CFF light hinter not doing horizontal hinting?

One day I decided to swap my system font with an OTF version and I 
noticed that it's noticably blurrier than the TTF version. So I tested 
both versions with ftdiff, and the results are attached.

The font being used here is Inter 3.15, which can be downloaded from 
[1]. The TTF variant is created from the same source as the OTF version 
and then processed with ttfautohint (which should give similar outputs 
as FreeType) [2].

The difference between the light auto-hinted outputs is that the TTF 
version gets it horizontal hinting (to be clear, I mean gridfitting 
things like horizontal lines), but the OTF version does not, giving a 
blurrier look overall. This is easy to notice in glyphs like 't' and 
'F'. I wonder if this is a bug since horizontal hinting is what the 
light hinter is exactly supposed to do.

The normal auto-hinted outputs looks fine in both cases, having hinting 
in both direction.

It also seems that the light-hinted version and unhinted version differs 
in vertical stems and glyph widths, although I don't get why it needs to 
happen.


One thing might be that the CFF fonts has no hinting instructions at 
all, so the hinter doesn't try to do its job. But as most of the OTF 
fonts I've seen doesn't provide that, I'd say that that behavior would 
be broken.

I've confirmed the same issue with another font (which uses a similar 
build process from Google Fonts), Inconsolata.


Tatsuyuki


[1] https://github.com/rsms/inter/releases/download/v3.15/Inter-3.15.zip

[2] https://github.com/rsms/inter/blob/master/Makefile