colorLib: add option to disable PaintColrLayers' reuse of layers from LayerList

When building COLR masters, layer reuse may lead to different number of layers between masters, becasue some master may reuse specific layers while others may not. Add a flag to disable this optimization; will need to be run again after the VF COLR font has been merged

Lib/fontTools/colorLib/builder.py

@@ -186,10 +186,12 @@ def populateCOLRv0(
 def buildCOLR(
     colorGlyphs: _ColorGlyphsDict,
     version: Optional[int] = None,
+    *,
     glyphMap: Optional[Mapping[str, int]] = None,
     varStore: Optional[ot.VarStore] = None,
     varIndexMap: Optional[ot.DeltaSetIndexMap] = None,
     clipBoxes: Optional[Dict[str, _ClipBoxInput]] = None,
+    allowLayerReuse: bool = True,
 ) -> C_O_L_R_.table_C_O_L_R_:
     """Build COLR table from color layers mapping.
 
@@ -231,7 +233,11 @@ def buildCOLR(
 
     populateCOLRv0(colr, colorGlyphsV0, glyphMap)
 
-    colr.LayerList, colr.BaseGlyphList = buildColrV1(colorGlyphsV1, glyphMap)
+    colr.LayerList, colr.BaseGlyphList = buildColrV1(
+        colorGlyphsV1,
+        glyphMap,
+        allowLayerReuse=allowLayerReuse,
+    )
 
     if version is None:
         version = 1 if (varStore or colorGlyphsV1) else 0
@@ -448,12 +454,14 @@ class LayerListBuilder:
     reusePool: Mapping[Tuple[Any, ...], int]
     tuples: Mapping[int, Tuple[Any, ...]]
     keepAlive: List[ot.Paint]  # we need id to remain valid
+    allowLayerReuse: bool
 
-    def __init__(self):
+    def __init__(self, *, allowLayerReuse=True):
         self.layers = []
         self.reusePool = {}
         self.tuples = {}
         self.keepAlive = []
+        self.allowLayerReuse = allowLayerReuse
 
         # We need to intercept construction of PaintColrLayers
         callbacks = _buildPaintCallbacks()
@@ -504,34 +512,35 @@ class LayerListBuilder:
         # Convert maps seqs or whatever into typed objects
         layers = [self.buildPaint(l) for l in layers]
 
-        # No reason to have a colr layers with just one entry
-        if len(layers) == 1:
-            return layers[0], {}
+        if self.allowLayerReuse:
+            # No reason to have a colr layers with just one entry
+            if len(layers) == 1:
+                return layers[0], {}
 
-        # Look for reuse, with preference to longer sequences
-        # This may make the layer list smaller
-        found_reuse = True
-        while found_reuse:
-            found_reuse = False
+            # Look for reuse, with preference to longer sequences
+            # This may make the layer list smaller
+            found_reuse = True
+            while found_reuse:
+                found_reuse = False
 
-            ranges = sorted(
-                _reuse_ranges(len(layers)),
-                key=lambda t: (t[1] - t[0], t[1], t[0]),
-                reverse=True,
-            )
-            for lbound, ubound in ranges:
-                reuse_lbound = self.reusePool.get(
-                    self._as_tuple(layers[lbound:ubound]), -1
+                ranges = sorted(
+                    _reuse_ranges(len(layers)),
+                    key=lambda t: (t[1] - t[0], t[1], t[0]),
+                    reverse=True,
                 )
-                if reuse_lbound == -1:
-                    continue
-                new_slice = ot.Paint()
-                new_slice.Format = int(ot.PaintFormat.PaintColrLayers)
-                new_slice.NumLayers = ubound - lbound
-                new_slice.FirstLayerIndex = reuse_lbound
-                layers = layers[:lbound] + [new_slice] + layers[ubound:]
-                found_reuse = True
-                break
+                for lbound, ubound in ranges:
+                    reuse_lbound = self.reusePool.get(
+                        self._as_tuple(layers[lbound:ubound]), -1
+                    )
+                    if reuse_lbound == -1:
+                        continue
+                    new_slice = ot.Paint()
+                    new_slice.Format = int(ot.PaintFormat.PaintColrLayers)
+                    new_slice.NumLayers = ubound - lbound
+                    new_slice.FirstLayerIndex = reuse_lbound
+                    layers = layers[:lbound] + [new_slice] + layers[ubound:]
+                    found_reuse = True
+                    break
 
         # The layer list is now final; if it's too big we need to tree it
         is_tree = len(layers) > MAX_PAINT_COLR_LAYER_COUNT
@@ -552,7 +561,7 @@ class LayerListBuilder:
         layers = [listToColrLayers(l) for l in layers]
 
         # No reason to have a colr layers with just one entry
-        if len(layers) == 1:
+        if self.allowLayerReuse and len(layers) == 1:
             return layers[0], {}
 
         paint = ot.Paint()
@@ -563,7 +572,7 @@ class LayerListBuilder:
 
         # Register our parts for reuse provided we aren't a tree
         # If we are a tree the leaves registered for reuse and that will suffice
-        if not is_tree:
+        if self.allowLayerReuse and not is_tree:
             for lbound, ubound in _reuse_ranges(len(layers)):
                 self.reusePool[self._as_tuple(layers[lbound:ubound])] = (
                     lbound + paint.FirstLayerIndex
@@ -603,6 +612,8 @@ def _format_glyph_errors(errors: Mapping[str, Exception]) -> str:
 def buildColrV1(
     colorGlyphs: _ColorGlyphsDict,
     glyphMap: Optional[Mapping[str, int]] = None,
+    *,
+    allowLayerReuse: bool = True,
 ) -> Tuple[Optional[ot.LayerList], ot.BaseGlyphList]:
     if glyphMap is not None:
         colorGlyphItems = sorted(
@@ -613,7 +624,7 @@ def buildColrV1(
 
     errors = {}
     baseGlyphs = []
-    layerBuilder = LayerListBuilder()
+    layerBuilder = LayerListBuilder(allowLayerReuse=allowLayerReuse)
     for baseGlyph, paint in colorGlyphItems:
         try:
             baseGlyphs.append(buildBaseGlyphPaintRecord(baseGlyph, layerBuilder, paint))

Lib/fontTools/fontBuilder.py

@@ -838,6 +838,7 @@ class FontBuilder(object):
         varStore=None,
         varIndexMap=None,
         clipBoxes=None,
+        allowLayerReuse=True,
     ):
         """Build new COLR table using color layers dictionary.
 
@@ -853,6 +854,7 @@ class FontBuilder(object):
             varStore=varStore,
             varIndexMap=varIndexMap,
             clipBoxes=clipBoxes,
+            allowLayerReuse=allowLayerReuse,
         )
 
     def setupCPAL(