10.1.4 Filtering curves

After generating the final pixel coordinates for each curve (see the previous sections), Limn next filters the curves to smooth them. Before this step, all the coordinates are on integer boundaries, which makes the curves rather bumpy and difficult to fit well.

To filter a point p, Limn does the following:

1. Computes the sum of the distances of n neighbors (points before and after p) to p. These neighbors are always taken from the original curve, since we don't want a newly filtered point to affect subsequent points as we continue along the curve; that leads to strange results.

2. Multiplies that sum by a weight, and adds the result to p. The weight is one-third by default; you can change this with the `-filter-percent' option, which takes an integer between zero and 100.

Repeatedly filtering a curve leads to even more smoothing, at the expense of fidelity to the original. By default, Limn filters each curve 4 times; you can change this with the `-filter-iterations' option.

If the curve has less than five points, filtering is omitted altogether, since such a short curve tends to collapse down to a single point.

The most important filtering parameter is the number n of surrounding points which are used to produce the new point. Limn has two different possibilities for this, to keep features from disappearing in the original curve. Let's call these possibilities n and alt_n; typically alt_n is smaller than n. Limn computes the total distance along the curve both coming into and going out of the point p for both n and alt_n surrounding points. Then it computes the angles between the in and out vectors for both. If those two angles differ by more than some threshold (10 degrees by default; you can change it with the `-filter-epsilon' option), then Limn uses alt_n to compute the new point; otherwise, it uses n.

Geometrically, this means that if using n points would result in a much different new point than using alt_n, use the latter, smaller number, thus (hopefully) distorting the curve less.

Limn uses 2 for n and 1 for alt_n by default. You can use the options `-filter-surround' and `-filter-alternative-surround' to change them. If the resolution of the input font is not 300dpi, you should scale them proportionately. (For a 1200dpi font, we've had good results with `-filter-surround=12' and `filter-alternative-surround= 6'.)