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Is Your Raster Image Suitable For Vectorization?

Raster Quality Checklist for raster to vector conversion

The most common reason for poor raster to vector conversion results is an unsuitable raster image.

Scan2CAD can only give results as good as the raster image you give it to vectorize. Nowhere is the saying “Garbage In, Garbage Out” truer than in raster to vector conversion!

To make sure that your image is suitable for raster to vector conversion, go through the Raster Quality Checklist below.

Do this even if your scanned image looks perfect when viewed full screen. Poor quality raster images often look fine when viewed full screen – it’s only when you zoom in that you can see there’s a problem.

Alternatively, work through Andrea’s Real World Guide to Vectorization, a PDF document written by Softcover’s Andrea Tribe. Andrea has used Scan2CAD to vectorize hundreds of images of different types and quality. 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.

The Real World Guide can be accessed by going to Scan2CAD’s Help Menu, then selecting Real World Guide.

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

Negative image Positive image
Negative images should be made positive before raster to vector conversion Positive image

Scan2CAD converts positive images (black or colored lines on white paper).

If your image is negative you need to click (Negate) to convert it to positive.

Is your image skew?

Skew image After deskew
Skewed images should be deskewed before raster to vector conversion After deskew

If your image is slightly skew, deskew it by clicking (Auto Deskew – black and white images only) or (Rotate by Line – any image).

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.

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.

Raster to vector conversion cannot convert very dirty images

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

To despeckle an image, click (Remove Speckles).

You can erase dirty areas using Scan2CAD’s raster erase tools, particularly the rectangular area erase (select ) and the irregular area erase (select ).

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

Now, zoom into your raster image …

Zoom into your raster image by placing your cursor over the image and pressing M on your keyboard. Keep pressing M until the lines are highly magnified. You can also zoom in by scrolling your mouse wheel forwards.

What do you see?

Good quality lines

Raster to vector conversion works great on good quality lines

The lines above are clean and strong and distinct. If the lines on your image look like this you will be able to get good raster to vector conversion results.

Hairy lines

Lines with small holes and crenellations should be smoothed before raster to vector conversion

If the lines on your image have “hairs” like the top line above, click (Smooth) to smooth them (bottom line above).

Dithered lines

Dithered lines must be mende before raster to vector conversion

If the lines on your image are dithered (made up of black speckles like the lines 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 using (Thicken Pixels).

You may need to use 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.

Lines with holes

Holes in lines should be filled before raster to vector conversion

If the lines on your image contain small holes you can remove them using (Remove Holes). To remove large holes use Scan2CAD’s Flood Fill command (see the Scan2CAD Help).

If your image has holes it may have been scanned at too high a resolution. You may want to try scanning your drawing again at a lower resolution.

Broken lines

Raster to vector conversion cannot convert lines that are not there

If the lines on your image are slightly broken you may be able to mend them automatically using one of or a combination of the following methods:

  • (Thicken Lines).
  • (Thicken Pixels).
  • Gap jumping. Before you vectorize the image:
    1. Select a Type from the Type Menu.
    2. Go to Type > Settings.
    3. Set a Gap Jump Distance.

    When you vectorize the image Scan2CAD will jump over breaks that are smaller than the distance you have specified, allowing it to produce continuous vectors despite the gaps.

If the lines on your image are very broken you will not be able to mend them 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. Your best bet is to rescan the drawing using our Scanning Checklist.

Small details

Raster to vector conversion cannot convert very small details

An image that has been scanned at a resolution that is optimal for most of the drawing may contain some small details that are made up of too few pixels to be sufficiently defined for raster to vector conversion.

Such details will vectorize to a mess of vectors. There is nothing you can do about this.

Dot and other non linear hatch patterns

Dot-type hatch patterns like the one in the drawing below will not vectorize well.

Raster to vector conversion cannot convert dot-type hatch patterns

It is better to replace hatches like these in your CAD program than to try to vectorize them. If your drawing contains dot-type hatch patterns, remove them by clicking (Remove Speckles).

Hatch patterns like the one below where non-linear hatch components are joined to each other and to the surrounding boundary cannot easily be removed and will vectorize to a mess of vectors.

Raster to vector conversion cannot convert messy hatch patterns

There is nothing you can do about this.

Touching parallel or concentric entities

Raster to vector conversion cannot separate touching entities Raster to vector conversion cannot separate touching entities

You will not get a good vectorization on parts of the image where parallel or concentric entities touch.

There is nothing you can do about this except rescan the drawing using our Scanning Checklist.

Merged entities

Raster to vector conversion cannot separate 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 except rescan the drawing using our Scanning Checklist.

Blurry lines

Raster to vector conversion cannot make sense of blurry lines

If the lines on your raster image are blurry as in the image above, the raster image is too poor quality for vectorization.

There is nothing you can do about this except rescan the drawing using our Scanning Checklist.

Blurriness is most common in JPEG images, so take care not to save your image in JPEG format.

Low resolution lines

Raster to vector conversion cannot convert images that are too low resolution

If shapes in your image are defined by only a few pixels and look jagged, as in the image above, your image is too low resolution. This is particularly common in logos and drawings that contain fine detail.

The drawing needs to be rescanned at a considerably higher resolution – aim for lines that are about 5 pixels thick.

Overlaid information

Raster to vector conversion cannot unscramble overlaid details Raster to vector conversion cannot unscramble overlaid details

The drawings above contain a lot of overlaid information.

Unfortunately, Scan2CAD is not human. It doesn’t know that it is looking at (for example) text and a wiring schedule overlaid on a building plan. All it sees are black patterns on a white background and it is not going to be able to unscramble the different components.

You are not going to get a sensible raster to vector conversion from an image with overlaid information. There is nothing you can do about this.

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