Adjust letterspacing for a portion of the text

Is it possible to adjust the letterspacing of the text for a specific portion of the text?

I do not think so. What is the purpose?

It may be desirable in some cases to adjust the letterspacing (or tracking) of text when the defaults are not correct. One common example is strings of capitals or small-caps. It’s always recommended that strings of capitals are letterspaced 5% or 10% wider than normal text (see for example R. Bringhurst “Elements of Typographic Style”).

All software allows for this, even Ms Word, so I think it should be added in TeXmacs.

I remember this mentioned in Mogan’s TODOs for CJK @darcy correct me if not

There must be some low level macro or command to do this, right? What controls letterspacing in TeXmacs normally?

I’m adjusting the spacing between the CJK characters and the English words.

I’ve checked the source code but there seems not to be any tunable parameter to modify the default letter spacing (which is a font-related parameter). TeXmacs does some tuning, but seems only related to other kind of spaces. See e.g. src/Graphics/Fonts/font_spacing.cpp . I guess would not be difficult to add, but there is not right now. I agree that we maybe can add an environment parameter to control a multiplier to the spacing. How LaTeX implement this?

In LaTeX this is done with \textls{} which I believe is part of the microtype package.

You are correct, letterspacing is part of the font informaton (along with kerning), but TeXmacs must deal with it during the line-breaking process. I.e. slightly adjust the space between letters to optimize the line-breaking processes. In fact, the threshold to which this is allowed is adjustable in Format > Paragraph > Advanced or Document > Paragraph > Advanced.

So it should be relatively easy to add the feature to manually adjust letterspacing.