Users sometimes send us photos of objects they’d like to vectorize the outline of using our Scan2CAD raster to vector conversion software.

Unfortunately, using raster to vector software to vectorize the outline of an object on a photo is not as simple as it sounds. One reason for this is that before you can vectorize the object outline you need to divide the photo into two discrete color areas: The object and the background. This is usually hard to do automatically because the object and the background tend to share colors that make it difficult or impossible to separate them.

If you just take any old snap of an object, like the one of the Nice Niece below, you won’t be able to separate the object from the background. In this instance it is quicker to simply trace around the outline you want manually.

There is too much commonality between the colors in the object and the background. It is impossible to separate them.

The trick is to photograph the object against a background which is a different color to the color of the object.

I got someone to hold a sheet of black card behind the Nice Niece while I took a photo. Now, the background and the Niece are largely different colors and it is possible to separate them using Scan2CAD’s thresholding or color palette tools.

The object and the background are different colors.		They can be separated into two discrete color areas …
… and vectorized.

If you are drawing an outline that you want to convert to CAD using raster to vector conversion, it needs to be drawn as a single, strong, unambiguous line.

Below left is a typical hand-drawn sketch. While you could scan and manually trace over this sketch in a raster to vector conversion program such as Scan2CAD using the program’s manual drawing tools, the sketch is not suitable for automatic raster to vector conversion for two reasons.

First, there are ambiguous lines where the artist has re-sketched parts of the drawing several times. An example is at the top of the left hand leaf (arrowed in red). Which line should the raster to vector converter follow?

Second, the sketch has been drawn in soft pencil using different pressures. This means that although parts of the sketch will come out black in the scan, others will be faint and broken (arrowed in blue).

Above right is the same sketch with uniformly strong and unambiguous lines. This is what it needs to look like if it is to be converted to CAD using automatic raster to vector conversion.

One of the images I was emailed this week to do a sample raster to vector conversion on with Scan2CAD was the outline of a spindle, some parts of which were very broken up.

If you tried to do a raster to vector conversion on this broken outline, the vectors would not continue smoothly over the breaks.

While it is possible to vectorize over small breaks, it is not possible to do so over large breaks where the original shape is represented by a few remnants.

One way to resolve the issue would have been to ask the sender to try rescanning the original drawing using a lower threshold. However, because the vast majority of the outline comprised good, solid, unbroken lines, I decided to fix it by tracing over the broken parts.

This is very easy to do using Scan2CAD’s raster Bezier Curve Draw Tool, as follows:

1. Select the Raster Edit, Draw and Bezier Curve Icons.
   
    
   

2. Select a pen size by going to Raster Edit Menu > Pen Size.
3. Make a series of left mouse button clicks along the broken line.
   
    
   

4. Click with your right mouse button to end. A raster bezier curve is drawn over the broken part of the image, mending it.
   
    
   

   (Note that you can use the Bezier Curve Draw Tool for tracing over broken lines and arcs as well as for tracing over broken curves.)

   Once the breaks are mended you will be able to achieve a smooth, unbroken raster to vector conversion.