Image tracing

An image does not have any structure: it is just a collection of marks on paper, grains in film, or pixels in a bitmap.

It is not examining the image and attempting to recognize or extract a three-dimensional model that may be depicted; i.e. it is not a vision system.

And, just as with these other two operations, while rasterization is fairly straightforward and algorithmic, vectorization involves the reconstruction of lost information and therefore requires heuristic methods.

Synthetic images such as maps, cartoons, logos, clip art, and technical drawings are suitable for vectorization.

Continuous tone photographs (such as live portraits) are not good candidates for vectorization.

Programs that do raster-to-vector conversion may accept bitmap formats such as TIFF, BMP and PNG.

Common vector formats are SVG, DXF, EPS, EMF and AI.

Personal computers often come with a simple paint program that produces a bitmap output file.

A person could look at the image, make some measurements, and then write the output file by hand.

If the image is not yet in machine readable form, then it has to be scanned into a usable file format.

Once there is a machine-readable bitmap, the image can be imported into a graphics editing program (such as Adobe Illustrator, CorelDRAW, or Inkscape).

Then a person can manually trace the elements of the image using the program's editing features.

[1] Once the black and white image was in the graphics program, some other elements were added and the figure was colored.

Ploch used a bitmap editor to remove the background and crop the more complex image components.

Some of these programs have a command line interface while others are interactive that allow the user to adjust the conversion settings and view the result.

Adobe Streamline is not only an interactive program, but it also allows a user to manually edit the input bitmap and the output curves.

The program did a good job on the map boundaries (the most tedious task in the tracing) and the settings dropped out all the text (small objects).

The results depend on having high-quality scans, reasonable settings, and good algorithms.

Corel advice: Put the image on a light table, cover it with vellum (tracing paper), and then manually ink the desired outlines.

If the image is of a fill-in form, then it will probably have just vertical and horizontal lines of a constant width.

Even images that were created as black on white drawings may end up with many shades of gray.

If the original image is on paper and is scanned, there is a similar result: edge pixels will be gray.

Many of the vectorization programs will group same-color pixels into lines, curves, or outlined shapes.

[6] For continuous tone images such as photographs, the result of color quantization is posterization.

Losing the letters is not a big issue; post conversion editing would want to delete the annotation and replace it with text rather than curves.

Vectorization is effective on single colored, nongradient input data, like signatures.

The resulting vector images are at least a factor of ten larger and may have pronounced posterization effects when a small number of colors are used.

This image illustrates the difference between bitmap and vector images. The bitmap image is composed of a fixed set of pixels, while the vector image is composed of a fixed set of shapes. In the picture, scaling the bitmap reveals the pixels while scaling the vector image preserves the shapes.
Original PNG file (16 kB)
Hand converted to SVG (47 kB)
Map in PNG format (13 kB)
Map after automated conversion and touch up to re-add the labels and adjust colors (18 kB)
Although these lines may look solid, they are far from perfect. The lines were drawn on vellum with a fountain pen, a 0.7 mm gel pen, and a 0.5 mm HB pencil. The images were scanned at 600 pixels per inch with 24-bit RGB. The line widths end up being 10 to 14 pixels wide. The ink colors are not uniform and have specular reflections that put light pixels within the lines. The pencil lines also have interior defects due to the roughness (tooth) of the paper. The scan also has some unsharp masking artifacts.