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Kerning in Metrics Window is Incorrectly Displayed #3400

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zachbogart opened this Issue Jan 9, 2019 · 6 comments

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zachbogart commented Jan 9, 2019

I am getting an odd behavior with displaying kerning between characters in the metrics view. If I kern by the negative of the letter width, the characters are not overlapping.

For example, here is a lowercase m:
font_m
I have the bearings set to zero on both sides so all of the spacing is from the kerning.
It is ~1100 in width, so I expect kerning a m/m pair with -1100 to overlap the two letters. But instead it overshoots:
overlap_m
Interestingly, the overlap can be reproduced if the kerning is set to 5/6ths of the letter width (or about -916):
overlap_cheating_m

Also, this behavior only occurs in the metrics window. For example, it does not occur in the kerning pair closeup dialogue.

I'm wondering why this is happening. Thanks for your help.

Zach

@zachbogart zachbogart closed this Jan 9, 2019

@zachbogart zachbogart reopened this Jan 9, 2019

@ctrlcctrlv

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ctrlcctrlv commented Jan 10, 2019

What happens in the generated font? (Try also comparing it with Windows-compatible kern checked to unchecked.)

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zachbogart commented Jan 10, 2019

The generated font shows them touching, not overlapping. My main confusion is the metric view is showing different visual kerning results compared with the kerning class page or kerning pair closeup page. Not even with a weird font.

Here's an example of it just using blocks I know are 100 by 100. If I kern them by 100, there is clearly too much space between them. Any ideas of the culprit?
screen shot 2019-01-09 at 9 08 22 pm

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ctrlcctrlv commented Jan 10, 2019

Like I said, what happens in the generated font? I need to know that to help you. Please post SFDs also.

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zachbogart commented Jan 10, 2019

The generated font seems to follow the intended kerning where it can (the dollar sign example boxes are perfectly spaced). The overlapping m doesn't seem to work and instead is generated without kerning.

See attached (I'm still trying to get the hang of FontForge, so I'm just trying to understand it with a toy example):
font_testing.sfd.zip

Again, it is a visual problem in the metrics window. The kerning seems to render fine, but I can't test out the font in the metrics pane without getting misleading results...

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ctrlcctrlv commented Jan 12, 2019

The behavior you describe does not happen in my build of FontForge.
mmm

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zachbogart commented Jan 12, 2019

Why is the width of the m above zero? In my example the four numbers are [1199, 50, 50, -1100].

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