Validation for open type

Eluard's picture

When saving a slightly edited font in Font Forge I ran the validation check and got a string of errors (not caused by me — they were in glyphs I hadn't touched). Almost all of them were of the kind "extremal points missing". Are these errors that I should endeavor to correct? Are they serious in any way?



dan_reynolds's picture

Yes, you should always draw your outlines with points on the extrema. It is good habit, and it makes curves much easier to tweak and edit later one (by you or by someone else… if you are using FontForge, you'd probably be OK with other people editing your fonts later on down the line…)

Also, it can play a role in hinting and on-screen rendering. No points at extrema = bad… unless you have a convincing design reason not to put them in a few particular spots… there are always exceptions to rules. But in general, it is good to do.

Eluard's picture

Thanks dan, that helps.

John Hudson's picture

Certain very small curve segments, e.g. at the tips of serifs, are sometimes left without extrema points, but otherwise you should always use them. As Dan notes, they are important for hinting, and also for calculating glyph bounding boxes accurately.

blank's picture

Let me make sure that I understand this correctly: if for some reason I draw a curve and the points are not all in extrema, will tossing in an extra extrema point improve screen rendering?

Eluard's picture

I should just clarify — even if it makes no difference to the good sense of the answers here — that the font that failed the validation test was a font that was purchased from Linotype. It's not one of my making, but I wondered if I should try to fix it if it's seriously broken.

DTY's picture

Points at extrema are useful in general, for the reasons Dan gave. There are some situations where they aren't always so useful, particularly in scripts and italics, e.g.:

Thomas Phinney's picture

James: assuming decent auto-hinting, and a renderer that is using hinting of some sort, yes. Of course, once you've added points at extrema, you may be able to delete some other points.

But as noted, some extrema are more important than others.


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