorized successfully. However, Thicken Pixels should be used with care as it can deteriorate the quality of the image by thickening lines too much and allowing lines that are close to each other to become joined.

 

8. Is your image broken?

Place your cursor over your image. Press M to Magnify. Press M again and again until your image is highly magnified. Or, zoom in by scrolling your mouse wheel forward. (To zoom out again, click or press the Home key.)

 

Broken up image

 

If your image is slightly broken you can probably mend it automatically using one of or a combination of the following methods:

Raster Effects Menu > Thicken > Lines ().
Raster Effects Menu > Thicken > Pixels ().
Before you vectorize the image set a Gap Jump Distance value. When you vectorize the image Scan2CAD will jump over breaks in the raster image that are smaller than the value you have specified, allowing it to produce continuous vectors despite the gaps in the image.

 

If your image is very broken you will not be able to mend it automatically or jump over the gaps using gap jumping. The only way to mend a very broken image is to draw new raster lines and arcs over the broken ones.

If your entire image is very broken it will take too long to improve its quality to the point where it can be successfully vectorized and there is probably no point trying to vectorize it.

 

9. Does your image contain overlaid information?

Place your cursor over your image. Press M to Magnify. Press M again and again until your image is highly magnified. Or, zoom in by scrolling your mouse wheel forward. (To zoom out again, click or press the Home key.)

 

Overlaid information

 

The drawing above contains a lot of overlaid information. Unfortunately, Scan2CAD is not human. It doesn’t know that it is looking at text and a wiring schedule overlaid on a building plan. All it sees is black patterns on a white background and it is not going to be able to separate the data. You are not going to be able to get a sensible vectorization from an image with overlaid information. There is nothing you can do about this.

 

10. Are the entities on your raster image merged together?

Place your cursor over your image. Press M to Magnify. Press M again and again until your image is highly magnified. Or, zoom in by scrolling your mouse wheel forward. (To zoom out again, click or press the Home key.)

 

Merged entities

 

If the entities on your raster image are merged together the raster image is too poor quality for vectorization. There is nothing you can do about this unless the original drawing shows white space between the entities. In this case it may be possible to separate the entities by scanning the drawing again at a higher resolution or with different scanner settings.

 

11. Is your raster image fuzzy?

Place your cursor over your image. Press M to Magnify. Press M again and again until your image is highly magnified. Or, zoom in by scrolling your mouse wheel forward. (To zoom out again, click or press the Home key.)

 

Fuzzy image

 

If your image is fuzzy it is too poor quality for vectorization. There is nothing you can do about this except rescan the drawing using the Scanning Checklist. Fuzziness is most common in JPEG images, so take care not to save your image in JPEG format.

 

12. Is your raster image too low resolution?

Place your cursor over your image. Press M to Magnify. Press M again and again until your image is highly magnified. Or, zoom in by scrolling your mouse wheel forward. (To zoom out again, click or press the Home key.)

 

If your image looks jagged the resolution is too low. The image needs to look smooth and well-defined.

If your image is a line drawing, aim for lines that are about 5 pixels thick.

 

Resolution too low – jagged image

Optimal resolution – smooth, well-defined image

 

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