font/opentype: implement font.Face interface

This CL adds the type Face that implements the font.Face interface.
This CL also adds tests using gofont/goregular as an input font, using
github.com/golang/freetype/truetype as reference values.

Updates golang/go#22451.

Change-Id: I2a6945309331b251ec2ddec95b6e809ad10aa116
Reviewed-on: https://go-review.googlesource.com/73870
Reviewed-by: Nigel Tao <nigeltao@golang.org>

font/opentype/face.go

@@ -0,0 +1,103 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package opentype
+
+import (
+	"image"
+
+	"golang.org/x/image/font"
+	"golang.org/x/image/font/sfnt"
+	"golang.org/x/image/math/fixed"
+)
+
+// FaceOptions describes the possible options given to NewFace when
+// creating a new font.Face from a sfnt.Font.
+type FaceOptions struct {
+	Size    float64      // Size is the font size in points
+	DPI     float64      // DPI is the dots per inch resolution
+	Hinting font.Hinting // Hinting selects how to quantize a vector font's glyph nodes
+}
+
+func defaultFaceOptions() *FaceOptions {
+	return &FaceOptions{
+		Size:    12,
+		DPI:     72,
+		Hinting: font.HintingNone,
+	}
+}
+
+// Face implements the font.Face interface for sfnt.Font values.
+type Face struct {
+	f       *sfnt.Font
+	hinting font.Hinting
+	scale   fixed.Int26_6
+
+	buf sfnt.Buffer
+}
+
+// NewFace returns a new font.Face for the given sfnt.Font.
+// if opts is nil, sensible defaults will be used.
+func NewFace(f *sfnt.Font, opts *FaceOptions) (font.Face, error) {
+	if opts == nil {
+		opts = defaultFaceOptions()
+	}
+	face := &Face{
+		f:       f,
+		hinting: opts.Hinting,
+		scale:   fixed.Int26_6(0.5 + (opts.Size * opts.DPI * 64 / 72)),
+	}
+	return face, nil
+}
+
+// Close satisfies the font.Face interface.
+func (f *Face) Close() error {
+	return nil
+}
+
+// Metrics satisfies the font.Face interface.
+func (f *Face) Metrics() font.Metrics {
+	m, err := f.f.Metrics(&f.buf, f.scale, f.hinting)
+	if err != nil {
+		return font.Metrics{}
+	}
+	return m
+}
+
+// Kern satisfies the font.Face interface.
+func (f *Face) Kern(r0, r1 rune) fixed.Int26_6 {
+	x0 := f.index(r0)
+	x1 := f.index(r1)
+	k, err := f.f.Kern(&f.buf, x0, x1, fixed.Int26_6(f.f.UnitsPerEm()), f.hinting)
+	if err != nil {
+		return 0
+	}
+	return k
+}
+
+// Glyph satisfies the font.Face interface.
+func (f *Face) Glyph(dot fixed.Point26_6, r rune) (dr image.Rectangle, mask image.Image, maskp image.Point, advance fixed.Int26_6, ok bool) {
+	panic("not implemented")
+}
+
+// GlyphBounds satisfies the font.Face interface.
+func (f *Face) GlyphBounds(r rune) (bounds fixed.Rectangle26_6, advance fixed.Int26_6, ok bool) {
+	advance, ok = f.GlyphAdvance(r)
+	if !ok {
+		return bounds, advance, ok
+	}
+	panic("not implemented")
+}
+
+// GlyphAdvance satisfies the font.Face interface.
+func (f *Face) GlyphAdvance(r rune) (advance fixed.Int26_6, ok bool) {
+	idx := f.index(r)
+	advance, err := f.f.GlyphAdvance(&f.buf, idx, f.scale, f.hinting)
+	return advance, err == nil
+}
+
+func (f *Face) index(r rune) sfnt.GlyphIndex {
+	x, _ := f.f.GlyphIndex(&f.buf, r)
+	return x
+}

font/opentype/face_test.go

@@ -0,0 +1,90 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package opentype
+
+import (
+	"testing"
+
+	"golang.org/x/image/font"
+	"golang.org/x/image/font/gofont/goregular"
+	"golang.org/x/image/font/sfnt"
+	"golang.org/x/image/math/fixed"
+)
+
+var (
+	regular font.Face
+)
+
+func init() {
+	font, err := sfnt.Parse(goregular.TTF)
+	if err != nil {
+		panic(err)
+	}
+
+	regular, err = NewFace(font, defaultFaceOptions())
+	if err != nil {
+		panic(err)
+	}
+}
+
+func TestFaceGlyphAdvance(t *testing.T) {
+	for _, test := range []struct {
+		r    rune
+		want fixed.Int26_6
+	}{
+		{' ', 213},
+		{'A', 512},
+		{'Á', 512},
+		{'Æ', 768},
+		{'i', 189},
+		{'x', 384},
+	} {
+		got, ok := regular.GlyphAdvance(test.r)
+		if !ok {
+			t.Errorf("could not get glyph advance width for %q", test.r)
+			continue
+		}
+
+		if got != test.want {
+			t.Errorf("%q: glyph advance width=%d. want=%d", test.r, got, test.want)
+			continue
+		}
+	}
+}
+
+func TestFaceKern(t *testing.T) {
+	// FIXME(sbinet) there is no kerning with gofont/goregular
+	for _, test := range []struct {
+		r1, r2 rune
+		want   fixed.Int26_6
+	}{
+		{'A', 'A', 0},
+		{'A', 'V', 0},
+		{'V', 'A', 0},
+		{'A', 'v', 0},
+		{'W', 'a', 0},
+		{'W', 'i', 0},
+		{'Y', 'i', 0},
+		{'f', '(', 0},
+		{'f', 'f', 0},
+		{'f', 'i', 0},
+		{'T', 'a', 0},
+		{'T', 'e', 0},
+	} {
+		got := regular.Kern(test.r1, test.r2)
+		if got != test.want {
+			t.Errorf("(%q, %q): glyph kerning=%d. want=%d", test.r1, test.r2, got, test.want)
+			continue
+		}
+	}
+}
+
+func TestFaceMetrics(t *testing.T) {
+	want := font.Metrics{Height: 768, Ascent: 726, Descent: 162}
+	got := regular.Metrics()
+	if got != want {
+		t.Fatalf("metrics failed. got=%#v. want=%#v", got, want)
+	}
+}

font/opentype/opentype.go

@@ -0,0 +1,7 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package opentype implements the font.Face interface based on SFNT
+// font file formats.
+package opentype // import "golang.org/x/image/font/opentype"