Merge pull request #482 from SixLabors/js/fix-479

Allow null AnchorTable

Author: JimBobSquarePants

src/SixLabors.Fonts/Tables/AdvancedTypographic/AdvancedTypographicUtils.cs

@@ -249,11 +249,19 @@ public static void ApplyAnchor(
         FontMetrics fontMetrics,
         GlyphPositioningCollection collection,
         int index,
-        AnchorTable baseAnchor,
+        AnchorTable? baseAnchor,
         MarkRecord markRecord,
         int baseGlyphIndex,
         Tag feature)
     {
+        // baseAnchor may be null because OpenType MarkToBase allows NULL anchor offsets
+        // in BaseArray/BaseRecord. A NULL offset means "this base glyph has no anchor
+        // for this mark class", and the lookup must be ignored for this mark–base pair.
+        if (baseAnchor is null)
+        {
+            return;
+        }
+
         GlyphShapingData baseData = collection[baseGlyphIndex];
         AnchorXY baseXY = baseAnchor.GetAnchor(fontMetrics, baseData, collection);
 

src/SixLabors.Fonts/Tables/AdvancedTypographic/GPos/LookupType4SubTable.cs

@@ -71,10 +71,10 @@ public static LookupType4Format1SubTable Load(BigEndianBinaryReader reader, long
             ushort markArrayOffset = reader.ReadOffset16();
             ushort baseArrayOffset = reader.ReadOffset16();
 
-            var markCoverage = CoverageTable.Load(reader, offset + markCoverageOffset);
-            var baseCoverage = CoverageTable.Load(reader, offset + baseCoverageOffset);
-            var markArrayTable = new MarkArrayTable(reader, offset + markArrayOffset);
-            var baseArrayTable = new BaseArrayTable(reader, offset + baseArrayOffset, markClassCount);
+            CoverageTable markCoverage = CoverageTable.Load(reader, offset + markCoverageOffset);
+            CoverageTable baseCoverage = CoverageTable.Load(reader, offset + baseCoverageOffset);
+            MarkArrayTable markArrayTable = new(reader, offset + markArrayOffset);
+            BaseArrayTable baseArrayTable = new(reader, offset + baseArrayOffset, markClassCount);
 
             return new LookupType4Format1SubTable(markCoverage, baseCoverage, markArrayTable, baseArrayTable, lookupFlags);
         }
@@ -106,7 +106,7 @@ public override bool TryUpdatePosition(
             while (--baseGlyphIndex >= 0)
             {
                 GlyphShapingData data = collection[baseGlyphIndex];
-                if (!AdvancedTypographicUtils.IsMarkGlyph(fontMetrics, data.GlyphId, data) && !(data.LigatureComponent > 0))
+                if (!AdvancedTypographicUtils.IsMarkGlyph(fontMetrics, data.GlyphId, data) && data.LigatureComponent <= 0)
                 {
                     break;
                 }