static fonts from single-master sources have overlaps, skewing glyf ttx diff

[anthrotype]

Following #971, ttx_diff.py now invokes fontmake -o ttf instead of -o variable when the input source contains only one master, in order to avoid fontmake adding unnecessary empty/noop variable tables (googlefonts/fontmake#1125).

But fontmake -o ttf also passes the source through the ufo2ft's remove overlaps filter (using booleanOperations by default, or skia-pathops if requested via --overlaps-backend pathops).

fontc being variable-first font compile does not attempt nor support removing the overlaps (as it would destroy master compatibility for a VF target).

Therefore comparing fontc-built single-master sources vs fontmake-built ones will usually produce several glyph diffs in glyph-related tables like glyf, maxp, etc. because the number of outline points when glyphs contain overlapping contours will be different between the two fonts.

For example, take this test font, containing only one glyph "T" with overlapping contours, and compare it with ttx_diff.py:

SingleMaster.glyphs.zip

--- build/default/fontc.ttf
+++ build/default/fontmake.ttf
@@ -12,12 +12,12 @@
     <!-- Most of this table will be recalculated by the compiler -->
     <tableVersion value="1.0"/>
     <fontRevision value="1.0"/>
-    <checkSumAdjustment value="0x0"/>
+    <checkSumAdjustment value="0xd88e9083"/>
     <magicNumber value="0x5f0f3cf5"/>
     <flags value="00000000 00000011"/>
     <unitsPerEm value="1000"/>
     <created value="Mon Sep 30 16:38:24 2024"/>
-    <modified value="Tue Oct  1 10:17:24 2024"/>
+    <modified value="Tue Oct  1 10:17:25 2024"/>
     <xMin value="26"/>
     <yMin value="-200"/>
     <xMax value="574"/>
@@ -53,7 +53,7 @@
     <!-- Most of this table will be recalculated by the compiler -->
     <tableVersion value="0x10000"/>
     <numGlyphs value="3"/>
-    <maxPoints value="10"/>
+    <maxPoints value="8"/>
     <maxContours value="2"/>
     <maxCompositePoints value="0"/>
     <maxCompositeContours value="0"/>
@@ -169,12 +169,10 @@
         <pt x="26" y="723" on="1"/>
         <pt x="574" y="723" on="1"/>
         <pt x="574" y="578" on="1"/>
-        <pt x="363" y="578" on="1"/>
-        <pt x="408" y="623" on="1"/>
+        <pt x="408" y="578" on="1"/>
         <pt x="408" y="0" on="1"/>
         <pt x="209" y="0" on="1"/>
-        <pt x="209" y="623" on="1"/>
-        <pt x="254" y="578" on="1"/>
+        <pt x="209" y="578" on="1"/>
         <pt x="26" y="578" on="1"/>
       </contour>
       <instructions/>

[jenskutilek]

Why not add --keep-overlaps to the fontmake call? It won't do any harm on the variable font build.

[anthrotype]

of couse we can do that for the time being, until we have remove overlaps in rust, but the comparison would not be fair/realistic, since most static builds do/should have overlaps removed.

[anthrotype]

after applying #979, ttx_diff'ing the above SingleMaster.glyphs test file yields "output is identical", though it feels a bit like cheating to me :)

I defer to @rsheeter

[rsheeter]

@raphlinus just noted https://github.com/GraphiteEditor/Graphite/tree/master/libraries/path-bool is now a thing, seems to be a port of a js pathops library. This one is new to me.