reader.go

// Copyright 2009 The Go Authors. All rights reserved.
// Copyright 2020 Mikhail Vladimirov
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package png implements a PNG image decoder and encoder. Only 1-bit images can
// be decoded (grayscale or paletted).
//
// The PNG specification is at https://www.w3.org/TR/PNG/.
package png

import (
	"compress/zlib"
	"encoding/binary"
	"fmt"
	"github.com/mi-v/img1b"
	"hash"
	"hash/crc32"
	"image"
	"image/color"
	"io"
)

// Color type, as per the PNG spec.
const (
	ctGrayscale      = 0
	ctTrueColor      = 2
	ctPaletted       = 3
	ctGrayscaleAlpha = 4
	ctTrueColorAlpha = 6
)

// A cb is a combination of color type and bit depth.
const (
	cbInvalid = iota
	cbG1
	cbG2
	cbG4
	cbG8
	cbGA8
	cbTC8
	cbP1
	cbP2
	cbP4
	cbP8
	cbTCA8
	cbG16
	cbGA16
	cbTC16
	cbTCA16
)

func cbPaletted(cb int) bool {
	return cbP1 <= cb && cb <= cbP8
}

// Filter type, as per the PNG spec.
const (
	ftNone    = 0
	ftSub     = 1
	ftUp      = 2
	ftAverage = 3
	ftPaeth   = 4
	nFilter   = 5
)

// Interlace type.
const (
	itNone  = 0
	itAdam7 = 1
)

// interlaceScan defines the placement and size of a pass for Adam7 interlacing.
type interlaceScan struct {
	xFactor, yFactor, xOffset, yOffset int
}

// interlacing defines Adam7 interlacing, with 7 passes of reduced images.
// See https://www.w3.org/TR/PNG/#8Interlace
var interlacing = []interlaceScan{
	{8, 8, 0, 0},
	{8, 8, 4, 0},
	{4, 8, 0, 4},
	{4, 4, 2, 0},
	{2, 4, 0, 2},
	{2, 2, 1, 0},
	{1, 2, 0, 1},
}

// Decoding stage.
// The PNG specification says that the IHDR, PLTE (if present), tRNS (if
// present), IDAT and IEND chunks must appear in that order. There may be
// multiple IDAT chunks, and IDAT chunks must be sequential (i.e. they may not
// have any other chunks between them).
// https://www.w3.org/TR/PNG/#5ChunkOrdering
const (
	dsStart = iota
	dsSeenIHDR
	dsSeenPLTE
	dsSeentRNS
	dsSeenIDAT
	dsSeenIEND
)

const pngHeader = "\x89PNG\r\n\x1a\n"

type decoder struct {
	r             io.Reader
	img           *img1b.Image
	crc           hash.Hash32
	width, height int
	depth         int
	palette       color.Palette
	cb            int
	stage         int
	idatLength    uint32
	tmp           [3 * 256]byte
	interlace     int
}

// A FormatError reports that the input is not a valid PNG.
type FormatError string

func (e FormatError) Error() string { return "png: invalid format: " + string(e) }

var chunkOrderError = FormatError("chunk out of order")

// An UnsupportedError reports that the input uses a valid but unimplemented PNG feature.
type UnsupportedError string

func (e UnsupportedError) Error() string { return "png: unsupported feature: " + string(e) }

func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}

func (d *decoder) parseIHDR(length uint32) error {
	if length != 13 {
		return FormatError("bad IHDR length")
	}
	if _, err := io.ReadFull(d.r, d.tmp[:13]); err != nil {
		return err
	}
	d.crc.Write(d.tmp[:13])
	if d.tmp[10] != 0 {
		return UnsupportedError("compression method")
	}
	if d.tmp[11] != 0 {
		return UnsupportedError("filter method")
	}
	if d.tmp[12] != itNone && d.tmp[12] != itAdam7 {
		return FormatError("invalid interlace method")
	}
	d.interlace = int(d.tmp[12])

	w := int32(binary.BigEndian.Uint32(d.tmp[0:4]))
	h := int32(binary.BigEndian.Uint32(d.tmp[4:8]))
	if w <= 0 || h <= 0 {
		return FormatError("non-positive dimension")
	}
	nPixels := int64(w) * int64(h)
	if nPixels != int64(int(nPixels)) {
		return UnsupportedError("dimension overflow")
	}

	d.cb = cbInvalid
	d.depth = int(d.tmp[8])
	if d.depth == 1 {
		switch d.tmp[9] {
		case ctGrayscale:
			d.cb = cbG1
		case ctPaletted:
			d.cb = cbP1
		}
	}
	if d.cb == cbInvalid {
		return UnsupportedError(fmt.Sprintf("bit depth %d, color type %d", d.tmp[8], d.tmp[9]))
	}
	d.width, d.height = int(w), int(h)
	return d.verifyChecksum()
}

func (d *decoder) parsePLTE(length uint32) error {
	np := int(length / 3) // The number of palette entries.
	if length%3 != 0 || np < 1 || np > 2 {
		return FormatError("bad PLTE length")
	}
	n, err := io.ReadFull(d.r, d.tmp[:3*np])
	if err != nil {
		return err
	}
	d.crc.Write(d.tmp[:n])

	if d.cb != cbP1 {
		return FormatError("PLTE, color type mismatch")
	}

	d.palette[0] = color.RGBA{d.tmp[0], d.tmp[1], d.tmp[2], 0xff}
	if np == 2 {
		d.palette[1] = color.RGBA{d.tmp[3], d.tmp[4], d.tmp[5], 0xff}
	} else {
		d.palette[1] = color.RGBA{0x00, 0x00, 0x00, 0xff}
	}

	return d.verifyChecksum()
}

func (d *decoder) parsetRNS(length uint32) error {
	switch d.cb {
	case cbG1:
		if length != 2 {
			return FormatError("bad tRNS length")
		}
		n, err := io.ReadFull(d.r, d.tmp[:length])
		if err != nil {
			return err
		}
		d.crc.Write(d.tmp[:n])

		rgba := d.palette[d.tmp[1]&1].(color.RGBA)
		d.palette[d.tmp[1]&1] = color.NRGBA{rgba.R, rgba.G, rgba.B, 0}

	case cbP1:
		if length > 2 {
			return FormatError("bad tRNS length")
		}
		n, err := io.ReadFull(d.r, d.tmp[:length])
		if err != nil {
			return err
		}
		d.crc.Write(d.tmp[:n])

		if len(d.palette) < n {
			d.palette = d.palette[:n]
		}
		for i := 0; i < n; i++ {
			rgba := d.palette[i].(color.RGBA)
			d.palette[i] = color.NRGBA{rgba.R, rgba.G, rgba.B, d.tmp[i]}
		}

	default:
		return FormatError("tRNS, color type mismatch")
	}
	return d.verifyChecksum()
}

// Read presents one or more IDAT chunks as one continuous stream (minus the
// intermediate chunk headers and footers). If the PNG data looked like:
//   ... len0 IDAT xxx crc0 len1 IDAT yy crc1 len2 IEND crc2
// then this reader presents xxxyy. For well-formed PNG data, the decoder state
// immediately before the first Read call is that d.r is positioned between the
// first IDAT and xxx, and the decoder state immediately after the last Read
// call is that d.r is positioned between yy and crc1.
func (d *decoder) Read(p []byte) (int, error) {
	if len(p) == 0 {
		return 0, nil
	}
	for d.idatLength == 0 {
		// We have exhausted an IDAT chunk. Verify the checksum of that chunk.
		if err := d.verifyChecksum(); err != nil {
			return 0, err
		}
		// Read the length and chunk type of the next chunk, and check that
		// it is an IDAT chunk.
		if _, err := io.ReadFull(d.r, d.tmp[:8]); err != nil {
			return 0, err
		}
		d.idatLength = binary.BigEndian.Uint32(d.tmp[:4])
		if string(d.tmp[4:8]) != "IDAT" {
			return 0, FormatError("not enough pixel data")
		}
		d.crc.Reset()
		d.crc.Write(d.tmp[4:8])
	}
	if int(d.idatLength) < 0 {
		return 0, UnsupportedError("IDAT chunk length overflow")
	}
	n, err := d.r.Read(p[:min(len(p), int(d.idatLength))])
	d.crc.Write(p[:n])
	d.idatLength -= uint32(n)
	return n, err
}

// decode decodes the IDAT data into an image.
func (d *decoder) decode() (*img1b.Image, error) {
	r, err := zlib.NewReader(d)
	if err != nil {
		return nil, err
	}
	defer r.Close()
	var img *img1b.Image
	if d.interlace == itNone {
		img, err = d.readImagePass(r, 0, false)
		if err != nil {
			return nil, err
		}
	} else if d.interlace == itAdam7 {
		// Allocate a blank image of the full size.
		img, err = d.readImagePass(nil, 0, true)
		if err != nil {
			return nil, err
		}
		for pass := 0; pass < 7; pass++ {
			imagePass, err := d.readImagePass(r, pass, false)
			if err != nil {
				return nil, err
			}
			if imagePass != nil {
				d.mergePassInto(img, imagePass, pass)
			}
		}
	}

	// Check for EOF, to verify the zlib checksum.
	n := 0
	for i := 0; n == 0 && err == nil; i++ {
		if i == 100 {
			return nil, io.ErrNoProgress
		}
		n, err = r.Read(d.tmp[:1])
	}
	if err != nil && err != io.EOF {
		return nil, FormatError(err.Error())
	}
	if n != 0 || d.idatLength != 0 {
		return nil, FormatError("too much pixel data")
	}

	return img, nil
}

// readImagePass reads a single image pass, sized according to the pass number.
func (d *decoder) readImagePass(r io.Reader, pass int, allocateOnly bool) (*img1b.Image, error) {
	pixOffset := 0
	var img *img1b.Image

	width, height := d.width, d.height
	if d.interlace == itAdam7 && !allocateOnly {
		p := interlacing[pass]
		// Add the multiplication factor and subtract one, effectively rounding up.
		width = (width - p.xOffset + p.xFactor - 1) / p.xFactor
		height = (height - p.yOffset + p.yFactor - 1) / p.yFactor
		// A PNG image can't have zero width or height, but for an interlaced
		// image, an individual pass might have zero width or height. If so, we
		// shouldn't even read a per-row filter type byte, so return early.
		if width == 0 || height == 0 {
			return nil, nil
		}
	}
	img = img1b.New(image.Rect(0, 0, width, height), d.palette)
	if allocateOnly {
		return img, nil
	}

	// The +1 is for the per-row filter type, which is at cr[0].
	rowSize := 1 + (width+7)/8
	// cr and pr are the bytes for the current and previous row.
	cr := make([]uint8, rowSize)
	pr := make([]uint8, rowSize)

	for y := 0; y < height; y++ {
		// Read the decompressed bytes.
		_, err := io.ReadFull(r, cr)
		if err != nil {
			if err == io.EOF || err == io.ErrUnexpectedEOF {
				return nil, FormatError("not enough pixel data")
			}
			return nil, err
		}

		// Apply the filter.
		cdat := cr[1:]
		pdat := pr[1:]
		switch cr[0] {
		case ftNone:
			// No-op.
		case ftSub:
			for i := 1; i < len(cdat); i++ {
				cdat[i] += cdat[i-1]
			}
		case ftUp:
			for i, p := range pdat {
				cdat[i] += p
			}
		case ftAverage:
			// The first column has no column to the left of it, so it is a
			// special case. We know that the first column exists because we
			// check above that width != 0, and so len(cdat) != 0.
			cdat[0] += pdat[0] / 2
			for i := 1; i < len(cdat); i++ {
				cdat[i] += uint8((int(cdat[i-1]) + int(pdat[i])) / 2)
			}
		case ftPaeth:
			filterPaeth(cdat, pdat, 1)
		default:
			return nil, FormatError("bad filter type")
		}

		copy(img.Pix[pixOffset:], cdat)
		pixOffset += img.Stride

		// The current row for y is the previous row for y+1.
		pr, cr = cr, pr
	}

	return img, nil
}

// mergePassInto merges a single pass into a full sized image.
func (d *decoder) mergePassInto(dst *img1b.Image, src *img1b.Image, pass int) {
	p := interlacing[pass]
	var (
		rect image.Rectangle
	)
	rect = dst.Rect
	bounds := src.Bounds()
	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
		dstY := y*p.yFactor + p.yOffset - rect.Min.Y
		for x := bounds.Min.X; x < bounds.Max.X; x++ {
			dstX := x*p.xFactor + p.xOffset - rect.Min.X
			dst.SetColorIndex(dstX, dstY, src.ColorIndexAt(x, y))
		}
	}
}

func (d *decoder) parseIDAT(length uint32) (err error) {
	d.idatLength = length
	d.img, err = d.decode()
	if err != nil {
		return err
	}
	return d.verifyChecksum()
}

func (d *decoder) parseIEND(length uint32) error {
	if length != 0 {
		return FormatError("bad IEND length")
	}
	return d.verifyChecksum()
}

func (d *decoder) parseChunk() error {
	// Read the length and chunk type.
	_, err := io.ReadFull(d.r, d.tmp[:8])
	if err != nil {
		return err
	}
	length := binary.BigEndian.Uint32(d.tmp[:4])
	d.crc.Reset()
	d.crc.Write(d.tmp[4:8])

	// Read the chunk data.
	switch string(d.tmp[4:8]) {
	case "IHDR":
		if d.stage != dsStart {
			return chunkOrderError
		}
		d.stage = dsSeenIHDR
		return d.parseIHDR(length)
	case "PLTE":
		if d.stage != dsSeenIHDR {
			return chunkOrderError
		}
		d.stage = dsSeenPLTE
		return d.parsePLTE(length)
	case "tRNS":
		if cbPaletted(d.cb) {
			if d.stage != dsSeenPLTE {
				return chunkOrderError
			}
		} else if d.stage != dsSeenIHDR {
			return chunkOrderError
		}
		d.stage = dsSeentRNS
		return d.parsetRNS(length)
	case "IDAT":
		if d.stage < dsSeenIHDR || d.stage > dsSeenIDAT || (d.stage == dsSeenIHDR && cbPaletted(d.cb)) {
			return chunkOrderError
		} else if d.stage == dsSeenIDAT {
			// Ignore trailing zero-length or garbage IDAT chunks.
			//
			// This does not affect valid PNG images that contain multiple IDAT
			// chunks, since the first call to parseIDAT below will consume all
			// consecutive IDAT chunks required for decoding the image.
			break
		}
		d.stage = dsSeenIDAT
		return d.parseIDAT(length)
	case "IEND":
		if d.stage != dsSeenIDAT {
			return chunkOrderError
		}
		d.stage = dsSeenIEND
		return d.parseIEND(length)
	}
	if length > 0x7fffffff {
		return FormatError(fmt.Sprintf("Bad chunk length: %d", length))
	}
	// Ignore this chunk (of a known length).
	var ignored [4096]byte
	for length > 0 {
		n, err := io.ReadFull(d.r, ignored[:min(len(ignored), int(length))])
		if err != nil {
			return err
		}
		d.crc.Write(ignored[:n])
		length -= uint32(n)
	}
	return d.verifyChecksum()
}

func (d *decoder) verifyChecksum() error {
	if _, err := io.ReadFull(d.r, d.tmp[:4]); err != nil {
		return err
	}
	if binary.BigEndian.Uint32(d.tmp[:4]) != d.crc.Sum32() {
		return FormatError("invalid checksum")
	}
	return nil
}

func (d *decoder) checkHeader() error {
	_, err := io.ReadFull(d.r, d.tmp[:len(pngHeader)])
	if err != nil {
		return err
	}
	if string(d.tmp[:len(pngHeader)]) != pngHeader {
		return FormatError("not a PNG file")
	}
	return nil
}

// Decode reads a PNG image from r and returns it as an img1b.Image.
func Decode(r io.Reader) (*img1b.Image, error) {
	d := &decoder{
		r:   r,
		crc: crc32.NewIEEE(),
		palette: color.Palette{
			color.RGBAModel.Convert(color.Black),
			color.RGBAModel.Convert(color.White),
		},
	}
	if err := d.checkHeader(); err != nil {
		if err == io.EOF {
			err = io.ErrUnexpectedEOF
		}
		return nil, err
	}
	for d.stage != dsSeenIEND {
		if err := d.parseChunk(); err != nil {
			if err == io.EOF {
				err = io.ErrUnexpectedEOF
			}
			return nil, err
		}
	}
	return d.img, nil
}

// DecodeConfig returns the color model and dimensions of a PNG image without
// decoding the entire image.
func DecodeConfig(r io.Reader) (image.Config, error) {
	d := &decoder{
		r:   r,
		crc: crc32.NewIEEE(),
		palette: color.Palette{
			color.RGBAModel.Convert(color.Black),
			color.RGBAModel.Convert(color.White),
		},
	}
	if err := d.checkHeader(); err != nil {
		if err == io.EOF {
			err = io.ErrUnexpectedEOF
		}
		return image.Config{}, err
	}
	for {
		if err := d.parseChunk(); err != nil {
			if err == io.EOF {
				err = io.ErrUnexpectedEOF
			}
			return image.Config{}, err
		}
		paletted := cbPaletted(d.cb)
		if d.stage == dsSeenIHDR && !paletted {
			break
		}
		if d.stage == dsSeenPLTE && paletted {
			break
		}
	}
	return image.Config{
		ColorModel: d.palette,
		Width:      d.width,
		Height:     d.height,
	}, nil
}