writer.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

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

// Encoder configures encoding PNG images.
type Encoder struct {
	CompressionLevel CompressionLevel

	// BufferPool optionally specifies a buffer pool to get temporary
	// EncoderBuffers when encoding an image.
	BufferPool EncoderBufferPool
}

// EncoderBufferPool is an interface for getting and returning temporary
// instances of the EncoderBuffer struct. This can be used to reuse buffers
// when encoding multiple images.
type EncoderBufferPool interface {
	Get() *EncoderBuffer
	Put(*EncoderBuffer)
}

// EncoderBuffer holds the buffers used for encoding PNG images.
type EncoderBuffer encoder

type encoder struct {
	enc     *Encoder
	w       io.Writer
	m       *img1b.Image
	cb      int
	err     error
	header  [8]byte
	footer  [4]byte
	tmp     [4 * 256]byte
	cr      []byte
	zw      *zlib.Writer
	zwLevel int
	bw      *bufio.Writer
}

type CompressionLevel int

const (
	DefaultCompression CompressionLevel = 0
	NoCompression      CompressionLevel = -1
	BestSpeed          CompressionLevel = -2
	BestCompression    CompressionLevel = -3

	// Positive CompressionLevel values are reserved to mean a numeric zlib
	// compression level, although that is not implemented yet.
)

func (e *encoder) writeChunk(b []byte, name string) {
	if e.err != nil {
		return
	}
	n := uint32(len(b))
	if int(n) != len(b) {
		e.err = UnsupportedError(name + " chunk is too large: " + strconv.Itoa(len(b)))
		return
	}
	binary.BigEndian.PutUint32(e.header[:4], n)
	e.header[4] = name[0]
	e.header[5] = name[1]
	e.header[6] = name[2]
	e.header[7] = name[3]
	crc := crc32.NewIEEE()
	crc.Write(e.header[4:8])
	crc.Write(b)
	binary.BigEndian.PutUint32(e.footer[:4], crc.Sum32())

	_, e.err = e.w.Write(e.header[:8])
	if e.err != nil {
		return
	}
	_, e.err = e.w.Write(b)
	if e.err != nil {
		return
	}
	_, e.err = e.w.Write(e.footer[:4])
}

func (e *encoder) writeIHDR() {
	b := e.m.Bounds()
	binary.BigEndian.PutUint32(e.tmp[0:4], uint32(b.Dx()))
	binary.BigEndian.PutUint32(e.tmp[4:8], uint32(b.Dy()))
	// Set bit depth and color type.
	switch e.cb {
	case cbG1:
		e.tmp[8] = 1
		e.tmp[9] = ctGrayscale
	case cbP1:
		e.tmp[8] = 1
		e.tmp[9] = ctPaletted
	}
	e.tmp[10] = 0 // default compression method
	e.tmp[11] = 0 // default filter method
	e.tmp[12] = 0 // non-interlaced
	e.writeChunk(e.tmp[:13], "IHDR")
}

func (e *encoder) writePLTEAndTRNS(p color.Palette) {
	if len(p) < 1 || len(p) > 2 {
		e.err = FormatError("bad palette length: " + strconv.Itoa(len(p)))
		return
	}
	last := -1
	for i, c := range p {
		c1 := color.NRGBAModel.Convert(c).(color.NRGBA)
		e.tmp[3*i+0] = c1.R
		e.tmp[3*i+1] = c1.G
		e.tmp[3*i+2] = c1.B
		if c1.A != 0xff {
			last = i
		}
		e.tmp[3*256+i] = c1.A
	}
	e.writeChunk(e.tmp[:3*len(p)], "PLTE")
	if last != -1 {
		e.writeChunk(e.tmp[3*256:3*256+1+last], "tRNS")
	}
}

// An encoder is an io.Writer that satisfies writes by writing PNG IDAT chunks,
// including an 8-byte header and 4-byte CRC checksum per Write call. Such calls
// should be relatively infrequent, since writeIDATs uses a bufio.Writer.
//
// This method should only be called from writeIDATs (via writeImage).
// No other code should treat an encoder as an io.Writer.
func (e *encoder) Write(b []byte) (int, error) {
	e.writeChunk(b, "IDAT")
	if e.err != nil {
		return 0, e.err
	}
	return len(b), nil
}

func (e *encoder) writeImage(w io.Writer, m *img1b.Image, cb int, level int) error {
	if e.zw == nil || e.zwLevel != level {
		zw, err := zlib.NewWriterLevel(w, level)
		if err != nil {
			return err
		}
		e.zw = zw
		e.zwLevel = level
	} else {
		e.zw.Reset(w)
	}
	defer e.zw.Close()

	b := m.Bounds()
	sz := 1 + (b.Dx()+7)/8
	if cap(e.cr) < sz {
		e.cr = make([]byte, sz)
	} else {
		e.cr = e.cr[:sz]
	}
	cr := e.cr

	// Mask to blank out of bounds bits.
	tm := byte(uint16(0xff00) >> ((b.Dx()-1)%8 + 1))
	lb := tm &^ (tm << 1)

	for y := b.Min.Y; y < b.Max.Y; y++ {
		offset := (y - b.Min.Y) * m.Stride
		copy(cr[1:], m.Pix[offset:offset+(b.Dx()+7)/8])
		// Extend the row last pixel till the end of the byte.
		// It seems to result in slightly better compression than just zeroing.
		if cr[sz-1]&lb == 0 {
			cr[sz-1] &= tm
		} else {
			cr[sz-1] |= ^tm
		}

		// Write the compressed bytes.
		if _, err := e.zw.Write(cr); err != nil {
			return err
		}
	}
	return nil
}

// Write the actual image data to one or more IDAT chunks.
func (e *encoder) writeIDATs() {
	if e.err != nil {
		return
	}
	if e.bw == nil {
		e.bw = bufio.NewWriterSize(e, 1<<15)
	} else {
		e.bw.Reset(e)
	}
	e.err = e.writeImage(e.bw, e.m, e.cb, levelToZlib(e.enc.CompressionLevel))
	if e.err != nil {
		return
	}
	e.err = e.bw.Flush()
}

// This function is required because we want the zero value of
// Encoder.CompressionLevel to map to zlib.DefaultCompression.
func levelToZlib(l CompressionLevel) int {
	switch l {
	case DefaultCompression:
		return zlib.DefaultCompression
	case NoCompression:
		return zlib.NoCompression
	case BestSpeed:
		return zlib.BestSpeed
	case BestCompression:
		return zlib.BestCompression
	default:
		return zlib.DefaultCompression
	}
}

func (e *encoder) writeIEND() { e.writeChunk(nil, "IEND") }

func isBlackWhite(pal color.Palette) bool {
	return len(pal) > 1 &&
		color.RGBAModel.Convert(pal[0]) == color.RGBAModel.Convert(color.Black) &&
		color.RGBAModel.Convert(pal[1]) == color.RGBAModel.Convert(color.White)
}

// Encode writes the Image m to w in PNG format.
func Encode(w io.Writer, m *img1b.Image) error {
	var e Encoder
	return e.Encode(w, m)
}

// Encode writes the Image m to w in PNG format.
func (enc *Encoder) Encode(w io.Writer, m *img1b.Image) error {
	// Obviously, negative widths and heights are invalid. Furthermore, the PNG
	// spec section 11.2.2 says that zero is invalid. Excessively large images are
	// also rejected.
	mw, mh := int64(m.Bounds().Dx()), int64(m.Bounds().Dy())
	if mw <= 0 || mh <= 0 || mw >= 1<<32 || mh >= 1<<32 {
		return FormatError("invalid image size: " + strconv.FormatInt(mw, 10) + "x" + strconv.FormatInt(mh, 10))
	}

	var e *encoder
	if enc.BufferPool != nil {
		buffer := enc.BufferPool.Get()
		e = (*encoder)(buffer)

	}
	if e == nil {
		e = &encoder{}
	}
	if enc.BufferPool != nil {
		defer enc.BufferPool.Put((*EncoderBuffer)(e))
	}

	e.enc = enc
	e.w = w
	e.m = m

	e.cb = cbP1
	pal := m.Palette
	if isBlackWhite(pal) {
		e.cb = cbG1
		pal = nil
	}

	_, e.err = io.WriteString(w, pngHeader)
	e.writeIHDR()
	if pal != nil {
		e.writePLTEAndTRNS(pal)
	}
	e.writeIDATs()
	e.writeIEND()
	return e.err
}