text: change line ascent and descent to fit font size

text/gotext.go

@@ -895,11 +895,18 @@ func toLine(faceToIndex map[font.Font]int, o shaping.Line, dir system.TextDirect
 		}
 		line.runeCount += run.Runes.Count
 		line.width += run.Advance
-		if line.ascent < run.LineBounds.Ascent {
-			line.ascent = run.LineBounds.Ascent
+
+		// fix ascent to make all glyphes (CJK or not) always in same baseline
+		// fixedDescent + fixedAscent should equal fontSize
+		scale := fixedToFloat(run.Size) / fixedToFloat(run.LineBounds.LineHeight())
+		fixedAscent := floatToFixed(fixedToFloat(run.LineBounds.Ascent) * scale)
+		fixedDescent := floatToFixed(fixedToFloat(-run.LineBounds.Descent+run.LineBounds.Gap) * scale)
+
+		if line.ascent < fixedAscent {
+			line.ascent = fixedAscent
 		}
-		if line.descent < -run.LineBounds.Descent+run.LineBounds.Gap {
-			line.descent = -run.LineBounds.Descent + run.LineBounds.Gap
+		if line.descent < fixedDescent {
+			line.descent = fixedDescent
 		}
 	}
 	line.lineHeight = maxSize

text/gotext_test.go

@@ -52,14 +52,44 @@ func TestEmptyString(t *testing.T) {
 		t.Fatalf("Layout returned no lines for empty string; expected 1")
 	}
 	l := lines.lines[0]
-	if expected := fixed.Int26_6(12094); l.ascent != expected {
+	if expected := fixed.Int26_6(10463); l.ascent != expected {
 		t.Errorf("unexpected ascent for empty string: %v, expected %v", l.ascent, expected)
 	}
-	if expected := fixed.Int26_6(2700); l.descent != expected {
+	if expected := fixed.Int26_6(2336); l.descent != expected {
 		t.Errorf("unexpected descent for empty string: %v, expected %v", l.descent, expected)
 	}
 }
 
+func TestCJKOrNotString(t *testing.T) {
+	ppem := fixed.I(200)
+	ltrFace, _ := opentype.Parse(goregular.TTF)
+	shaper := testShaper(ltrFace)
+
+	for _, r := range [][]rune{
+		[]rune("Sa"),
+		[]rune("SayHi"),
+		[]rune("SayHi 您好"),
+		[]rune("✨ⷽℎ↞⋇ⱜ⪫⢡⽛⣦␆Ⱨⳏ⳯⒛⭣╎⌞⟻⢇┃➡⬎⩱⸇ⷎ⟅▤⼶⇺⩳⎏⤬⬞ⴈ⋠⿶⢒₍☟⽂ⶦ⫰⭢⌹∼▀⾯⧂❽⩏ⓖ⟅⤔⍇␋⽓ₑ⢳⠑❂⊪⢘⽨⃯▴ⷿ"),
+	} {
+		t.Run("ascent/descent should fit font size", func(t *testing.T) {
+			lines := shaper.LayoutRunes(Parameters{
+				PxPerEm:  ppem,
+				MaxWidth: 2000,
+				Locale:   english,
+			}, r)
+
+			l := lines.lines[0]
+
+			if expected := fixed.Int26_6(10463); l.ascent != expected {
+				t.Errorf("unexpected ascent for string: %v, expected %v", l.ascent, expected)
+			}
+			if expected := fixed.Int26_6(2336); l.descent != expected {
+				t.Errorf("unexpected descent for empty string: %v, expected %v", l.descent, expected)
+			}
+		})
+	}
+}
+
 func TestNoFaces(t *testing.T) {
 	ppem := fixed.I(200)
 	shaper := testShaper()