aw/golang-image
1
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

font/sfnt: new package for parsing SFNT fonts.

Browse Source 
Updates golang/go#16904

Change-Id: Ic4c55865741b712f7d82448276fc357f2edf9b45
Reviewed-on: https://go-review.googlesource.com/33417
Reviewed-by: Dave Day <djd@golang.org>
This commit is contained in:
Nigel Tao 2016-11-20 14:06:55 +11:00
parent d2f07f8aaa
commit c39d899a5b
2 changed files with 344 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

304
font/sfnt/sfnt.go Normal file
View File

@ -0,0 +1,304 @@
// 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/
import (
"errors"
"io"
)
// These constants are not part of the specifications, but are limitations used
// by this implementation.
const (
maxNumTables = 256
// (maxTableOffset + maxTableLength) will not overflow an int32.
maxTableLength = 1 << 29
maxTableOffset = 1 << 29
)
var (
errInvalidBounds = errors.New("sfnt: invalid bounds")
errInvalidHeadTable = errors.New("sfnt: invalid head table")
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")
errUnsupportedNumberOfTables = errors.New("sfnt: unsupported number of tables")
errUnsupportedTableOffsetLength = errors.New("sfnt: unsupported table offset or length")
errUnsupportedVersion = errors.New("sfnt: unsupported version")
)
// 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)
}
// 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.
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: cff, cff2, vorg?
// 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 {
numGlyphs int
unitsPerEm Units
}
}
// NumGlyphs returns the number of glyphs in f.
func (f *Font) NumGlyphs() int { return f.cached.numGlyphs }
// 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".
// TODO: support CFF fonts.
return errUnsupportedVersion
}
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 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.maxp.length != 32 {
return errInvalidMaxpTable
}
u, err = f.src.u16(buf, f.maxp, 4)
if err != nil {
return err
}
f.cached.numGlyphs = int(u)
return nil
}

40
font/sfnt/sfnt_test.go Normal file
View File

@ -0,0 +1,40 @@
// 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
import (
"bytes"
"testing"
"golang.org/x/image/font/gofont/goregular"
)
func TestParse(t *testing.T) {
f, err := Parse(goregular.TTF)
if err != nil {
t.Fatalf("Parse: %v", err)
}
testFont(t, f)
}
func TestParseReaderAt(t *testing.T) {
f, err := ParseReaderAt(bytes.NewReader(goregular.TTF))
if err != nil {
t.Fatalf("ParseReaderAt: %v", err)
}
testFont(t, f)
}
func testFont(t *testing.T, f *Font) {
if got, want := f.UnitsPerEm(), Units(2048); got != want {
t.Errorf("UnitsPerEm: got %d, want %d", got, want)
}
// The exact number of glyphs in goregular.TTF can vary, and future
// versions may add more glyphs, but https://blog.golang.org/go-fonts says
// that "The WGL4 character set... [has] more than 650 characters in all.
if got, want := f.NumGlyphs(), 650; got <= want {
t.Errorf("NumGlyphs: got %d, want > %d", got, want)
}
}