Glyph tracing

While bitmap fonts consist of 8-byte glyphs that themselves consist of 1-bit encoded pixels, TrueType fonts work with outlines. To convert a bitmap font to TrueType, a glyph needs to be traced to obtain a set of closed paths that represent the outlines of the glyph.

Suppose we have a glyph b'\xff\x81\xbd\xa5\xa5\xbd\x81\xff', that looks as follows.

########
#      #
# #### #
# #  # #
# #  # #
# #### #
#      #
########

The Tracer class can be used to convert this bitmap into a list of closed paths.

from retrofont.trace import Tracer

tracer = Tracer()
tracer.load(b'\xff\x81\xbd\xa5\xa5\xbd\x81\xff')
tracer.trace()
paths = tracer.get_paths()

This produces a list of four paths, one for the outline of the large square, one for the outline of the small square, one for the inner contours of the small square and one for the inner contours of the large square.

print(paths[1])
print(paths[2])

Here we see the paths of the small square. The first path is the outline, the second one is the inner contour (hole).

[(2, 2), (2, 6), (6, 6), (6, 2)]
[(5, 3), (5, 5), (3, 5), (3, 3)]

Notice that the outline runs clockwise, while the inner contour runs counter clockwise. This is how outlines are distinguished from holes.