Potrace: a polygon-based tracing algorithm

Black-on-white images can be represented either as a bitmap or as a vector outline. A bitmap represents an image as a grid of black or white pixels. A vector outline describes an image via an algebraic description of its contours, typically in the form of Bezier curves. The advantage of representing an image as a vector outline is that it can be scaled to any size without loss of quality. Outline images are independent of the resolution of any particular output device. They are particularly popular in the description of fonts, which must be reproducible at many different sizes. Examples of outline font formats include PostScript Type 1 fonts, TrueType, and Metafont. On the other hand, most actual input and output devices, such as scanners, displays, and printers, ultimately produce or consume bitmaps. The process of converting a vector outline to a bitmap is called rendering. The converse process of turning bitmaps into outlines is called tracing. It is clear that no tracing algorithm can be perfect in an absolute sense, as there are in general many possible outlines that can give rise to the same bitmap. The process of tracing cannot be used to generate information that is not already present. On the other hand, out of the many possible outlines that could give rise to a given bitmap, clearly some are more plausible or aesthetically pleasing than others. For example, a common way of rendering bitmaps at a high resolution is to draw each black pixel as a precise square, which gives rise to “jaggies” or staircase patterns. Clearly, jaggies are neither pleasant to look at, nor are they particularly plausible interpretations of the original image. There is probably no absolute measure of what constitutes a good tracing algorithm, but it seems clear that some algorithms give better results than others. In this paper, we describe a tracing algorithm which is simple, efficient, and tends to produce excellent results. The algorithm is called potrace, which stands for polygon tracer. However, the output of the algorithm is not a polygon, but a smooth contour made from Bezier curves. The name of the algorithm derives from the fact that it uses polygons as an intermediate representation of images. The potrace algorithm is designed to work well on high resolution images. Thus, a typical application is to produce a vector outline from a company or university logo that has been scanned at a high resolution. Another possible application is the conversion of bitmapped fonts to outline fonts, if the original bitmapped fonts are available at a high enough resolution. No tracing algorithm will work well on very small scales, such as