Low resolution raster images are one of the most frequent causes of poor raster to vector conversion results.
As stated in a previous post, raster images are made up of little squares called pixels.
The number of pixels per inch of a raster image is called its resolution. Resolution is measured in dpi (dots per inch). An image with a resolution of 300 dpi has 300 pixels along each inch. An image with a resolution of 600 dpi has 600 pixels along each inch, etc.
The term dots per inch comes from printer terminology – printers print dots. Because raster images are made up of square pixels rather than round dots, a more accurate term would be ppi, or pixels per inch. However, although the term ppi does exist, no-one uses it.
The resolution of a raster image is very important for raster to vector conversion because it determines how well the shapes on the image are defined. The better the shapes are defined, the better the raster to vector conversion will be.
For example, consider the curve below.
On a raster image, the curve must be represented by square pixels.
The images below show the pixel representation of the curve at lower and lower resolutions.
At the higher resolutions, the pixels are small and are able to represent the shape of the curve well. If you were to vectorize these images, you would get a vector curve.
As the resolution gets lower and lower, the pixels become too large to be able to represent the curve and no longer form a curve shape. If you were to vectorize these images, you would not get a vector curve, because there is no curve shape on the image.
If you want to do a raster to vector conversion, you must make sure that the resolution of the raster image you want to vectorize is high enough to adequately represent the shapes on your drawing. (But not too high – that’s a subject for another post!)
In my next posts I’ll be looking at what resolution is right for raster to vector conversion, how to tell if your image is too low resolution for raster to vector conversion and other resolution related issues.
{ 2 comments… read them below or add one }
I also find that for some drawings too high a resolution can be a problem, if you scan a printed sheet at 400 dpi it can be resolved as many small unconnected dots, this causes problems with vector conversions (and file size) leaving some thick lines represented as a mess of small lines in all random directions, there are two ways around this, one to scan at 300 dpi, which seems to give a better more solid result on our Xerox 510 scanner, or secondly to remember to use the thicken commands in scan2cad before vectorising, its also worth noting that for thick raster lines you can tweak the line width detection settings to make a line come out as a single vector line rather than a pair of parallel lines reflecting each side of the thick raster line.
Another clear, concise and wonderful explanation of this topic. Gratefully, L.