Add interpolation of cvt/cvar table

Lib/fontTools/varLib/mutator.py

@@ -152,6 +152,21 @@ def main(args=None):
 			coordinates += GlyphCoordinates(delta) * scalar
 		_SetCoordinates(varfont, glyphname, coordinates)
 
+	# Interpolate cvt
+
+	if 'cvar' in varfont:
+		cvar = varfont['cvar']
+		cvt = varfont['cvt ']
+		deltas = {}
+		for var in cvar.variations:
+			scalar = supportScalar(loc, var.axes)
+			if not scalar: continue
+			for i, c in enumerate(var.coordinates):
+				if c is not None:
+					deltas[i] = deltas.get(i, 0) + scalar * c
+		for i, delta in deltas.items():
+			cvt[i] += int(round(delta))
+
 	print("Removing variable tables")
 	for tag in ('avar','cvar','fvar','gvar','HVAR','MVAR','VVAR','STAT'):
 		if tag in varfont:

Tests/varLib/data/test_results/BuildMain.ttx

@@ -128,6 +128,11 @@
     </cmap_format_4>
   </cmap>
 
+  <cvt>
+    <cv index="0" value="474"/>
+    <cv index="1" value="677"/>
+  </cvt>
+
   <loca>
     <!-- The 'loca' table will be calculated by the compiler -->
   </loca>
@@ -848,6 +853,23 @@
     </ValueRecord>
   </MVAR>
 
+  <cvar>
+    <version major="1" minor="0"/>
+    <tuple>
+      <coord axis="wght" value="-1.0"/>
+      <delta cvt="0" value="-4"/>
+    </tuple>
+    <tuple>
+      <coord axis="wght" value="1.0"/>
+      <delta cvt="0" value="13"/>
+    </tuple>
+    <tuple>
+      <coord axis="wght" value="1.0"/>
+      <coord axis="cntr" value="1.0"/>
+      <delta cvt="0" value="13"/>
+    </tuple>
+  </cvar>
+
   <fvar>
 
     <!-- Weight -->