Originally Posted by TechniSol
Does anyone have any idea of how the Kobo fonts have variable stroke weight or how we might find or create/modify other fonts with that feature or what might be involved?
My impression is that whether advanced font settings are available or not is not in each case dependent on a special property of the font but rather is decided somehow by the software. This impression is based on the following observations:
1) If a font is embedded
in an epub it is impossible to override the font face by the font settings. You can open the font settings menu and change the font face to whatever you want the text is displayed with the embedded font. But if you select a font face that allows advanced font settings and make changes to them those advanced features get applied to the embedded font, cf. this post
2) If there are characters (e.g. from an Indian script) in an epub.kepub
that are not supported by the selected font those characters get displayed in another font that supports them provided such a font is installed. If the selected font allows advanced features then they work also on the replacement font, independent whether these features were selectable for the replacement font originally. Also the height, for instance, of the Indian characters varies according to the selected font (They become bigger with Malabar selected, smaller with Amasis).
Among the fonts that come with the Kobo only the Japanese fonts do not allow selection of advanced features. In order to test whether one can make advanced features work for Japanese text with the method described in 2) I took a kepub.epub with Japanese text, selected a non-Japanese Kobo font and changed the advanced font settings. To my disappointment, I was unable to see any change in the display, I mean concerning the advanced features. There was, however, some change. The Japanese characters became extremely small, changing of the font size did not have any effect. Even CSS font size settings for this passages got ignored if a non-Japanese font was selected. It seems understandable that monospaced fonts are resistant against advanced features.
So my guess is that every non-monospaced font is, theoretically, suitable for advanced font features, but the software decides whether they are applied and allows user selection only for Kobo fonts. As reported in another thread, it does not help to copy a font into the font folder of the system partition.