[cff] properly skip blend arguments when no variations are set up by …

…the user (#123)

opentype/api/font/cff/charstring.go

@@ -186,13 +186,11 @@ type cff2CharstringHandler struct {
 
 func (cff2CharstringHandler) Context() ps.Context { return ps.Type2Charstring }
 
-// returns true if variations are activated
-func (met *cff2CharstringHandler) isVar() bool {
-	return len(met.coords) != 0 && len(met.vars.ItemVariationDatas) != 0
-}
-
 func (met *cff2CharstringHandler) setVSIndex(index int) error {
-	if !met.isVar() {
+	// if the font has variations, always build the scalar
+	// slice, even if no variations are activated by the user:
+	// the blend operator needs to know how many args to skip.
+	if len(met.vars.ItemVariationDatas) == 0 {
 		return nil
 	}
 
@@ -221,7 +219,8 @@ func (met *cff2CharstringHandler) blend(state *ps.Machine) error {
 		return errors.New("missing arguments for blend operator")
 	}
 
-	if met.isVar() {
+	// actually apply the deltas only if the user has activated variations
+	if len(met.coords) != 0 {
 		args := state.ArgStack.Vals[state.ArgStack.Top-n*(k+1) : state.ArgStack.Top]
 		// the first n values are the 'default' arguments
 		for i := int32(0); i < n; i++ {

opentype/api/font/cff/parser_test.go

@@ -136,3 +136,22 @@ func TestParseCFF2(t *testing.T) {
 		tu.AssertNoErr(t, err)
 	}
 }
+
+func TestIssue122(t *testing.T) {
+	b, err := td.Files.ReadFile("common/NotoSansCJKjp-VF.otf")
+	tu.AssertNoErr(t, err)
+
+	ft, err := loader.NewLoader(bytes.NewReader(b))
+	tu.AssertNoErr(t, err)
+
+	table, err := ft.RawTable(loader.MustNewTag("CFF2"))
+	tu.AssertNoErr(t, err)
+
+	out, err := ParseCFF2(table)
+	tu.AssertNoErr(t, err)
+
+	// check that the correct number of segments are
+	// computed, even if the user has not activated variations
+	segments, _, _ := out.LoadGlyph(38, nil)
+	tu.Assert(t, len(segments) == 12)
+}