Tracing images (from bitmap to vector)

Since vector files are much more flexible at the time of editing the elements of an image, an illustration is often prepared as vectorial objects, even if the original reference is a bitmap image. For example, a vector in the shape of circle is constructed over a circular shape; starting from an irregular contour, a filled shape which reproduces this same contour is created. This process is called vectorial tracing or vectorizing / vectorisation. In order to trace this way, the resulting vector shape must fit, as perfectly as possible, the shape of the bitmap with a combination of straight and curved shapes.

This process can be carried out manually , with the tools of the illustration program, especially the freehand pen. This task is, needless to say, tedious. As a compensation, the designer is deciding all the time the exact position of each anchor point, and the precision can be the best possible. Alternatively, and very recommendable for drawings of a certain complexity, we can perform an automatic tracing or autotracing: this can be done either within the illustration program, or using some auxiliary program.

Automatic tracing

The most trouble-free way of tracing a bitmap is the one which programs such as Xara, Freehand, StarDraw, Inkscape, Illustrator CS2 or Flash allow: within the same program an option exists to trace which recognizes the shapes and places them as newly created vector objects in the workspace.

Other programs, like Illustrator until its version 11 (CS) only include a very basic system of vectorising, which recognizes shapes one by one. The solution in these cases is to use an auxiliary application, as Corel Trace (included in the CorelDraw package) or Adobe Streamline. This last one must be purchased separately, and it has finally been incorporated (and perfectioned) in the latest Illustrator version, CS2. In order to transfer the vector created by Streamline or CorelTrace to the illustration program, it is necessary to save it in a suitable format (normally like AI — EPS), or sometimes you can use the copy and paste function (via Windows clipboard.)

Autotrace and POTrace are open-source programs with a similar functionality, which can be used to convert bitmap artwork to vector files in the same way. Inkscape, the recent open-source vector illustration program, includes an excellent autotracer based on POTrace.

If you wish, it is also possible to create vector shapes within a bitmap based application such as Photoshop. You only need to use the Paths palette and tools. A path can be manually drawn with the path drawing tools, similar to the ones in Illustrator; you can also automatically create a path from a selection, a mask or a channel. The paths can be saved within any Photoshop native file or you can export them as Adobe Illustrator files. If you include a path with a Photoshop file or with a PDF or EPS exported from Photoshop, it may work as a clipping path.

Tracing systems.

Two main methods of vector tracing exist: the method of centered line and the method of outline (although these names may vary depending on the program you use.) The results in both case are sensibly different. In the outline method lines are drawn following the contours of the drawing and it creates a full series of black forms, white or in colour, without external line. This process is useful for images in colour, with areas covered by black (or colour) and for characters. For most illustrations, then, this is the method of choice.

The centered line method practically works in an opposite way. It generates empty shapes, with no fill, and it only creates straight or curved lines. It is used almost exclusively for line only drawings, like linear maps, technical drawings... In addition, this modality is also suitable to create images to which some kind of “natural” brush can be applied.

Two examples of a special effect tracing. The vectorised drawing consists of a series of lines in different angles, which resemble a crosshatched ink drawing or an etching.

There are additional tracing methods, alternative possibilities to vectorise with a combination of techniques, for example creating filled shapes together with lines and open vectorial curves. Some programs have special, alternative options in the tracing system. These methods also may have their interest. For example, they can combine both previously described systems, for drawings that alternate great areas of flat colour with line only parts.

You may find some special functions useful in certain cases, like the wood engraving tracing in CorelTrace, which produces shapes of variable thickness, reminiscing of the linoleum prints, and the sketch function, which creates a crosshatching similar to an etching or pen drawing. Illustrator incorporates an automatic tracing much like the sketch option in CorelTrace, not surprisingly called crosshatch. This sketch / crosshatch filter is well worth exploring, since in just a moment we obtain a fresh “ink drawing” or “etching” from any original. The images enclosed here might serve as an example.