diff --git a/src/OT/Layout/GSUB/SingleSubstFormat1.hh b/src/OT/Layout/GSUB/SingleSubstFormat1.hh
index 5b54fdb07..850be86c0 100644
--- a/src/OT/Layout/GSUB/SingleSubstFormat1.hh
+++ b/src/OT/Layout/GSUB/SingleSubstFormat1.hh
@@ -95,6 +95,34 @@ struct SingleSubstFormat1_3
   bool would_apply (hb_would_apply_context_t *c) const
   { return c->len == 1 && (this+coverage).get_coverage (c->glyphs[0]) != NOT_COVERED; }
 
+  unsigned
+  get_glyph_alternates (hb_codepoint_t  glyph_id,
+                        unsigned        start_offset,
+                        unsigned       *alternate_count  /* IN/OUT.  May be NULL. */,
+                        hb_codepoint_t *alternate_glyphs /* OUT.     May be NULL. */) const
+  {
+    unsigned int index = (this+coverage).get_coverage (glyph_id);
+    if (likely (index == NOT_COVERED))
+    {
+      if (alternate_count)
+        *alternate_count = 0;
+      return 0;
+    }
+
+    if (alternate_count && *alternate_count)
+    {
+      hb_codepoint_t d = deltaGlyphID;
+      hb_codepoint_t mask = get_mask ();
+
+      glyph_id = (glyph_id + d) & mask;
+
+      *alternate_glyphs = glyph_id;
+      *alternate_count = 1;
+    }
+
+    return 1;
+  }
+
   bool apply (hb_ot_apply_context_t *c) const
   {
     TRACE_APPLY (this);
diff --git a/src/OT/Layout/GSUB/SingleSubstFormat2.hh b/src/OT/Layout/GSUB/SingleSubstFormat2.hh
index 17aa08736..9c651abe7 100644
--- a/src/OT/Layout/GSUB/SingleSubstFormat2.hh
+++ b/src/OT/Layout/GSUB/SingleSubstFormat2.hh
@@ -75,6 +75,31 @@ struct SingleSubstFormat2_4
   bool would_apply (hb_would_apply_context_t *c) const
   { return c->len == 1 && (this+coverage).get_coverage (c->glyphs[0]) != NOT_COVERED; }
 
+  unsigned
+  get_glyph_alternates (hb_codepoint_t  glyph_id,
+                        unsigned        start_offset,
+                        unsigned       *alternate_count  /* IN/OUT.  May be NULL. */,
+                        hb_codepoint_t *alternate_glyphs /* OUT.     May be NULL. */) const
+  {
+    unsigned int index = (this+coverage).get_coverage (glyph_id);
+    if (likely (index == NOT_COVERED))
+    {
+      if (alternate_count)
+        *alternate_count = 0;
+      return 0;
+    }
+
+    if (alternate_count && *alternate_count)
+    {
+      glyph_id = substitute[index];
+
+      *alternate_glyphs = glyph_id;
+      *alternate_count = 1;
+    }
+
+    return 1;
+  }
+
   bool apply (hb_ot_apply_context_t *c) const
   {
     TRACE_APPLY (this);