Why is 4 the magical Path Minimum Length?

Dave Bailey

 According to Font Audit (FL5) and Red Arrow Plug-In for Glyphs http://glyphsextensions.com/redarrow/ a path of length <4 is flagged as an error. I know fonts have been produced with smaller stem widths than 4, so what's the deal? Is it a legacy issue?

Chris Lozos

at what UPM?

Dave Bailey

1000 UPM

Craig Eliason

Bear in mind these programs are signaling potential mistakes to check, not necessarily errors in the technical sense. E.g. there's nothing invalid about a slightly-off-vertical line.
I'd guess it's just an estimate of what length would be more likely an unwanted accident rather than an intentional shape. At what length would you want the warning to show up? 

Dave Bailey

Thanks for the explanation, Craig, I was thinking about the scenario in a technical issue frame of mind instead of using those arrows as a way of spotting potential mistakes in the drawings.

Thomas Phinney

Craig is exactly right.

SOME FontAudit checks are outright errors. Some are things that would likely be an accident or error in most fonts, but are not invalid things to do and could be deliberate—such as a line that is one off of being perfectly vertical, or a path that is 2 units long.

Dave Bailey

@Thomas Phinney
As a follow up question, which of the FontAudit items are outright errors? I now understand about 'Short Paths' and 'Slightly skewed horizontal or vertical paths'

Thomas Phinney

@Dave Bailey

Well, this is a matter of debate, and really there is a continuum rather than "outright error" vs "minor issues"

I would say that outside of a variable font the following are errors (and in a variable font there would have to be a reason based on what they transform to). They result in added data without affecting rendering:
- Empty lines and curves
- Flat curves

These are outright errors that are likely to cause rendering problems or not render at all:
- Loop
- Contour is not closed *

These are errors that may cause problems depending on output format, etc.
- vectors on ClosePaths

Most everything else is flagging likely errors in design, but not errors that will cause complete havoc or data that literally can't do anything. Yes, there are lots of things that will cause minor rendering issues.

* Open contours that appear correctly filled using the paint bucket are OK, FontLab will correct the contour automatically.

I'm super jet lagged right now, so odds of error in the above are substantial. Probably missed something.

Dave Bailey

Thanks for the further explanation, Thomas!