[instancer] Start avar2 instancing

[behdad]

Just some code for now to get min/max delta of a
varIdx from a varStore...

[behdad]

Just thinking out loud, this is what I think so far:

• Say we're dealing with a HOI-style avar2. That is, there's wght and wgt2, the latter being completely driven by wght; and hidden. Both have a range of 100:400:1000; Now, if you instantiate to limit wght to, say, 400..700, we, ideally, will deduce that wgt2 can only vary from 400..700 as well, and change it's fvar to reflect that.

Because of the above, I wrote the code that taken axis limits, tells you what the min/max of each axis can be in the VarStore. Eg.:

$ time ~/fonttools/fonttools varLib.instancer.avar RobotoFlex-avar2.ttf slnt=.4 wght=500
{'slnt': (0, 0, 0, 10, 0), 'wght': (0.200012, 0.200012, 0.200012, 300, 500)}
GRAD 0.11226271092891693 (-1.44036865234375, 0.56378173828125)
slnt 0.0 (0.0, 0.0)
XOPQ 0.11226271092891693 (-1.44036865234375, 0.56378173828125)
YOPQ 0.09091131016612053 (-1.34844970703125, 0.52490234375)
XTRA -0.016883354634046555 (-1.40167236328125, 2.11279296875)
YTUC 0.0 (-1.0, 1.0)
YTLC 0.01033998653292656 (-1.7003173828125, 1.362060546875)
YTAS -0.009705182164907455 (-1.380615234375, 1.00970458984375)
YTDE 0.0 (-1.10980224609375, 1.02593994140625)
YTFI 0.0 (-1.0, 1.0)
wght 0.0 (0.0, 0.0)
wdth 0.0 (0.0, 0.0)
opsz 0.0 (0.0, 0.0)

Problem arises if the user wants to modify these hidden axes as well. Then it's not clear if we should limit their range or not :(. But I'd argue that the parametric axes in RobotoFlex and alike should be public, not private.

The code above is not complete to compute new axis extents though. Remember that we add the delta for the axis to its currently set value. So the code should take that into consideration. In which case, eg, wdth above should take (-1.0, 1.0) as its extents.

After deciding the normalized new range of each axis, we need to work them back from the avar1-segments to get new fvar values. And then build a new varStore for axes that scales the individual values based on the new axis extents.

That's my thinking.

[behdad]

@davelab6 We really need those avar2 test fonts for various use-cases...

[behdad]

cc @anthrotype

[behdad]

@justvanrossum

[behdad]

This kinda seems to work now, even though it's not complete. I need testers like @Lorp