[MPEG-OTSPEC] 'opbd' feature

[John Hudson]

On 16092020 3:30 pm, Peter Constable wrote:
>
> “In recommended usage, the application applies this feature 
> automatically to characters at the right end of a horizontal line when 
> the 'opbd' feature has been applied to those characters. See 
> the the ['opbd'](#opbd) feature for additional information.”
>
> To allow for usage of ‘lfbd’ or ‘rtbd’ independent of ‘opbd’, I’ve 
> have to revise that, but that would be doable.
>
Note that an actual implementation would throw up a number of 
interesting practical issues. The lfbd and rtbd features should be 
applied, respectively, when text is left-aligned or right-aligned. That 
would imply not only margin alignment, but also column alignment, and 
even tab alignment. While margin and column widths may reasonably be 
assumed to be wide enough to merit text boundary alignment, tab 
distances may be close, and it may be necessary for software to use some 
kind of tolerance when determining whether to apply a boundary alignment 
feature.

If text is justified (i.e. both left and right boundaries are aligned to 
margin, column edges, etc..) the output of the lfbd and rtbd features 
needs to be taken into account for linebreaking and other justification 
functions such as wordspacing tolerances or kashida insertion.


Because of the amount of work involved in defining lfbd and rtbd GPOS 
lookups (and the current lack of any very convenient visual editing 
tool), I don't imagine it being something a lot of font makers would do 
for a lot of fonts (although that may be different if automated (paging 
Mr Cozens)). I can imagine it being most desirable in some display fonts 
with unusual glyph shapes, extending left and right swashes intended for 
line initial and final use, and similar things that don't submit well to 
algorithmic text boundary alignment.

JH

-- 

John Hudson
Tiro Typeworks Ltd    www.tiro.com
Salish Sea, BC        tiro at tiro.com

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