[HarfBuzz] Alternate/random glyphs

[Aleš Mlakar]

Thanks guys, I've tried to turn off the rand feature (which is on by
default in pango it seems) and it really did start working differently.
The example image that I've sent earlier now works identically!
My actual text doesn't seem to work 100% correctly yet, but there's some
(per glyph) open type features there (ss01 etc) so I have to triple check
it that it gets through correctly.

Thanks for your help!

Best regards,
Ales



On Fri, Feb 14, 2020 at 6:21 PM Jonathan Kew <jfkthame at gmail.com> wrote:

> On 14/02/2020 15:50, Aleš Mlakar wrote:
> > Hey Simon,
> > I think it doesn't use rand feature, because it never gets to that part
> > of the code or at least not to the random_number() function which is the
> > only rng function I could find in Harfbuzz :D.
>
> HarfBuzz will only use this if the font has a 'rand' feature with an
> AlternateSubstFormat1 subtable, to pick from among the available
> alternates for a glyph. You could try dumping the font's GSUB table
> (e.g. with fonttools) to see what's actually in there.
>
> It's entirely possible for a font's 'rand' feature to be implemented in
> some other way, such as a collection of contextual substitutions; then
> no real (pseudo-)randomness is involved.
>
> JK
>
> > I did some debugging and it goes into some coverage functions inside
> > hb-ot-layout-common.hh from hb-ot-layout-gsub-table.hh, it's too deep
> > for me to be able to understand it though.
> > So, as far as I can understand all of this either one of Indesign or
> > Harfbuzz is doing it wrong.
> >
> > Best,
> > Ales
> >
> >
> > On Fri, Feb 14, 2020 at 1:04 PM Simon Cozens <simon at simon-cozens.org
> > <mailto:simon at simon-cozens.org>> wrote:
> >
> >     On 12/02/2020 19:28, Aleš Mlakar wrote:
> >      > I did a quick debug through that part of HarfBuzz and it seems
> it's
> >      > doing lookups and never gets to the random code.
> >
> >     OpenType randomization on the whole isn't *really* random. Most fonts
> >     implement pseudo-random selection of alternate glyphs by going
> >     through a
> >     series of lookup rules (see e.g. the bottom of
> >     http://opentypecookbook.com/common-techniques.html ). As you can
> see,
> >     none of the techniques there actually use any source of randomness;
> >     they
> >     just apply deterministic rules to the input stream.
> >
> >     There *is* a "rand" feature in the OpenType standard, which asks the
> >     shaper for non-deterministic random selection, but for this to work,
> >     two
> >     things need to be in place: the shaper needs to support it, and the
> >     font
> >     needs to define it. Harfbuzz as a shaper supports it, but no others
> do.
> >     Because of this, few fonts define it.
> >
> >     Even then, Harfbuzz's random number implementation is only
> pseudorandom
> >     with a fixed random seed, so you get the same results each time
> anyway.
> >
> >     I don't know whether Daft Brush uses the "rand" feature. It probably
> >     doesn't. Although if you are seeing differences between harfbuzz and
> >     InDesign, it might be that it does.
> >
> >     In short it is down to the font.
> >
> >     S
> >
> >
> >
> > --
> > Aleš Mlakar,
> > Programmer/Consultant
> > *am.bits*
> >
> > _______________________________________________
> > HarfBuzz mailing list
> > HarfBuzz at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/harfbuzz
> >
>
>

-- 
Aleš Mlakar,
Programmer/Consultant
*am.bits*
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