[HarfBuzz] Alternate/random glyphs

[Simon Cozens]

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