[HarfBuzz] Alternate/random glyphs

[Jonathan Kew]

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*
> 
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