[mutator] Add instantiateVariableFont public function

Towards addressing https://github.com/fonttools/fonttools/issues/1070

The new instantiateVariableFont function takes a variable TTFont and a
dictionary of axes locations and returns a static TTFont instance.

The current code modifies the varfont in-place. To adapt it for
generating multiple instances from the same varfont, for now I simply
resorted to making a copy (can be optionally disabled by setting
inplace=True).

Also, replaced print() with log.info().

Lib/fontTools/varLib/mutator.py

@@ -10,6 +10,10 @@ from fontTools.ttLib.tables._g_l_y_f import GlyphCoordinates
 from fontTools.varLib import _GetCoordinates, _SetCoordinates
 from fontTools.varLib.models import supportScalar, normalizeLocation
 import os.path
+import logging
+
+
+log = logging.getLogger("fontTools.varlib.mutator")
 
 
 def _iup_segment(coords, rc1, rd1, rc2, rd2):
@@ -99,33 +103,30 @@ def _iup_delta(delta, coords, ends):
 	return out
 
 
-def main(args=None):
+def instantiateVariableFont(varfont, location, inplace=False):
+	""" Generate a static instance from a variable TTFont and a dictionary
+	defining the desired location along the variable font's axes.
+	The location values must be specified as user-space coordinates, e.g.:
 
-	if args is None:
+		{'wght': 400, 'wdth': 100}
-		import sys
-		args = sys.argv[1:]
 
-	varfilename = args[0]
+	By default, a new TTFont object is returned. If ``inplace`` is True, the
-	locargs = args[1:]
+	input varfont is modified and reduced to a static font.
-	outfile = os.path.splitext(varfilename)[0] + '-instance.ttf'
+	"""
-
+	if not inplace:
-	loc = {}
+		# make a copy to leave input varfont unmodified
-	for arg in locargs:
+		stream = BytesIO()
-		tag,val = arg.split('=')
+		varfont.save(stream)
-		assert len(tag) <= 4
+		stream.seek(0)
-		loc[tag.ljust(4)] = float(val)
+		varfont = TTFont(stream)
-	print("Location:", loc)
-
-	print("Loading variable font")
-	varfont = TTFont(varfilename)
 
 	fvar = varfont['fvar']
 	axes = {a.axisTag:(a.minValue,a.defaultValue,a.maxValue) for a in fvar.axes}
 	# TODO Apply avar
 	# TODO Round to F2Dot14?
-	loc = normalizeLocation(loc, axes)
+	loc = normalizeLocation(location, axes)
 	# Location is normalized now
-	print("Normalized location:", loc)
+	log.info("Normalized location: %s", loc)
 
 	gvar = varfont['gvar']
 	glyf = varfont['glyf']
@@ -167,12 +168,41 @@ def main(args=None):
 		for i, delta in deltas.items():
 			cvt[i] += int(round(delta))
 
-	print("Removing variable tables")
+	log.info("Removing variable tables")
 	for tag in ('avar','cvar','fvar','gvar','HVAR','MVAR','VVAR','STAT'):
 		if tag in varfont:
 			del varfont[tag]
 
-	print("Saving instance font", outfile)
+	return varfont
+
+
+def main(args=None):
+	from fontTools import configLogger
+
+	if args is None:
+		import sys
+		args = sys.argv[1:]
+
+	varfilename = args[0]
+	locargs = args[1:]
+	outfile = os.path.splitext(varfilename)[0] + '-instance.ttf'
+
+	# TODO Allow to specify logging verbosity as command line option
+	configLogger(level=logging.INFO)
+
+	loc = {}
+	for arg in locargs:
+		tag,val = arg.split('=')
+		assert len(tag) <= 4
+		loc[tag.ljust(4)] = float(val)
+	log.info("Location: %s", loc)
+
+	log.info("Loading variable font")
+	varfont = TTFont(varfilename)
+
+	instantiateVariableFont(varfont, loc, inplace=True)
+
+	log.info("Saving instance font %s", outfile)
 	varfont.save(outfile)