Potrace Library API

For Potrace, a bitmap of size w × h is embedded in a cartesian coordinate system where each pixel takes up the space of one unit square. The pixels are positioned so that the corners of pixels (and not their centers) lie at points with integer coordinates, as illustrated in Figure 1. The origin of the coordinate system is at the lower left corner of the bitmap. The four corners of the bitmaps have coordinates (0, 0), (0, h), (w, h), and (w, 0). Sometimes we need to refer to a specific pixel (as opposed to a point in the plane). When we speak of “pixel [i, j]”, we mean the pixel whose corners have coordinates (i, j), (i, j + 1), (i + 1, j + 1), (i + 1, j) in Potrace’s coordinate system. Thus, pixel [i, j] is the pixel whose center is at coordinates (i + 0.5, j + 0.5). To avoid confusion, we use square brackets to refer to the pixel [i, j], and round brackets to refer to the point (i, j).