otTables: make DeltaSetIndexMap TTX dump less verbose by omitting no-op entries

though we still emit the empty <Map index=.../> elements; they help identify chunks of delta-set indices, and define the length of the array upon loading from XML

Lib/fontTools/ttLib/tables/otTables.py

@@ -600,6 +600,11 @@ class Coverage(FormatSwitchingBaseTable):
 		glyphs.append(attrs["value"])
 
 
+# The special 0xFFFFFFFF delta-set index is used to indicate that there
+# is no variation data in the ItemVariationStore for a given variable field
+NO_VARIATION_INDEX = 0xFFFFFFFF
+
+
 class DeltaSetIndexMap(getFormatSwitchingBaseTableClass("uint8")):
 
 	def populateDefaults(self, propagator=None):
@@ -647,12 +652,19 @@ class DeltaSetIndexMap(getFormatSwitchingBaseTableClass("uint8")):
 		return rawTable
 
 	def toXML2(self, xmlWriter, font):
+		# Make xml dump less verbose, by omitting no-op entries like:
+		#   <Map index="..." outer="65535" inner="65535"/>
+		xmlWriter.comment(
+			"Omitted values default to 0xFFFF/0xFFFF (no variations)"
+		)
+		xmlWriter.newline()
 		for i, value in enumerate(getattr(self, "mapping", [])):
-			attrs = (
+			attrs = [('index', i)]
-				('index', i),
+			if value != NO_VARIATION_INDEX:
+				attrs.extend([
 					('outer', value >> 16),
 					('inner', value & 0xFFFF),
-			)
+				])
 			xmlWriter.simpletag("Map", attrs)
 			xmlWriter.newline()
 
@@ -661,8 +673,8 @@ class DeltaSetIndexMap(getFormatSwitchingBaseTableClass("uint8")):
 		if mapping is None:
 			self.mapping = mapping = []
 		index = safeEval(attrs['index'])
-		outer = safeEval(attrs['outer'])
+		outer = safeEval(attrs.get('outer', '0xFFFF'))
-		inner = safeEval(attrs['inner'])
+		inner = safeEval(attrs.get('inner', '0xFFFF'))
 		assert inner <= 0xFFFF
 		mapping.insert(index, (outer << 16) | inner)
 

Tests/ttLib/tables/C_O_L_R_test.py

@@ -532,17 +532,18 @@ COLR_V1_XML = [
 
 COLR_V1_VAR_XML = [
     '<VarIndexMap Format="0">',
-    '  <Map index="0" outer="1" inner="1"/>',
+    '  <!-- Omitted values default to 0xFFFF/0xFFFF (no variations) -->',
-    '  <Map index="1" outer="1" inner="0"/>',
+    '  <Map index="0" outer="1" inner="0"/>',
-    '  <Map index="2" outer="1" inner="0"/>',
+    '  <Map index="1"/>',
-    '  <Map index="3" outer="1" inner="1"/>',
+    '  <Map index="2"/>',
-    '  <Map index="4" outer="1" inner="0"/>',
+    '  <Map index="3" outer="1" inner="0"/>',
-    '  <Map index="5" outer="1" inner="0"/>',
+    '  <Map index="4"/>',
+    '  <Map index="5"/>',
     '  <Map index="6" outer="0" inner="2"/>',
     '  <Map index="7" outer="0" inner="0"/>',
     '  <Map index="8" outer="0" inner="1"/>',
-    '  <Map index="9" outer="1" inner="0"/>',
+    '  <Map index="9"/>',
-    '  <Map index="10" outer="1" inner="0"/>',
+    '  <Map index="10"/>',
     '  <Map index="11" outer="0" inner="3"/>',
     '  <Map index="12" outer="0" inner="3"/>',
     "</VarIndexMap>",
@@ -571,12 +572,11 @@ COLR_V1_VAR_XML = [
     '    <Item index="3" value="[500]"/>',
     "  </VarData>",
     '  <VarData index="1">',
-    "    <!-- ItemCount=2 -->",
+    "    <!-- ItemCount=1 -->",
     '    <NumShorts value="32769"/>',
     "    <!-- VarRegionCount=1 -->",
     '    <VarRegionIndex index="0" value="0"/>',
-    '    <Item index="0" value="[0]"/>',
+    '    <Item index="0" value="[65536]"/>',
-    '    <Item index="1" value="[65536]"/>',
     "  </VarData>",
     "</VarStore>",
 ]