[NTG-context] Re: OpenType contextual alternates disabled by default

[Hans Hagen via ntg-context <ntg-context@ntg.nl>]

On 9/9/2025 9:34 AM, Max Chernoff via ntg-context wrote:
> Hi Hans,
> 
> OpenType contextual alternates ("calt") seem to be disabled by default
> in ConTeXt. However, the OpenType spec recommends that this is enabled
> by default
> 
>      https://learn.microsoft.com/en-us/typography/opentype/spec/features_ae#tag-calt
> 
> and it's enabled by default in both HTML and HarfBuzz
> 
>      https://drafts.csswg.org/css-fonts/#default-features
> 
>      https://harfbuzz.github.io/shaping-opentype-features.html
> 
> Changing this default in ConTeXt probably isn't possible due to
> backwards compatibility concerns, so this is more of an FYI than a
> request for any changes to be made.
> 
> Background: this came up in a recent TeX.SE question
> 
>      https://tex.stackexchange.com/a/750767
> 
> where a new user was surprised by ConTeXt's default setting since it
> makes some fonts render differently than other programs.

We started with opentype support (2005) when there wasn't much info 
about it. The reference implementation was uniscribe so we had to guess 
and compare with that. The xetex engine was using some libraries and 
changed them later on. So we couldn't check that out either.

Now, when it comes to features, those experiences, also with specific 
fonts, made us decide for the defaults. Over time the specification 
became better and more complete but as you say: we want to remain 
compatibility. (I assume that ltx folk spend substantial time on 
checking changes in hb every time there is an update in tex live, which 
is quite often if i look at git syncing.)

An omission in otf is that the font doesn't specify what features are to 
be on by default. Combine that with font usage where users have no 
control over features, and we see why soem are just turned on. Probably 
the same for some (fixing) heuristics. It's now how tex works: there we 
*ask* for something, not being *enforced* to.

If I could motivate myself we could of course make such a list of 
default features per font.

Of course one can wonder why we have rlig/lig/clig as well as kern/dist 
split into several features when they are supposed to be always on. But 
I will not waste time on pondering that.

Hans

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