Add a face.Metrics method, and parse hhea ascent and descent.

A recent change added a Metrics method to the font.Face interface.

example/truetype/main.go

@@ -77,4 +77,12 @@ func main() {
 	printGlyph(g)
 	i1 := f.Index(c1)
 	fmt.Printf("\n'%c', '%c' Kern:%d\n", c0, c1, f.Kern(fupe, i0, i1))
+
+	fmt.Printf("\nThe numbers above are in FUnits.\n" +
+		"The numbers below are in 26.6 fixed point pixels, at 12pt and 72dpi.\n\n")
+	a := truetype.NewFace(f, &truetype.Options{
+		Size: 12,
+		DPI:  72,
+	})
+	fmt.Printf("%#v\n", a.Metrics())
 }

truetype/face.go

@@ -7,6 +7,7 @@ package truetype
 
 import (
 	"image"
+	"math"
 
 	"github.com/golang/freetype/raster"
 	"golang.org/x/image/font"
@@ -249,6 +250,17 @@ func (a *face) index(r rune) Index {
 // Close satisfies the font.Face interface.
 func (a *face) Close() error { return nil }
 
+// Metrics satisfies the font.Face interface.
+func (a *face) Metrics() font.Metrics {
+	scale := float64(a.scale)
+	fupe := float64(a.f.FUnitsPerEm())
+	return font.Metrics{
+		Height:  a.scale,
+		Ascent:  fixed.Int26_6(math.Ceil(scale * float64(+a.f.ascent) / fupe)),
+		Descent: fixed.Int26_6(math.Ceil(scale * float64(-a.f.descent) / fupe)),
+	}
+}
+
 // Kern satisfies the font.Face interface.
 func (a *face) Kern(r0, r1 rune) fixed.Int26_6 {
 	i0 := a.index(r0)

truetype/truetype.go

@@ -184,7 +184,9 @@ type Font struct {
 	locaOffsetFormat        int
 	nGlyph, nHMetric, nKern int
 	fUnitsPerEm             int32
-	bounds                  fixed.Rectangle26_6
+	ascent                  int32               // In FUnits.
+	descent                 int32               // In FUnits; typically negative.
+	bounds                  fixed.Rectangle26_6 // In FUnits.
 	// Values from the maxp section.
 	maxTwilightPoints, maxStorage, maxFunctionDefs, maxStackElements uint16
 }
@@ -289,6 +291,8 @@ func (f *Font) parseHhea() error {
 	if len(f.hhea) != 36 {
 		return FormatError(fmt.Sprintf("bad hhea length: %d", len(f.hhea)))
 	}
+	f.ascent = int32(int16(u16(f.hhea, 4)))
+	f.descent = int32(int16(u16(f.hhea, 6)))
 	f.nHMetric = int(u16(f.hhea, 34))
 	if 4*f.nHMetric+2*(f.nGlyph-f.nHMetric) != len(f.hmtx) {
 		return FormatError(fmt.Sprintf("bad hmtx length: %d", len(f.hmtx)))