I've gone ahead and pushed this.
I will try to open an issue for a regtest soon, when I've given some thought
how to formulate an issue. I'm in a little doubt, however, over what needs to
be tested. There are many dashed lines in the regtests already, and a
comparison of changes shows a general improvement (for example, the horizontal
line of ottavas connecting with the vertical stroke at the end, not always the
case before, and the number of dashes drawn matching the number calculated).
Also, LilyPond's use of Postscript to draw these lines seems straightforward,
so my example further above is simply verifying something about PS itself with
which LilyPond doesn't tamper.
---
** [issues:#5086] Fix dashed line errors**
**Status:** Fixed
**Labels:** Fixed_2_19_57
**Created:** Tue Mar 07, 2017 04:45 AM UTC by David Nalesnik
**Last Updated:** Tue Mar 14, 2017 10:35 PM UTC
**Owner:** David Nalesnik
Issue 5086: Fix dashed line errors
There are two errors in lily/line-interface.cc which cause
dashed lines to be drawn inaccurately.
(1) Line-thickness is improperly subtracted from the "off"
value in Line_interface::make_dashed_line. This was done,
presumably, to account for the line cap of each dash. In fact,
PostScript discounts the line cap when constructing the pattern.
(2) Dashed lines are constructed so that they begin and end
with a dash. Thus, a dashed line is built of dash + whitespace
units and a last lone dash. Period is not adjusted in
Line_interface::line for this last dash, which can lead to a
noticeable difference in its length compared to other dashes.
Correcting these flaws ensures that dashed lines end with a
dash rather than whitespace, and that terminating dashes are
not clipped. Also, the number of dashes drawn reflects the
number calculated.
http://codereview.appspot.com/320320043
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