Coloured Characters

[Richard Wordingham via Unicode]

On Thu, 22 Feb 2018 10:55:23 +0000 (GMT)
William_J_G Overington <wjgo_10009 at btinternet.com> wrote:

> Richard Wordingham wrote:
> 
> > 'Foreground' and 'background' are the only externally defined
> > colours. There's no ability to explicitly choose, say 'text stroked
> > sable and dotted gules'.  Instead, it's 'text stroked sable and
> > dotted proper', with a choice of palettes to define 'proper'.  

> External selection of decoration colours would theroretically be
> possible, I do not know how difficult this would be to implement.

The problem lies in changing existing interfaces.  I can only speak
with any real knowledge for the OpenType COLR/CPAL method.

The change would be a major pain in programming languages with
obligatory (even if implicit) typing.  At present, foreground and
background need to be specified (if only be default) and passed into
the painting routines. You now want to expand the foreground argument
into a list of colours - or possibly a callback routine.

The next issue is what is to happen when the list provided is too
short.  Without suitable handling, this may cause problems with fonts
that already work in applications that at one interface level know
nothing about colour fonts.  For example, the HTML code that I have been
using with my font knows nothing about colour fonts as such.  To get
colour with my web page, I just select a coloured font.

The final issue that springs to mind is that the COLR table of OpenType
allows for 65,535 different colours in glyphs; 0xFFFF is the only
reserved colour ID.  It represents the foreground colour.  If there is
only one palette in the font, 0xFFFE can be a legitimate
user-defined colour ID.  I wouldn't be surprised if such an
assignment survived the transition from a proof-of-principle font to
a released font.

A less painful method for interfaces might be the selection of
palettes by name.  However, there are rather more possible colour
combinations than can be accommodated in an sfnt name table, so an
approximation algorithm would be required.  It would also make the CPAL
tables larger and much more difficult to generate.  There are also 30
unassigned bits left in the palette's type attribute.

Of course, Unicode is not constrained by what is currently available,
and as an entity is interested at most in what is feasible rather than
the precise mechanisms.  Several full members, though, will care about
precise mechanisms.

Richard.