Raster Quality Checklist for raster to vector conversion

Nowhere is the saying Garbage In, Garbage Out truer than in raster to vector conversion!

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

A raster to vector converter 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.

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, your raster to vector converter will probably contain a deskew function for straightening it out. However, 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 probably be able to clean it. Most raster to vector converters contain erase tools and automatic despeckle tools.

There are often dirty areas around the edges of raster images. Most raster to vector converters include a crop tool that will allow you to remove the dirty edges.

Now, zoom into your raster image ... 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, assuming you are using a capable raster to vector converter.

Dithered lines

Dithered lines must be mended 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 (see How to scan a drawing for raster to vector conversion ).

If rescanning is not an option, your raster to vector converter may include tools for mending the dithered lines.

Lines with holes

Holes in lines should be filled before raster to vector conversion

If the lines on your image contain holes your raster to vector converter will almost certainly include an automatic tool for removing them.

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

Most raster to vector converters include technology called "gap jumping". This means that when they vectorize your image they are able to "jump" over small breaks automatically.

However if the lines on your image are very broken you will not be able to 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. See How to scan a drawing for raster to vector conversion.

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, it may be best to remove them using your converter's despeckle command.

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. See How to scan a drawing for raster to vector conversion.

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. See How to scan a drawing for raster to vector conversion.

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. See How to scan a drawing for raster to vector conversion.

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.

For more information on resolution, see the articles about resolution listed here.

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, raster to vector converters are not human. They do not know that they are looking at (for example) text and a wiring schedule overlaid on a building plan. All they see are black patterns on a white background and they are 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.