COLRVariationMerger: implement reuse of VarIndexBase between tables with same varIdxes

When multiple variable tables refer to the same delta-sets they can now share the same VarIndexBase so the resulting DeltaSetIndexMap is a bit smaller.
For simplicity, we only reuse VarIndexBase when variable tables fully share (ie. same, and same number of) varIdxes; potentially we could reuse subsets of varIdxes (e.g. a VarColoStop.Alpha has a +0.5 delta, and later on elsewhere a PaintVarSolid.Alpha has a similar +0.5 delta; the latter could have a VarIndexBase that reuses an existing DeltaSetIndexMap entry for the former), but for now this I think is good enough.

Lib/fontTools/varLib/__init__.py

@@ -723,7 +723,6 @@ def _add_COLR(font, model, master_fonts, axisTags, colr_layer_reuse=True):
 		mapping = store.optimize()
 		colr.VarStore = store
 		varIdxes = [mapping[v] for v in merger.varIdxes]
-		# TODO: Optimize reusable runs of delta-set indices from multiple paints
 		colr.VarIndexMap = builder.buildDeltaSetIndexMap(varIdxes)
 
 	# rebuild LayerList to optimize PaintColrLayers layer reuse

Lib/fontTools/varLib/merger.py

@@ -4,6 +4,7 @@ Merge OpenType Layout tables (GDEF / GPOS / GSUB).
 import os
 import copy
 import enum
+import itertools
 from operator import ior
 import logging
 from fontTools.misc import classifyTools
@@ -1122,7 +1123,11 @@ class COLRVariationMerger(VariationMerger):
 
 	def __init__(self, model, axisTags, font):
 		VariationMerger.__init__(self, model, axisTags, font)
-		self.varIdxes = []
+		# maps {tuple(varIdxes): VarIndexBase} to facilitate reuse of VarIndexBase
+		# between variable tables with same varIdxes.
+		self.varIndexCache = {}
+		# total number of varIdxes (i.e. sum(len(vs) for vs in self.varIndexCache))
+		self.varIndexCount = 0
 		# set of id()s of the subtables that contain variations after merging
 		# and need to be upgraded to the associated VarType.
 		self.varTableIds = set()
@@ -1130,6 +1135,18 @@ class COLRVariationMerger(VariationMerger):
 	def mergeTables(self, font, master_ttfs, tableTags=("COLR",)):
 		VariationMerger.mergeTables(self, font, master_ttfs, tableTags)
 
+	@property
+	def varIdxes(self):
+		"""Return flat list of all the varIdxes generated while merging.
+
+		To be called after mergeTables() for building a DeltaSetIndexMap.
+		"""
+		# dict remembers insertion order (as of py37+), and we extend the cache
+		# of VarIndexBases incrementally as new, unique tuples of varIdxes are found
+		# while merging, thus we don't need to sort but we just unpack the keys and
+		# chain the tuples
+		return [v for v in itertools.chain(*self.varIndexCache.keys())]
+
 	def checkFormatEnum(self, out, lst, validate=lambda _: True):
 		fmt = out.Format
 		formatEnum = out.formatEnum
@@ -1209,10 +1226,14 @@ class COLRVariationMerger(VariationMerger):
 			baseValue, varIdx = self.storeMastersForAttr(out, lst, attr)
 			setattr(out, attr, baseValue)
 			varIdxes.append(varIdx)
+		varIdxes = tuple(varIdxes)
 
 		if any(v != ot.NO_VARIATION_INDEX for v in varIdxes):
-			varIndexBase = len(self.varIdxes)
+			# try to reuse an existing VarIndexBase for the same varIdxes, or else
-			self.varIdxes.extend(varIdxes)
+			# create a new one at the end of self.varIndexCache (py37+ dicts are ordered)
+			varIndexBase = self.varIndexCache.setdefault(varIdxes, self.varIndexCount)
+			if varIndexBase == self.varIndexCount:
+				self.varIndexCount += len(varIdxes)
 
 		return varIndexBase
 

Tests/varLib/merger_test.py

@@ -422,6 +422,65 @@ class COLRVariationMergerTest:
                 [NO_VARIATION_INDEX, NO_VARIATION_INDEX, 0, NO_VARIATION_INDEX],
                 id="sweep_grad-startAngle",
             ),
+            pytest.param(
+                [
+                    {
+                        "Format": int(ot.PaintFormat.PaintSweepGradient),
+                        "ColorLine": {
+                            "Extend": int(ot.ExtendMode.PAD),
+                            "ColorStop": [
+                                {"StopOffset": 0.0, "PaletteIndex": 0, "Alpha": 1.0},
+                                {"StopOffset": 1.0, "PaletteIndex": 1, "Alpha": 1.0},
+                            ],
+                        },
+                        "centerX": 0,
+                        "centerY": 0,
+                        "startAngle": 0.0,
+                        "endAngle": 180.0,
+                    },
+                    {
+                        "Format": int(ot.PaintFormat.PaintSweepGradient),
+                        "ColorLine": {
+                            "Extend": int(ot.ExtendMode.PAD),
+                            "ColorStop": [
+                                {"StopOffset": 0.0, "PaletteIndex": 0, "Alpha": 0.5},
+                                {"StopOffset": 1.0, "PaletteIndex": 1, "Alpha": 0.5},
+                            ],
+                        },
+                        "centerX": 0,
+                        "centerY": 0,
+                        "startAngle": 0.0,
+                        "endAngle": 180.0,
+                    },
+                ],
+                [
+                    '<Paint Format="9"><!-- PaintVarSweepGradient -->',
+                    "  <ColorLine>",
+                    '    <Extend value="pad"/>',
+                    "    <!-- StopCount=2 -->",
+                    '    <ColorStop index="0">',
+                    '      <StopOffset value="0.0"/>',
+                    '      <PaletteIndex value="0"/>',
+                    '      <Alpha value="1.0"/>',
+                    '      <VarIndexBase value="0"/>',
+                    "    </ColorStop>",
+                    '    <ColorStop index="1">',
+                    '      <StopOffset value="1.0"/>',
+                    '      <PaletteIndex value="1"/>',
+                    '      <Alpha value="1.0"/>',
+                    '      <VarIndexBase value="0"/>',
+                    "    </ColorStop>",
+                    "  </ColorLine>",
+                    '  <centerX value="0"/>',
+                    '  <centerY value="0"/>',
+                    '  <startAngle value="0.0"/>',
+                    '  <endAngle value="180.0"/>',
+                    '  <VarIndexBase/>',
+                    "</Paint>",
+                ],
+                [NO_VARIATION_INDEX, 0],
+                id="sweep_grad-stops-alpha-reuse-varidxbase",
+            ),
             pytest.param(
                 [
                     {