[instancer] Setting a new default location seems to disrupt axis mapping in a "binary" axis

[arrowtype]

I have set up a “binary” on/off variable axis with axis mapping in a designspace. It seems to work well!

    <axis name="Italic" tag="ital" minimum="0" maximum="1" default="0">
      <labels>
          <label uservalue="0" name="Roman" elidable="true" />
          <label uservalue="1" name="Italic" />
      </labels>
      <map input="1" output="1"/>
      <map input="0.5" output="1"/>
      <map input="0.4999999" output="0"/>
      <map input="0" output="0"/>
    </axis>

However, if I then use the fontTools.instancer to add a new default location to this variable font, the ital axis suddenly goes back to being continuous. Here is a comparison:

Screen.Recording.2023-07-05.at.10.42.48.AM.mov

The bottom line is acting as expected, with the ital axis snapping from upright to italic at 0.5. The top line is instanced to have a default wght of 400, but its ital axis is acting as a continuous/smooth axis, which is not intended. I have replicated this result with https://wakamaifondue.com/beta/, as well.

This is especially strange because if I use TTX to look at the avar tables of each font (before/after the instancer), there is no change.

  <avar>

    <!-- (wght and opsz mapping are here) -->

    <segment axis="ital">
      <mapping from="-1.0" to="-1.0"/>
      <mapping from="0.0" to="0.0"/>
      <mapping from="0.5" to="1.0"/>
      <mapping from="1.0" to="1.0"/>
    </segment>
  </avar>

Here’s the code I’m using to set this instance:

import sys
from fontTools.ttLib import TTFont
from fontTools.varLib import instancer

# get fontpath passed in
fontpath = sys.argv[-1]

# open the font in memory
varfont = TTFont(fontpath)

# determine the min and max wght axis values
for axis in varfont["fvar"].axes:
    if axis.axisTag == "wght":
        minWght = axis.minValue
        maxWght = axis.maxValue
        oldDflt = axis.defaultValue

# create a new font with a default of 400 for the wght axis
newFont = instancer.instantiateVariableFont(varfont, {"wght": (minWght, 400, maxWght)})

## (some stuff to edit the name table)

# save the new font, with same path as input font
newFont.save(fontpath)

Version: fonttools==4.40.1.dev0. Once I file this issue, I’ll poke around and see if I can update to the latest development branch, and I’ll note if that makes a difference. Update: To confirm, I just pip installed the latest in the L4-fixes branch, and got the same result.

I’m happy to send along fonts in an email, if helpful! Let me know. Thanks!

[anthrotype]

are you sure the avar tables before/after are the same? I would expect that for a discrete change to happen the original avar that you say is working should have a mapping for <map input="0.4999999" output="0"/> as in the input DS axis, e.g.:

   <avar>

     <!-- (wght and opsz mapping are here) -->

     <segment axis="ital">
       <mapping from="-1.0" to="-1.0"/>
       <mapping from="0.0" to="0.0"/>
+      <mapping from="0.4999" to="0.0"/>
       <mapping from="0.5" to="1.0"/>
       <mapping from="1.0" to="1.0"/>
     </segment>
   </avar>

maybe that gets lost somehow in the instancing. Have you tried to use fewer decimal places? Maybe your "0.4999999" gets rounded to "0.5" as it goes through float => Fixed2Dot14.
Indeed floatToFixed(0.5, 14) and floatToFixed(0.4999999, 14) return 8192, whereas you want the second to be just one unit apart, 8191. With 14 bits for the fractional part, the granularity is I believe up to 4 decimal places, 1/(2<<13) to be precise. So I believe 0.4999 should do the trick.

[anthrotype]

0.49994 to be precise

from fontTools.misc.fixedTools import floatToFixed, fixedToStr

bits = 14
v = 0.5
fv = floatToFixed(v, bits)  # 8192
v2 = float(fixedToStr(fv - 1, bits))
print(v2)  # 0.49994

[arrowtype]

Wow, thanks for figuring out and sharing the specifics on decimal accuracy!

And actually, yes, when I make that change to my mapping attributes, the instancer works out without any issues!

I feel a bit silly... I had actually figured out that the most accurate I could get in the designspace, and have the discrete axis work in the non-instanced output, is 6 decimal places. The code I copy-pasted was from a note to myself about what I was initially doing wrong. But, it’s great to know more specifically that I should probably go to .49994.

And, after making that adjustment, things work well even after the instancer step. Perfect!

Screen.Recording.2023-07-05.at.2.12.49.PM.mov

Thanks so much for the help here! I don’t see an issue here, so please feel free to close this unless you want to take any further action.

[Lorp]

Note that there is acceptance of the need to be able to express "immediately after N" or "immediately before N", meaning the value (in normalised coordinates) 1/16384 more or less than N. According to the spec, axis values in variable fonts are rounded to the nearest 1/16384 before being used in the variation algorithm. AFAIK Samsa is the only app where you can step through a designspace in 1/16384 units.

[behdad]

In theory we can extend the designspace format to allow for .5+epsilon / .5-epsilon syntax. It's a bit ugly to implement though.