font/sfnt/sfnt.go - image - Git at Google

 // Copyright 2016 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 sfnt implements a decoder for SFNT font file formats, including
 // TrueType and OpenType.
 package sfnt // import "golang.org/x/image/font/sfnt"

 // This implementation was written primarily to the
 // https://www.microsoft.com/en-us/Typography/OpenTypeSpecification.aspx
 // specification. Additional documentation is at
 // http://developer.apple.com/fonts/TTRefMan/
 //
 // The pyftinspect tool from https://github.com/fonttools/fonttools is useful
 // for inspecting SFNT fonts.

 import (
 	"errors"
 	"io"

 	"golang.org/x/image/math/fixed"
 )

 // These constants are not part of the specifications, but are limitations used
 // by this implementation.
 const (
 	maxHintBits         = 256
 	maxNumTables        = 256
 	maxRealNumberStrLen = 64 // Maximum length in bytes of the "-123.456E-7" representation.

 	// (maxTableOffset + maxTableLength) will not overflow an int32.
 	maxTableLength = 1 << 29
 	maxTableOffset = 1 << 29
 )

 var (
 	errGlyphIndexOutOfRange = errors.New("sfnt: glyph index out of range")

 	errInvalidBounds        = errors.New("sfnt: invalid bounds")
 	errInvalidCFFTable      = errors.New("sfnt: invalid CFF table")
 	errInvalidHeadTable     = errors.New("sfnt: invalid head table")
 	errInvalidLocationData  = errors.New("sfnt: invalid location data")
 	errInvalidMaxpTable     = errors.New("sfnt: invalid maxp table")
 	errInvalidSourceData    = errors.New("sfnt: invalid source data")
 	errInvalidTableOffset   = errors.New("sfnt: invalid table offset")
 	errInvalidTableTagOrder = errors.New("sfnt: invalid table tag order")
 	errInvalidVersion       = errors.New("sfnt: invalid version")

 	errUnsupportedCFFVersion         = errors.New("sfnt: unsupported CFF version")
 	errUnsupportedRealNumberEncoding = errors.New("sfnt: unsupported real number encoding")
 	errUnsupportedNumberOfHints      = errors.New("sfnt: unsupported number of hints")
 	errUnsupportedNumberOfTables     = errors.New("sfnt: unsupported number of tables")
 	errUnsupportedTableOffsetLength  = errors.New("sfnt: unsupported table offset or length")
 	errUnsupportedType2Charstring    = errors.New("sfnt: unsupported Type 2 Charstring")
 )

 // GlyphIndex is a glyph index in a Font.
 type GlyphIndex uint16

 // Units are an integral number of abstract, scalable "font units". The em
 // square is typically 1000 or 2048 "font units". This would map to a certain
 // number (e.g. 30 pixels) of physical pixels, depending on things like the
 // display resolution (DPI) and font size (e.g. a 12 point font).
 type Units int32

 func u16(b []byte) uint16 {
 	_ = b[1] // Bounds check hint to compiler.
 	return uint16(b[0])<<8 | uint16(b[1])<<0
 }

 func u32(b []byte) uint32 {
 	_ = b[3] // Bounds check hint to compiler.
 	return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3])<<0
 }

 // source is a source of byte data. Conceptually, it is like an io.ReaderAt,
 // except that a common source of SFNT font data is in-memory instead of
 // on-disk: a []byte containing the entire data, either as a global variable
 // (e.g. "goregular.TTF") or the result of an ioutil.ReadFile call. In such
 // cases, as an optimization, we skip the io.Reader / io.ReaderAt model of
 // copying from the source to a caller-supplied buffer, and instead provide
 // direct access to the underlying []byte data.
 type source struct {
 	b []byte
 	r io.ReaderAt

 	// TODO: add a caching layer, if we're using the io.ReaderAt? Note that
 	// this might make a source no longer safe to use concurrently.
 }

 // valid returns whether exactly one of s.b and s.r is nil.
 func (s *source) valid() bool {
 	return (s.b == nil) != (s.r == nil)
 }

 // viewBufferWritable returns whether the []byte returned by source.view can be
 // written to by the caller, including by passing it to the same method
 // (source.view) on other receivers (i.e. different sources).
 //
 // In other words, it returns whether the source's underlying data is an
 // io.ReaderAt, not a []byte.
 func (s *source) viewBufferWritable() bool {
 	return s.b == nil
 }

 // view returns the length bytes at the given offset. buf is an optional
 // scratch buffer to reduce allocations when calling view multiple times. A nil
 // buf is valid. The []byte returned may be a sub-slice of buf[:cap(buf)], or
 // it may be an unrelated slice. In any case, the caller should not modify the
 // contents of the returned []byte, other than passing that []byte back to this
 // method on the same source s.
 func (s *source) view(buf []byte, offset, length int) ([]byte, error) {
 	if 0 > offset || offset > offset+length {
 		return nil, errInvalidBounds
 	}

 	// Try reading from the []byte.
 	if s.b != nil {
 		if offset+length > len(s.b) {
 			return nil, errInvalidBounds
 		}
 		return s.b[offset : offset+length], nil
 	}

 	// Read from the io.ReaderAt.
 	if length <= cap(buf) {
 		buf = buf[:length]
 	} else {
 		// Round length up to the nearest KiB. The slack can lead to fewer
 		// allocations if the buffer is re-used for multiple source.view calls.
 		n := length
 		n += 1023
 		n &^= 1023
 		buf = make([]byte, length, n)
 	}
 	if n, err := s.r.ReadAt(buf, int64(offset)); n != length {
 		return nil, err
 	}
 	return buf, nil
 }

 // u16 returns the uint16 in the table t at the relative offset i.
 //
 // buf is an optional scratch buffer as per the source.view method.
 func (s *source) u16(buf []byte, t table, i int) (uint16, error) {
 	if i < 0 || uint(t.length) < uint(i+2) {
 		return 0, errInvalidBounds
 	}
 	buf, err := s.view(buf, int(t.offset)+i, 2)
 	if err != nil {
 		return 0, err
 	}
 	return u16(buf), nil
 }

 // table is a section of the font data.
 type table struct {
 	offset, length uint32
 }

 // Parse parses an SFNT font from a []byte data source.
 func Parse(src []byte) (*Font, error) {
 	f := &Font{src: source{b: src}}
 	if err := f.initialize(); err != nil {
 		return nil, err
 	}
 	return f, nil
 }

 // ParseReaderAt parses an SFNT font from an io.ReaderAt data source.
 func ParseReaderAt(src io.ReaderAt) (*Font, error) {
 	f := &Font{src: source{r: src}}
 	if err := f.initialize(); err != nil {
 		return nil, err
 	}
 	return f, nil
 }

 // Font is an SFNT font.
 //
 // All of its methods are safe to call concurrently, although the method
 // arguments may have further restrictions. For example, it is valid to have
 // multiple concurrent Font.LoadGlyph calls to the same *Font receiver, as long
 // as each call has a different Buffer.
 type Font struct {
 	src source

 	// https://www.microsoft.com/typography/otspec/otff.htm#otttables
 	// "Required Tables".
 	cmap table
 	head table
 	hhea table
 	hmtx table
 	maxp table
 	name table
 	os2  table
 	post table

 	// https://www.microsoft.com/typography/otspec/otff.htm#otttables
 	// "Tables Related to TrueType Outlines".
 	//
 	// This implementation does not support hinting, so it does not read the
 	// cvt, fpgm gasp or prep tables.
 	glyf table
 	loca table

 	// https://www.microsoft.com/typography/otspec/otff.htm#otttables
 	// "Tables Related to PostScript Outlines".
 	//
 	// TODO: cff2, vorg?
 	cff table

 	// https://www.microsoft.com/typography/otspec/otff.htm#otttables
 	// "Advanced Typographic Tables".
 	//
 	// TODO: base, gdef, gpos, gsub, jstf, math?

 	// https://www.microsoft.com/typography/otspec/otff.htm#otttables
 	// "Other OpenType Tables".
 	//
 	// TODO: hdmx, kern, vmtx? Others?

 	cached struct {
 		isPostScript bool
 		unitsPerEm   Units

 		// The glyph data for the glyph index i is in
 		// src[locations[i+0]:locations[i+1]].
 		locations []uint32
 	}
 }

 // NumGlyphs returns the number of glyphs in f.
 func (f *Font) NumGlyphs() int { return len(f.cached.locations) - 1 }

 // UnitsPerEm returns the number of units per em for f.
 func (f *Font) UnitsPerEm() Units { return f.cached.unitsPerEm }

 func (f *Font) initialize() error {
 	if !f.src.valid() {
 		return errInvalidSourceData
 	}
 	var buf []byte

 	// https://www.microsoft.com/typography/otspec/otff.htm "Organization of an
 	// OpenType Font" says that "The OpenType font starts with the Offset
 	// Table", which is 12 bytes.
 	buf, err := f.src.view(buf, 0, 12)
 	if err != nil {
 		return err
 	}
 	switch u32(buf) {
 	default:
 		return errInvalidVersion
 	case 0x00010000:
 		// No-op.
 	case 0x4f54544f: // "OTTO".
 		f.cached.isPostScript = true
 	}
 	numTables := int(u16(buf[4:]))
 	if numTables > maxNumTables {
 		return errUnsupportedNumberOfTables
 	}

 	// "The Offset Table is followed immediately by the Table Record entries...
 	// sorted in ascending order by tag", 16 bytes each.
 	buf, err = f.src.view(buf, 12, 16*numTables)
 	if err != nil {
 		return err
 	}
 	for b, first, prevTag := buf, true, uint32(0); len(b) > 0; b = b[16:] {
 		tag := u32(b)
 		if first {
 			first = false
 		} else if tag <= prevTag {
 			return errInvalidTableTagOrder
 		}
 		prevTag = tag

 		o, n := u32(b[8:12]), u32(b[12:16])
 		if o > maxTableOffset || n > maxTableLength {
 			return errUnsupportedTableOffsetLength
 		}
 		// We ignore the checksums, but "all tables must begin on four byte
 		// boundries [sic]".
 		if o&3 != 0 {
 			return errInvalidTableOffset
 		}

 		// Match the 4-byte tag as a uint32. For example, "OS/2" is 0x4f532f32.
 		switch tag {
 		case 0x43464620:
 			f.cff = table{o, n}
 		case 0x4f532f32:
 			f.os2 = table{o, n}
 		case 0x636d6170:
 			f.cmap = table{o, n}
 		case 0x676c7966:
 			f.glyf = table{o, n}
 		case 0x68656164:
 			f.head = table{o, n}
 		case 0x68686561:
 			f.hhea = table{o, n}
 		case 0x686d7478:
 			f.hmtx = table{o, n}
 		case 0x6c6f6361:
 			f.loca = table{o, n}
 		case 0x6d617870:
 			f.maxp = table{o, n}
 		case 0x6e616d65:
 			f.name = table{o, n}
 		case 0x706f7374:
 			f.post = table{o, n}
 		}
 	}

 	var u uint16

 	// https://www.microsoft.com/typography/otspec/head.htm
 	if f.head.length != 54 {
 		return errInvalidHeadTable
 	}
 	u, err = f.src.u16(buf, f.head, 18)
 	if err != nil {
 		return err
 	}
 	if u == 0 {
 		return errInvalidHeadTable
 	}
 	f.cached.unitsPerEm = Units(u)

 	// https://www.microsoft.com/typography/otspec/maxp.htm
 	if f.cached.isPostScript {
 		if f.maxp.length != 6 {
 			return errInvalidMaxpTable
 		}
 	} else {
 		if f.maxp.length != 32 {
 			return errInvalidMaxpTable
 		}
 	}
 	u, err = f.src.u16(buf, f.maxp, 4)
 	if err != nil {
 		return err
 	}
 	numGlyphs := int(u)

 	if f.cached.isPostScript {
 		p := cffParser{
 			src:    &f.src,
 			base:   int(f.cff.offset),
 			offset: int(f.cff.offset),
 			end:    int(f.cff.offset + f.cff.length),
 		}
 		f.cached.locations, err = p.parse()
 		if err != nil {
 			return err
 		}
 	} else {
 		// TODO: locaParser for TrueType fonts.
 		f.cached.locations = make([]uint32, numGlyphs+1)
 	}
 	if len(f.cached.locations) != numGlyphs+1 {
 		return errInvalidLocationData
 	}
 	return nil
 }

 // TODO: func (f *Font) GlyphIndex(r rune) (x GlyphIndex, ok bool)
 // This will require parsing the cmap table.

 func (f *Font) viewGlyphData(buf []byte, x GlyphIndex) ([]byte, error) {
 	xx := int(x)
 	if f.NumGlyphs() <= xx {
 		return nil, errGlyphIndexOutOfRange
 	}
 	i := f.cached.locations[xx+0]
 	j := f.cached.locations[xx+1]
 	return f.src.view(buf, int(i), int(j-i))
 }

 // LoadGlyphOptions are the options to the Font.LoadGlyph method.
 type LoadGlyphOptions struct {
 	// TODO: scale / transform / hinting.
 }

 // LoadGlyph loads the glyphs vectors for the x'th glyph into b.Segments.
 func (f *Font) LoadGlyph(b *Buffer, x GlyphIndex, opts *LoadGlyphOptions) error {
 	buf, err := f.viewGlyphData(b.buf, x)
 	if err != nil {
 		return err
 	}
 	// Only update b.buf if it is safe to re-use buf.
 	if f.src.viewBufferWritable() {
 		b.buf = buf
 	}

 	b.Segments = b.Segments[:0]
 	if f.cached.isPostScript {
 		b.psi.type2Charstrings.initialize(b.Segments)
 		if err := b.psi.run(psContextType2Charstring, buf); err != nil {
 			return err
 		}
 		b.Segments = b.psi.type2Charstrings.segments
 	} else {
 		return errors.New("sfnt: TODO: load glyf data")
 	}

 	// TODO: look at opts to scale / transform / hint the Buffer.Segments.

 	return nil
 }

 // Buffer holds the result of the Font.LoadGlyph method. It is valid to re-use
 // a Buffer with multiple Font.LoadGlyph calls, even with different *Font
 // receivers, as long as they are not concurrent calls.
 //
 // It is also valid to have multiple concurrent Font.LoadGlyph calls to the
 // same *Font receiver, as long as each call has a different Buffer.
 type Buffer struct {
 	Segments []Segment
 	// buf is a byte buffer for when a Font's source is an io.ReaderAt.
 	buf []byte
 	// psi is a PostScript interpreter for when the Font is an OpenType/CFF
 	// font.
 	psi psInterpreter
 }

 // Segment is a segment of a vector path.
 type Segment struct {
 	Op   SegmentOp
 	Args [6]fixed.Int26_6
 }

 // SegmentOp is a vector path segment's operator.
 type SegmentOp uint32

 const (
 	SegmentOpMoveTo SegmentOp = iota
 	SegmentOpLineTo
 	SegmentOpQuadTo
 	SegmentOpCubeTo
 )
	// Copyright 2016 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 sfnt implements a decoder for SFNT font file formats, including
	// TrueType and OpenType.
	package sfnt // import "golang.org/x/image/font/sfnt"

	// This implementation was written primarily to the
	// https://www.microsoft.com/en-us/Typography/OpenTypeSpecification.aspx
	// specification. Additional documentation is at
	// http://developer.apple.com/fonts/TTRefMan/
	//
	// The pyftinspect tool from https://github.com/fonttools/fonttools is useful
	// for inspecting SFNT fonts.

	import (
	"errors"
	"io"

	"golang.org/x/image/math/fixed"
	)

	// These constants are not part of the specifications, but are limitations used
	// by this implementation.
	const (
	maxHintBits = 256
	maxNumTables = 256
	maxRealNumberStrLen = 64 // Maximum length in bytes of the "-123.456E-7" representation.

	// (maxTableOffset + maxTableLength) will not overflow an int32.
	maxTableLength = 1 << 29
	maxTableOffset = 1 << 29
	)

	var (
	errGlyphIndexOutOfRange = errors.New("sfnt: glyph index out of range")

	errInvalidBounds = errors.New("sfnt: invalid bounds")
	errInvalidCFFTable = errors.New("sfnt: invalid CFF table")
	errInvalidHeadTable = errors.New("sfnt: invalid head table")
	errInvalidLocationData = errors.New("sfnt: invalid location data")
	errInvalidMaxpTable = errors.New("sfnt: invalid maxp table")
	errInvalidSourceData = errors.New("sfnt: invalid source data")
	errInvalidTableOffset = errors.New("sfnt: invalid table offset")
	errInvalidTableTagOrder = errors.New("sfnt: invalid table tag order")
	errInvalidVersion = errors.New("sfnt: invalid version")

	errUnsupportedCFFVersion = errors.New("sfnt: unsupported CFF version")
	errUnsupportedRealNumberEncoding = errors.New("sfnt: unsupported real number encoding")
	errUnsupportedNumberOfHints = errors.New("sfnt: unsupported number of hints")
	errUnsupportedNumberOfTables = errors.New("sfnt: unsupported number of tables")
	errUnsupportedTableOffsetLength = errors.New("sfnt: unsupported table offset or length")
	errUnsupportedType2Charstring = errors.New("sfnt: unsupported Type 2 Charstring")
	)

	// GlyphIndex is a glyph index in a Font.
	type GlyphIndex uint16

	// Units are an integral number of abstract, scalable "font units". The em
	// square is typically 1000 or 2048 "font units". This would map to a certain
	// number (e.g. 30 pixels) of physical pixels, depending on things like the
	// display resolution (DPI) and font size (e.g. a 12 point font).
	type Units int32

	func u16(b []byte) uint16 {
	_ = b[1] // Bounds check hint to compiler.
	return uint16(b[0])<<8 \| uint16(b[1])<<0
	}

	func u32(b []byte) uint32 {
	_ = b[3] // Bounds check hint to compiler.
	return uint32(b[0])<<24 \| uint32(b[1])<<16 \| uint32(b[2])<<8 \| uint32(b[3])<<0
	}

	// source is a source of byte data. Conceptually, it is like an io.ReaderAt,
	// except that a common source of SFNT font data is in-memory instead of
	// on-disk: a []byte containing the entire data, either as a global variable
	// (e.g. "goregular.TTF") or the result of an ioutil.ReadFile call. In such
	// cases, as an optimization, we skip the io.Reader / io.ReaderAt model of
	// copying from the source to a caller-supplied buffer, and instead provide
	// direct access to the underlying []byte data.
	type source struct {
	b []byte
	r io.ReaderAt

	// TODO: add a caching layer, if we're using the io.ReaderAt? Note that
	// this might make a source no longer safe to use concurrently.
	}

	// valid returns whether exactly one of s.b and s.r is nil.
	func (s *source) valid() bool {
	return (s.b == nil) != (s.r == nil)
	}

	// viewBufferWritable returns whether the []byte returned by source.view can be
	// written to by the caller, including by passing it to the same method
	// (source.view) on other receivers (i.e. different sources).
	//
	// In other words, it returns whether the source's underlying data is an
	// io.ReaderAt, not a []byte.
	func (s *source) viewBufferWritable() bool {
	return s.b == nil
	}

	// view returns the length bytes at the given offset. buf is an optional
	// scratch buffer to reduce allocations when calling view multiple times. A nil
	// buf is valid. The []byte returned may be a sub-slice of buf[:cap(buf)], or
	// it may be an unrelated slice. In any case, the caller should not modify the
	// contents of the returned []byte, other than passing that []byte back to this
	// method on the same source s.
	func (s *source) view(buf []byte, offset, length int) ([]byte, error) {
	if 0 > offset \|\| offset > offset+length {
	return nil, errInvalidBounds
	}

	// Try reading from the []byte.
	if s.b != nil {
	if offset+length > len(s.b) {
	return nil, errInvalidBounds
	}
	return s.b[offset : offset+length], nil
	}

	// Read from the io.ReaderAt.
	if length <= cap(buf) {
	buf = buf[:length]
	} else {
	// Round length up to the nearest KiB. The slack can lead to fewer
	// allocations if the buffer is re-used for multiple source.view calls.
	n := length
	n += 1023
	n &^= 1023
	buf = make([]byte, length, n)
	}
	if n, err := s.r.ReadAt(buf, int64(offset)); n != length {
	return nil, err
	}
	return buf, nil
	}

	// u16 returns the uint16 in the table t at the relative offset i.
	//
	// buf is an optional scratch buffer as per the source.view method.
	func (s *source) u16(buf []byte, t table, i int) (uint16, error) {
	if i < 0 \|\| uint(t.length) < uint(i+2) {
	return 0, errInvalidBounds
	}
	buf, err := s.view(buf, int(t.offset)+i, 2)
	if err != nil {
	return 0, err
	}
	return u16(buf), nil
	}

	// table is a section of the font data.
	type table struct {
	offset, length uint32
	}

	// Parse parses an SFNT font from a []byte data source.
	func Parse(src []byte) (*Font, error) {
	f := &Font{src: source{b: src}}
	if err := f.initialize(); err != nil {
	return nil, err
	}
	return f, nil
	}

	// ParseReaderAt parses an SFNT font from an io.ReaderAt data source.
	func ParseReaderAt(src io.ReaderAt) (*Font, error) {
	f := &Font{src: source{r: src}}
	if err := f.initialize(); err != nil {
	return nil, err
	}
	return f, nil
	}

	// Font is an SFNT font.
	//
	// All of its methods are safe to call concurrently, although the method
	// arguments may have further restrictions. For example, it is valid to have
	// multiple concurrent Font.LoadGlyph calls to the same *Font receiver, as long
	// as each call has a different Buffer.
	type Font struct {
	src source

	// https://www.microsoft.com/typography/otspec/otff.htm#otttables
	// "Required Tables".
	cmap table
	head table
	hhea table
	hmtx table
	maxp table
	name table
	os2 table
	post table

	// https://www.microsoft.com/typography/otspec/otff.htm#otttables
	// "Tables Related to TrueType Outlines".
	//
	// This implementation does not support hinting, so it does not read the
	// cvt, fpgm gasp or prep tables.
	glyf table
	loca table

	// https://www.microsoft.com/typography/otspec/otff.htm#otttables
	// "Tables Related to PostScript Outlines".
	//
	// TODO: cff2, vorg?
	cff table

	// https://www.microsoft.com/typography/otspec/otff.htm#otttables
	// "Advanced Typographic Tables".
	//
	// TODO: base, gdef, gpos, gsub, jstf, math?

	// https://www.microsoft.com/typography/otspec/otff.htm#otttables
	// "Other OpenType Tables".
	//
	// TODO: hdmx, kern, vmtx? Others?

	cached struct {
	isPostScript bool
	unitsPerEm Units

	// The glyph data for the glyph index i is in
	// src[locations[i+0]:locations[i+1]].
	locations []uint32
	}
	}

	// NumGlyphs returns the number of glyphs in f.
	func (f *Font) NumGlyphs() int { return len(f.cached.locations) - 1 }

	// UnitsPerEm returns the number of units per em for f.
	func (f *Font) UnitsPerEm() Units { return f.cached.unitsPerEm }

	func (f *Font) initialize() error {
	if !f.src.valid() {
	return errInvalidSourceData
	}
	var buf []byte

	// https://www.microsoft.com/typography/otspec/otff.htm "Organization of an
	// OpenType Font" says that "The OpenType font starts with the Offset
	// Table", which is 12 bytes.
	buf, err := f.src.view(buf, 0, 12)
	if err != nil {
	return err
	}
	switch u32(buf) {
	default:
	return errInvalidVersion
	case 0x00010000:
	// No-op.
	case 0x4f54544f: // "OTTO".
	f.cached.isPostScript = true
	}
	numTables := int(u16(buf[4:]))
	if numTables > maxNumTables {
	return errUnsupportedNumberOfTables
	}

	// "The Offset Table is followed immediately by the Table Record entries...
	// sorted in ascending order by tag", 16 bytes each.
	buf, err = f.src.view(buf, 12, 16*numTables)
	if err != nil {
	return err
	}
	for b, first, prevTag := buf, true, uint32(0); len(b) > 0; b = b[16:] {
	tag := u32(b)
	if first {
	first = false
	} else if tag <= prevTag {
	return errInvalidTableTagOrder
	}
	prevTag = tag

	o, n := u32(b[8:12]), u32(b[12:16])
	if o > maxTableOffset \|\| n > maxTableLength {
	return errUnsupportedTableOffsetLength
	}
	// We ignore the checksums, but "all tables must begin on four byte
	// boundries [sic]".
	if o&3 != 0 {
	return errInvalidTableOffset
	}

	// Match the 4-byte tag as a uint32. For example, "OS/2" is 0x4f532f32.
	switch tag {
	case 0x43464620:
	f.cff = table{o, n}
	case 0x4f532f32:
	f.os2 = table{o, n}
	case 0x636d6170:
	f.cmap = table{o, n}
	case 0x676c7966:
	f.glyf = table{o, n}
	case 0x68656164:
	f.head = table{o, n}
	case 0x68686561:
	f.hhea = table{o, n}
	case 0x686d7478:
	f.hmtx = table{o, n}
	case 0x6c6f6361:
	f.loca = table{o, n}
	case 0x6d617870:
	f.maxp = table{o, n}
	case 0x6e616d65:
	f.name = table{o, n}
	case 0x706f7374:
	f.post = table{o, n}
	}
	}

	var u uint16

	// https://www.microsoft.com/typography/otspec/head.htm
	if f.head.length != 54 {
	return errInvalidHeadTable
	}
	u, err = f.src.u16(buf, f.head, 18)
	if err != nil {
	return err
	}
	if u == 0 {
	return errInvalidHeadTable
	}
	f.cached.unitsPerEm = Units(u)

	// https://www.microsoft.com/typography/otspec/maxp.htm
	if f.cached.isPostScript {
	if f.maxp.length != 6 {
	return errInvalidMaxpTable
	}
	} else {
	if f.maxp.length != 32 {
	return errInvalidMaxpTable
	}
	}
	u, err = f.src.u16(buf, f.maxp, 4)
	if err != nil {
	return err
	}
	numGlyphs := int(u)

	if f.cached.isPostScript {
	p := cffParser{
	src: &f.src,
	base: int(f.cff.offset),
	offset: int(f.cff.offset),
	end: int(f.cff.offset + f.cff.length),
	}
	f.cached.locations, err = p.parse()
	if err != nil {
	return err
	}
	} else {
	// TODO: locaParser for TrueType fonts.
	f.cached.locations = make([]uint32, numGlyphs+1)
	}
	if len(f.cached.locations) != numGlyphs+1 {
	return errInvalidLocationData
	}
	return nil
	}

	// TODO: func (f *Font) GlyphIndex(r rune) (x GlyphIndex, ok bool)
	// This will require parsing the cmap table.

	func (f *Font) viewGlyphData(buf []byte, x GlyphIndex) ([]byte, error) {
	xx := int(x)
	if f.NumGlyphs() <= xx {
	return nil, errGlyphIndexOutOfRange
	}
	i := f.cached.locations[xx+0]
	j := f.cached.locations[xx+1]
	return f.src.view(buf, int(i), int(j-i))
	}

	// LoadGlyphOptions are the options to the Font.LoadGlyph method.
	type LoadGlyphOptions struct {
	// TODO: scale / transform / hinting.
	}

	// LoadGlyph loads the glyphs vectors for the x'th glyph into b.Segments.
	func (f Font) LoadGlyph(b Buffer, x GlyphIndex, opts *LoadGlyphOptions) error {
	buf, err := f.viewGlyphData(b.buf, x)
	if err != nil {
	return err
	}
	// Only update b.buf if it is safe to re-use buf.
	if f.src.viewBufferWritable() {
	b.buf = buf
	}

	b.Segments = b.Segments[:0]
	if f.cached.isPostScript {
	b.psi.type2Charstrings.initialize(b.Segments)
	if err := b.psi.run(psContextType2Charstring, buf); err != nil {
	return err
	}
	b.Segments = b.psi.type2Charstrings.segments
	} else {
	return errors.New("sfnt: TODO: load glyf data")
	}

	// TODO: look at opts to scale / transform / hint the Buffer.Segments.

	return nil
	}

	// Buffer holds the result of the Font.LoadGlyph method. It is valid to re-use
	// a Buffer with multiple Font.LoadGlyph calls, even with different *Font
	// receivers, as long as they are not concurrent calls.
	//
	// It is also valid to have multiple concurrent Font.LoadGlyph calls to the
	// same *Font receiver, as long as each call has a different Buffer.
	type Buffer struct {
	Segments []Segment
	// buf is a byte buffer for when a Font's source is an io.ReaderAt.
	buf []byte
	// psi is a PostScript interpreter for when the Font is an OpenType/CFF
	// font.
	psi psInterpreter
	}

	// Segment is a segment of a vector path.
	type Segment struct {
	Op SegmentOp
	Args [6]fixed.Int26_6
	}

	// SegmentOp is a vector path segment's operator.
	type SegmentOp uint32

	const (
	SegmentOpMoveTo SegmentOp = iota
	SegmentOpLineTo
	SegmentOpQuadTo
	SegmentOpCubeTo
	)