10.1.1 Finding pixel outlines

The first step in the conversion from a character shape represented as a bitmap to a list of mathematical curves is to find all the cyclical outlines (i.e., closed curves) in the bitmap image. The resulting list is called a pixel outline list. Each pixel outline in the list consists of the pixel coordinates of each edge on the outline.

For example, the pixel outline list for an `i' has two elements: one for the dot, and one for the stem. The pixel outline list for an `o' also has two elements: one for the outside of the shape, and one for the inside.

But we must differentiate between an outside outline (whose interior is to be filled with black to render the character) and an inside outline (whose interior is to be filled with white). Limn's convention is to write the pixel coordinates for outside outlines in counterclockwise order, and those for inside outlines in clockwise order.

This counterclockwise movement of outside outlines is required by the Type 1 format used for PostScript fonts, which is why we adopted that convention for Limn.

For example, consider a pixel outline consisting of a single black pixel at the origin. The pixel has four corners, and hence the outline will have four coordinates. Limn looks for starting pixels from top to bottom, left to right, within a bitmap image. Thus, the list of pixel coordinates will start at (0,1) and proceed counterclockwise: (0,0) (1,0) (1,1). Here is a picture:

start => (0,1)<-(1,1) | ^ v | (0,0)->(0,1)

Because finding pixel outlines does not involve approximation or estimation, there are no options to control the process. Put another way, Limn will always find the correct pixel coordinates for each outline.

Once these pixel outlines have been found, each is then processed independently; i.e., all the remaining steps, described in the following sections, operate on each pixel outline individually.

The source code for this is in `limn/pxl-outline.c' and `lib/edge.c'.