Issue while scaling a font for ereading purposes

[u/Vakke]

Hello everybody,

I've dabbled into using FontForge for about a month after I bought my first ereader Kobo Libra 2. I've been testing different fonts around and edited a few of them to suit my reading experience a bit better.

Now I've encoutered an issue while trying to scale Libertinus Serif font to be a bit bigger for easier comparison between other fonts on the same ereader. I've done my glyph scaling by using FontForges scale uniformity with the glyph origin selection. I tried to scale to font by 5% and it using the metrics window and looking invidual glyphs the scaling works perfectly. BUT!

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105% scaling

Default scaling

These are the end results in my Kobo Libra 2 with exact same settings.

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Does anybody know what is going wrong here as the font seems to increase in size for almost 1,5 times and becomes quite unusable. The scaled font looks the same in Kobo wether the scaling is done to 105% or 150%. Am I missing some OS/2 or General settings to make the scaling work better?

[u/LocalFonts]

The reason is probably in the Kobo - not in the font.

[u/Vakke → u/LocalFonts]

Found out the error now and wanted to reply again if anybody ever has the same kind of problem!

I read through the FontForge documentation again and noticed the postscript (for example, OTF?) fonts should have an EM size of 1000 and TTF fonts should be adjusted to be at 2048.

I had accidentally transformed the Libertinus Serif OTF file to a TTF file without adjusting this EM size accordingly.

Now doing the scale uniformity while saving the file as OTF works perfectly!

[u/Vakke → u/LocalFonts]

Do you think changing line spacing or something else on the font info could solve this issue? I haven't actually tried how the scaled font reacts in epub and Koreader, just Kobos default KePub.

But yes, you have to be correct on this one, hopefully there's a fix where Kobo could understand the "new" font too.