// Copyright 2019 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.
//go:generate go run gen.go
// Package ccitt implements a CCITT (fax) image decoder.
package ccitt
import (
"encoding/binary"
"errors"
"image"
"io"
"math/bits"
)
var (
errIncompleteCode = errors.New("ccitt: incomplete code")
errInvalidBounds = errors.New("ccitt: invalid bounds")
errInvalidCode = errors.New("ccitt: invalid code")
errInvalidMode = errors.New("ccitt: invalid mode")
errInvalidOffset = errors.New("ccitt: invalid offset")
errMissingEOL = errors.New("ccitt: missing End-of-Line")
errRunLengthOverflowsWidth = errors.New("ccitt: run length overflows width")
errRunLengthTooLong = errors.New("ccitt: run length too long")
errUnsupportedMode = errors.New("ccitt: unsupported mode")
errUnsupportedSubFormat = errors.New("ccitt: unsupported sub-format")
errUnsupportedWidth = errors.New("ccitt: unsupported width")
)
// Order specifies the bit ordering in a CCITT data stream.
type Order uint32
const (
// LSB means Least Significant Bits first.
LSB Order = iota
// MSB means Most Significant Bits first.
MSB
)
// SubFormat represents that the CCITT format consists of a number of
// sub-formats. Decoding or encoding a CCITT data stream requires knowing the
// sub-format context. It is not represented in the data stream per se.
type SubFormat uint32
const (
Group3 SubFormat = iota
Group4
)
// AutoDetectHeight is passed as the height argument to NewReader to indicate
// that the image height (the number of rows) is not known in advance.
const AutoDetectHeight = -1
// Options are optional parameters.
type Options struct {
// Align means that some variable-bit-width codes are byte-aligned.
Align bool
// Invert means that black is the 1 bit or 0xFF byte, and white is 0.
Invert bool
}
// maxWidth is the maximum (inclusive) supported width. This is a limitation of
// this implementation, to guard against integer overflow, and not anything
// inherent to the CCITT format.
const maxWidth = 1 << 20
func invertBytes(b []byte) {
for i, c := range b {
b[i] = ^c
}
}
func reverseBitsWithinBytes(b []byte) {
for i, c := range b {
b[i] = bits.Reverse8(c)
}
}
// highBits writes to dst (1 bit per pixel, most significant bit first) the
// high (0x80) bits from src (1 byte per pixel). It returns the number of bytes
// written and read such that dst[:d] is the packed form of src[:s].
//
// For example, if src starts with the 8 bytes [0x7D, 0x7E, 0x7F, 0x80, 0x81,
// 0x82, 0x00, 0xFF] then 0x1D will be written to dst[0].
//
// If src has (8 * len(dst)) or more bytes then only len(dst) bytes are
// written, (8 * len(dst)) bytes are read, and invert is ignored.
//
// Otherwise, if len(src) is not a multiple of 8 then the final byte written to
// dst is padded with 1 bits (if invert is true) or 0 bits. If inverted, the 1s
// are typically temporary, e.g. they will be flipped back to 0s by an
// invertBytes call in the highBits caller, reader.Read.
func highBits(dst []byte, src []byte, invert bool) (d int, s int) {
// Pack as many complete groups of 8 src bytes as we can.
n := len(src) / 8
if n > len(dst) {
n = len(dst)
}
dstN := dst[:n]
for i := range dstN {
src8 := src[i*8 : i*8+8]
dstN[i] = ((src8[0] & 0x80) >> 0) |
((src8[1] & 0x80) >> 1) |
((src8[2] & 0x80) >> 2) |
((src8[3] & 0x80) >> 3) |
((src8[4] & 0x80) >> 4) |
((src8[5] & 0x80) >> 5) |
((src8[6] & 0x80) >> 6) |
((src8[7] & 0x80) >> 7)
}
d, s = n, 8*n
dst, src = dst[d:], src[s:]
// Pack up to 7 remaining src bytes, if there's room in dst.
if (len(dst) > 0) && (len(src) > 0) {
dstByte := byte(0)
if invert {
dstByte = 0xFF >> uint(len(src))
}
for n, srcByte := range src {
dstByte |= (srcByte & 0x80) >> uint(n)
}
dst[0] = dstByte
d, s = d+1, s+len(src)
}
return d, s
}
type bitReader struct {
r io.Reader
// readErr is the error returned from the most recent r.Read call. As the
// io.Reader documentation says, when r.Read returns (n, err), "always
// process the n > 0 bytes returned before considering the error err".
readErr error
// order is whether to process r's bytes LSB first or MSB first.
order Order
// The high nBits bits of the bits field hold upcoming bits in MSB order.
bits uint64
nBits uint32
// bytes[br:bw] holds bytes read from r but not yet loaded into bits.
br uint32
bw uint32
bytes [1024]uint8
}
func (b *bitReader) alignToByteBoundary() {
n := b.nBits & 7
b.bits <<= n
b.nBits -= n
}
// nextBitMaxNBits is the maximum possible value of bitReader.nBits after a
// bitReader.nextBit call, provided that bitReader.nBits was not more than this
// value before that call.
//
// Note that the decode function can unread bits, which can temporarily set the
// bitReader.nBits value above nextBitMaxNBits.
const nextBitMaxNBits = 31
func (b *bitReader) nextBit() (uint64, error) {
for {
if b.nBits > 0 {
bit := b.bits >> 63
b.bits <<= 1
b.nBits--
return bit, nil
}
if available := b.bw - b.br; available >= 4 {
// Read 32 bits, even though b.bits is a uint64, since the decode
// function may need to unread up to maxCodeLength bits, putting
// them back in the remaining (64 - 32) bits. TestMaxCodeLength
// checks that the generated maxCodeLength constant fits.
//
// If changing the Uint32 call, also change nextBitMaxNBits.
b.bits = uint64(binary.BigEndian.Uint32(b.bytes[b.br:])) << 32
b.br += 4
b.nBits = 32
continue
} else if available > 0 {
b.bits = uint64(b.bytes[b.br]) << (7 * 8)
b.br++
b.nBits = 8
continue
}
if b.readErr != nil {
return 0, b.readErr
}
n, err := b.r.Read(b.bytes[:])
b.br = 0
b.bw = uint32(n)
b.readErr = err
if b.order != MSB {
reverseBitsWithinBytes(b.bytes[:b.bw])
}
}
}
func decode(b *bitReader, decodeTable [][2]int16) (uint32, error) {
nBitsRead, bitsRead, state := uint32(0), uint64(0), int32(1)
for {
bit, err := b.nextBit()
if err != nil {
if err == io.EOF {
err = errIncompleteCode
}
return 0, err
}
bitsRead |= bit << (63 - nBitsRead)
nBitsRead++
// The "&1" is redundant, but can eliminate a bounds check.
state = int32(decodeTable[state][bit&1])
if state < 0 {
return uint32(^state), nil
} else if state == 0 {
// Unread the bits we've read, then return errInvalidCode.
b.bits = (b.bits >> nBitsRead) | bitsRead
b.nBits += nBitsRead
return 0, errInvalidCode
}
}
}
// decodeEOL decodes the 12-bit EOL code 0000_0000_0001.
func decodeEOL(b *bitReader) error {
nBitsRead, bitsRead := uint32(0), uint64(0)
for {
bit, err := b.nextBit()
if err != nil {
if err == io.EOF {
err = errMissingEOL
}
return err
}
bitsRead |= bit << (63 - nBitsRead)
nBitsRead++
if nBitsRead < 12 {
if bit&1 == 0 {
continue
}
} else if bit&1 != 0 {
return nil
}
// Unread the bits we've read, then return errMissingEOL.
b.bits = (b.bits >> nBitsRead) | bitsRead
b.nBits += nBitsRead
return errMissingEOL
}
}
type reader struct {
br bitReader
subFormat SubFormat
// width is the image width in pixels.
width int
// rowsRemaining starts at the image height in pixels, when the reader is
// driven through the io.Reader interface, and decrements to zero as rows
// are decoded. Alternatively, it may be negative if the image height is
// not known in advance at the time of the NewReader call.
//
// When driven through DecodeIntoGray, this field is unused.
rowsRemaining int
// curr and prev hold the current and previous rows. Each element is either
// 0x00 (black) or 0xFF (white).
//
// prev may be nil, when processing the first row.
curr []byte
prev []byte
// ri is the read index. curr[:ri] are those bytes of curr that have been
// passed along via the Read method.
//
// When the reader is driven through DecodeIntoGray, instead of through the
// io.Reader interface, this field is unused.
ri int
// wi is the write index. curr[:wi] are those bytes of curr that have
// already been decoded via the decodeRow method.
//
// What this implementation calls wi is roughly equivalent to what the spec
// calls the a0 index.
wi int
// These fields are copied from the *Options (which may be nil).
align bool
invert bool
// atStartOfRow is whether we have just started the row. Some parts of the
// spec say to treat this situation as if "wi = -1".
atStartOfRow bool
// penColorIsWhite is whether the next run is black or white.
penColorIsWhite bool
// seenStartOfImage is whether we've called the startDecode method.
seenStartOfImage bool
// truncated is whether the input is missing the final 6 consecutive EOL's
// (for Group3) or 2 consecutive EOL's (for Group4). Omitting that trailer
// (but otherwise padding to a byte boundary, with either all 0 bits or all
// 1 bits) is invalid according to the spec, but happens in practice when
// exporting from Adobe Acrobat to TIFF + CCITT. This package silently
// ignores the format error for CCITT input that has been truncated in that
// fashion, returning the full decoded image.
//
// Detecting trailer truncation (just after the final row of pixels)
// requires knowing which row is the final row, and therefore does not
// trigger if the image height is not known in advance.
truncated bool
// readErr is a sticky error for the Read method.
readErr error
}
func (z *reader) Read(p []byte) (int, error) {
if z.readErr != nil {
return 0, z.readErr
}
originalP := p
for len(p) > 0 {
// Allocate buffers (and decode any start-of-image codes), if
// processing the first or second row.
if z.curr == nil {
if !z.seenStartOfImage {
if z.readErr = z.startDecode(); z.readErr != nil {
break
}
z.atStartOfRow = true
}
z.curr = make([]byte, z.width)
}
// Decode the next row, if necessary.
if z.atStartOfRow {
if z.rowsRemaining < 0 {
// We do not know the image height in advance. See if the next
// code is an EOL. If it is, it is consumed. If it isn't, the
// bitReader shouldn't advance along the bit stream, and we
// simply decode another row of pixel data.
//
// For the Group4 subFormat, we may need to align to a byte
// boundary. For the Group3 subFormat, the previous z.decodeRow
// call (or z.startDecode call) has already consumed one of the
// 6 consecutive EOL's. The next EOL is actually the second of
// 6, in the middle, and we shouldn't align at that point.
if z.align && (z.subFormat == Group4) {
z.br.alignToByteBoundary()
}
if err := z.decodeEOL(); err == errMissingEOL {
// No-op. It's another row of pixel data.
} else if err != nil {
z.readErr = err
break
} else {
if z.readErr = z.finishDecode(true); z.readErr != nil {
break
}
z.readErr = io.EOF
break
}
} else if z.rowsRemaining == 0 {
// We do know the image height in advance, and we have already
// decoded exactly that many rows.
if z.readErr = z.finishDecode(false); z.readErr != nil {
break
}
z.readErr = io.EOF
break
} else {
z.rowsRemaining--
}
if z.readErr = z.decodeRow(z.rowsRemaining == 0); z.readErr != nil {
break
}
}
// Pack from z.curr (1 byte per pixel) to p (1 bit per pixel).
packD, packS := highBits(p, z.curr[z.ri:], z.invert)
p = p[packD:]
z.ri += packS
// Prepare to decode the next row, if necessary.
if z.ri == len(z.curr) {
z.ri, z.curr, z.prev = 0, z.prev, z.curr
z.atStartOfRow = true
}
}
n := len(originalP) - len(p)
if z.invert {
invertBytes(originalP[:n])
}
return n, z.readErr
}
func (z *reader) penColor() byte {
if z.penColorIsWhite {
return 0xFF
}
return 0x00
}
func (z *reader) startDecode() error {
switch z.subFormat {
case Group3:
if err := z.decodeEOL(); err != nil {
return err
}
case Group4:
// No-op.
default:
return errUnsupportedSubFormat
}
z.seenStartOfImage = true
return nil
}
func (z *reader) finishDecode(alreadySeenEOL bool) error {
numberOfEOLs := 0
switch z.subFormat {
case Group3:
if z.truncated {
return nil
}
// The stream ends with a RTC (Return To Control) of 6 consecutive
// EOL's, but we should have already just seen an EOL, either in
// z.startDecode (for a zero-height image) or in z.decodeRow.
numberOfEOLs = 5
case Group4:
autoDetectHeight := z.rowsRemaining < 0
if autoDetectHeight {
// Aligning to a byte boundary was already handled by reader.Read.
} else if z.align {
z.br.alignToByteBoundary()
}
// The stream ends with two EOL's. If the first one is missing, and we
// had an explicit image height, we just assume that the trailing two
// EOL's were truncated and return a nil error.
if err := z.decodeEOL(); err != nil {
if (err == errMissingEOL) && !autoDetectHeight {
z.truncated = true
return nil
}
return err
}
numberOfEOLs = 1
default:
return errUnsupportedSubFormat
}
if alreadySeenEOL {
numberOfEOLs--
}
for ; numberOfEOLs > 0; numberOfEOLs-- {
if err := z.decodeEOL(); err != nil {
return err
}
}
return nil
}
func (z *reader) decodeEOL() error {
return decodeEOL(&z.br)
}
func (z *reader) decodeRow(finalRow bool) error {
z.wi = 0
z.atStartOfRow = true
z.penColorIsWhite = true
if z.align {
z.br.alignToByteBoundary()
}
switch z.subFormat {
case Group3:
for ; z.wi < len(z.curr); z.atStartOfRow = false {
if err := z.decodeRun(); err != nil {
return err
}
}
err := z.decodeEOL()
if finalRow && (err == errMissingEOL) {
z.truncated = true
return nil
}
return err
case Group4:
for ; z.wi < len(z.curr); z.atStartOfRow = false {
mode, err := decode(&z.br, modeDecodeTable[:])
if err != nil {
return err
}
rm := readerMode{}
if mode < uint32(len(readerModes)) {
rm = readerModes[mode]
}
if rm.function == nil {
return errInvalidMode
}
if err := rm.function(z, rm.arg); err != nil {
return err
}
}
return nil
}
return errUnsupportedSubFormat
}
func (z *reader) decodeRun() error {
table := blackDecodeTable[:]
if z.penColorIsWhite {
table = whiteDecodeTable[:]
}
total := 0
for {
n, err := decode(&z.br, table)
if err != nil {
return err
}
if n > maxWidth {
panic("unreachable")
}
total += int(n)
if total > maxWidth {
return errRunLengthTooLong
}
// Anything 0x3F or below is a terminal code.
if n <= 0x3F {
break
}
}
if total > (len(z.curr) - z.wi) {
return errRunLengthOverflowsWidth
}
dst := z.curr[z.wi : z.wi+total]
penColor := z.penColor()
for i := range dst {
dst[i] = penColor
}
z.wi += total
z.penColorIsWhite = !z.penColorIsWhite
return nil
}
// The various modes' semantics are based on determining a row of pixels'
// "changing elements": those pixels whose color differs from the one on its
// immediate left.
//
// The row above the first row is implicitly all white. Similarly, the column
// to the left of the first column is implicitly all white.
//
// For example, here's Figure 1 in "ITU-T Recommendation T.6", where the
// current and previous rows contain black (B) and white (w) pixels. The a?
// indexes point into curr, the b? indexes point into prev.
//
// b1 b2
// v v
// prev: BBBBBwwwwwBBBwwwww
// curr: BBBwwwwwBBBBBBwwww
// ^ ^ ^
// a0 a1 a2
//
// a0 is the "reference element" or current decoder position, roughly
// equivalent to what this implementation calls reader.wi.
//
// a1 is the next changing element to the right of a0, on the "coding line"
// (the current row).
//
// a2 is the next changing element to the right of a1, again on curr.
//
// b1 is the first changing element on the "reference line" (the previous row)
// to the right of a0 and of opposite color to a0.
//
// b2 is the next changing element to the right of b1, again on prev.
//
// The various modes calculate a1 (and a2, for modeH):
// - modePass calculates that a1 is at or to the right of b2.
// - modeH calculates a1 and a2 without considering b1 or b2.
// - modeV* calculates a1 to be b1 plus an adjustment (between -3 and +3).
const (
findB1 = false
findB2 = true
)
// findB finds either the b1 or b2 value.
func (z *reader) findB(whichB bool) int {
// The initial row is a special case. The previous row is implicitly all
// white, so that there are no changing pixel elements. We return b1 or b2
// to be at the end of the row.
if len(z.prev) != len(z.curr) {
return len(z.curr)
}
i := z.wi
if z.atStartOfRow {
// a0 is implicitly at -1, on a white pixel. b1 is the first black
// pixel in the previous row. b2 is the first white pixel after that.
for ; (i < len(z.prev)) && (z.prev[i] == 0xFF); i++ {
}
if whichB == findB2 {
for ; (i < len(z.prev)) && (z.prev[i] == 0x00); i++ {
}
}
return i
}
// As per figure 1 above, assume that the current pen color is white.
// First, walk past every contiguous black pixel in prev, starting at a0.
oppositeColor := ^z.penColor()
for ; (i < len(z.prev)) && (z.prev[i] == oppositeColor); i++ {
}
// Then walk past every contiguous white pixel.
penColor := ^oppositeColor
for ; (i < len(z.prev)) && (z.prev[i] == penColor); i++ {
}
// We're now at a black pixel (or at the end of the row). That's b1.
if whichB == findB2 {
// If we're looking for b2, walk past every contiguous black pixel
// again.
oppositeColor := ^penColor
for ; (i < len(z.prev)) && (z.prev[i] == oppositeColor); i++ {
}
}
return i
}
type readerMode struct {
function func(z *reader, arg int) error
arg int
}
var readerModes = [...]readerMode{
modePass: {function: readerModePass},
modeH: {function: readerModeH},
modeV0: {function: readerModeV, arg: +0},
modeVR1: {function: readerModeV, arg: +1},
modeVR2: {function: readerModeV, arg: +2},
modeVR3: {function: readerModeV, arg: +3},
modeVL1: {function: readerModeV, arg: -1},
modeVL2: {function: readerModeV, arg: -2},
modeVL3: {function: readerModeV, arg: -3},
modeExt: {function: readerModeExt},
}
func readerModePass(z *reader, arg int) error {
b2 := z.findB(findB2)
if (b2 < z.wi) || (len(z.curr) < b2) {
return errInvalidOffset
}
dst := z.curr[z.wi:b2]
penColor := z.penColor()
for i := range dst {
dst[i] = penColor
}
z.wi = b2
return nil
}
func readerModeH(z *reader, arg int) error {
// The first iteration finds a1. The second finds a2.
for i := 0; i < 2; i++ {
if err := z.decodeRun(); err != nil {
return err
}
}
return nil
}
func readerModeV(z *reader, arg int) error {
a1 := z.findB(findB1) + arg
if (a1 < z.wi) || (len(z.curr) < a1) {
return errInvalidOffset
}
dst := z.curr[z.wi:a1]
penColor := z.penColor()
for i := range dst {
dst[i] = penColor
}
z.wi = a1
z.penColorIsWhite = !z.penColorIsWhite
return nil
}
func readerModeExt(z *reader, arg int) error {
return errUnsupportedMode
}
// DecodeIntoGray decodes the CCITT-formatted data in r into dst.
//
// It returns an error if dst's width and height don't match the implied width
// and height of CCITT-formatted data.
func DecodeIntoGray(dst *image.Gray, r io.Reader, order Order, sf SubFormat, opts *Options) error {
bounds := dst.Bounds()
if (bounds.Dx() < 0) || (bounds.Dy() < 0) {
return errInvalidBounds
}
if bounds.Dx() > maxWidth {
return errUnsupportedWidth
}
z := reader{
br: bitReader{r: r, order: order},
subFormat: sf,
align: (opts != nil) && opts.Align,
invert: (opts != nil) && opts.Invert,
width: bounds.Dx(),
}
if err := z.startDecode(); err != nil {
return err
}
width := bounds.Dx()
for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
p := (y - bounds.Min.Y) * dst.Stride
z.curr = dst.Pix[p : p+width]
if err := z.decodeRow(y+1 == bounds.Max.Y); err != nil {
return err
}
z.curr, z.prev = nil, z.curr
}
if err := z.finishDecode(false); err != nil {
return err
}
if z.invert {
for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
p := (y - bounds.Min.Y) * dst.Stride
invertBytes(dst.Pix[p : p+width])
}
}
return nil
}
// NewReader returns an io.Reader that decodes the CCITT-formatted data in r.
// The resultant byte stream is one bit per pixel (MSB first), with 1 meaning
// white and 0 meaning black. Each row in the result is byte-aligned.
//
// A negative height, such as passing AutoDetectHeight, means that the image
// height is not known in advance. A negative width is invalid.
func NewReader(r io.Reader, order Order, sf SubFormat, width int, height int, opts *Options) io.Reader {
readErr := error(nil)
if width < 0 {
readErr = errInvalidBounds
} else if width > maxWidth {
readErr = errUnsupportedWidth
}
return &reader{
br: bitReader{r: r, order: order},
subFormat: sf,
align: (opts != nil) && opts.Align,
invert: (opts != nil) && opts.Invert,
width: width,
rowsRemaining: height,
readErr: readErr,
}
}
