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Once an entire curve (i.e., the bitmap outline between two corners; see 10.1.2 Finding corners) has been fit, Limn checks for straight lines that are adjacent to splines. Unless such lines fit the bitmap extremely well, they must be changed to splines.
The reason is that the point at which the line and spline meet will be a visible "bump" in the typeset character unless the two blend smoothly. Where two splines meet, the continuity is guaranteed from the way we constructed the splines (see section 10.1.5.2 Finding tangents). But where a line and spline meet, nothing makes the join smooth.
For example, if the outline of a `o' has been subdivided many times (as typically happens), a spline may end up fitting just a few pixels--so few that a line would fit just as well. The actions described in the previous section will therefore change the spline to a line. But since the adjacent parts of the `o' are being fit with curves, that line will result in a noticeable flat spot in the final output. So we must change it back to a spline.
We want this reversion to be more likely for short curves than long curves, since short curves are more likely to be the result of a small piece of a curved shape. So Limn divides the total distance between the fitted line and the bitmap curve by the square of the curve length, and compares the result to a threshold, .01 by default. You can change this with the `-line-reversion-threshold' option.